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<p class="section-title">STDLIB</p>
<p class="section-subtitle">Reference Manual</p>
<p class="section-version">Version 3.7</p>
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<h3>Table of Contents</h3>
<ul class="flipMenu">
<li title="STDLIB (App)"><a href="STDLIB_app.html">STDLIB (App)
</a></li>
<li id="no" title="array " expanded="false">array<ul>
<li><a href="array.html">
Top of manual page
</a></li>
<li title="default-1"><a href="array.html#default-1">default/1</a></li>
<li title="fix-1"><a href="array.html#fix-1">fix/1</a></li>
<li title="foldl-3"><a href="array.html#foldl-3">foldl/3</a></li>
<li title="foldr-3"><a href="array.html#foldr-3">foldr/3</a></li>
<li title="from_list-1"><a href="array.html#from_list-1">from_list/1</a></li>
<li title="from_list-2"><a href="array.html#from_list-2">from_list/2</a></li>
<li title="from_orddict-1"><a href="array.html#from_orddict-1">from_orddict/1</a></li>
<li title="from_orddict-2"><a href="array.html#from_orddict-2">from_orddict/2</a></li>
<li title="get-2"><a href="array.html#get-2">get/2</a></li>
<li title="is_array-1"><a href="array.html#is_array-1">is_array/1</a></li>
<li title="is_fix-1"><a href="array.html#is_fix-1">is_fix/1</a></li>
<li title="map-2"><a href="array.html#map-2">map/2</a></li>
<li title="new-0"><a href="array.html#new-0">new/0</a></li>
<li title="new-1"><a href="array.html#new-1">new/1</a></li>
<li title="new-2"><a href="array.html#new-2">new/2</a></li>
<li title="relax-1"><a href="array.html#relax-1">relax/1</a></li>
<li title="reset-2"><a href="array.html#reset-2">reset/2</a></li>
<li title="resize-1"><a href="array.html#resize-1">resize/1</a></li>
<li title="resize-2"><a href="array.html#resize-2">resize/2</a></li>
<li title="set-3"><a href="array.html#set-3">set/3</a></li>
<li title="size-1"><a href="array.html#size-1">size/1</a></li>
<li title="sparse_foldl-3"><a href="array.html#sparse_foldl-3">sparse_foldl/3</a></li>
<li title="sparse_foldr-3"><a href="array.html#sparse_foldr-3">sparse_foldr/3</a></li>
<li title="sparse_map-2"><a href="array.html#sparse_map-2">sparse_map/2</a></li>
<li title="sparse_size-1"><a href="array.html#sparse_size-1">sparse_size/1</a></li>
<li title="sparse_to_list-1"><a href="array.html#sparse_to_list-1">sparse_to_list/1</a></li>
<li title="sparse_to_orddict-1"><a href="array.html#sparse_to_orddict-1">sparse_to_orddict/1</a></li>
<li title="to_list-1"><a href="array.html#to_list-1">to_list/1</a></li>
<li title="to_orddict-1"><a href="array.html#to_orddict-1">to_orddict/1</a></li>
</ul>
</li>
<li title="assert.hrl"><a href="assert.hrl.html">assert.hrl</a></li>
<li id="no" title="base64 " expanded="false">base64<ul>
<li><a href="base64.html">
Top of manual page
</a></li>
<li title="decode-1"><a href="base64.html#decode-1">decode/1</a></li>
<li title="decode_to_string-1"><a href="base64.html#decode_to_string-1">decode_to_string/1</a></li>
<li title="encode-1"><a href="base64.html#encode-1">encode/1</a></li>
<li title="encode_to_string-1"><a href="base64.html#encode_to_string-1">encode_to_string/1</a></li>
<li title="mime_decode-1"><a href="base64.html#mime_decode-1">mime_decode/1</a></li>
<li title="mime_decode_to_string-1"><a href="base64.html#mime_decode_to_string-1">mime_decode_to_string/1</a></li>
</ul>
</li>
<li id="no" title="beam_lib " expanded="false">beam_lib<ul>
<li><a href="beam_lib.html">
Top of manual page
</a></li>
<li title="all_chunks-1"><a href="beam_lib.html#all_chunks-1">all_chunks/1</a></li>
<li title="build_module-1"><a href="beam_lib.html#build_module-1">build_module/1</a></li>
<li title="chunks-2"><a href="beam_lib.html#chunks-2">chunks/2</a></li>
<li title="chunks-3"><a href="beam_lib.html#chunks-3">chunks/3</a></li>
<li title="clear_crypto_key_fun-0"><a href="beam_lib.html#clear_crypto_key_fun-0">clear_crypto_key_fun/0</a></li>
<li title="cmp-2"><a href="beam_lib.html#cmp-2">cmp/2</a></li>
<li title="cmp_dirs-2"><a href="beam_lib.html#cmp_dirs-2">cmp_dirs/2</a></li>
<li title="crypto_key_fun-1"><a href="beam_lib.html#crypto_key_fun-1">crypto_key_fun/1</a></li>
<li title="diff_dirs-2"><a href="beam_lib.html#diff_dirs-2">diff_dirs/2</a></li>
<li title="format_error-1"><a href="beam_lib.html#format_error-1">format_error/1</a></li>
<li title="info-1"><a href="beam_lib.html#info-1">info/1</a></li>
<li title="md5-1"><a href="beam_lib.html#md5-1">md5/1</a></li>
<li title="strip-1"><a href="beam_lib.html#strip-1">strip/1</a></li>
<li title="strip_files-1"><a href="beam_lib.html#strip_files-1">strip_files/1</a></li>
<li title="strip_release-1"><a href="beam_lib.html#strip_release-1">strip_release/1</a></li>
<li title="version-1"><a href="beam_lib.html#version-1">version/1</a></li>
</ul>
</li>
<li id="no" title="binary " expanded="false">binary<ul>
<li><a href="binary.html">
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<li title="at-2"><a href="binary.html#at-2">at/2</a></li>
<li title="bin_to_list-1"><a href="binary.html#bin_to_list-1">bin_to_list/1</a></li>
<li title="bin_to_list-2"><a href="binary.html#bin_to_list-2">bin_to_list/2</a></li>
<li title="bin_to_list-3"><a href="binary.html#bin_to_list-3">bin_to_list/3</a></li>
<li title="compile_pattern-1"><a href="binary.html#compile_pattern-1">compile_pattern/1</a></li>
<li title="copy-1"><a href="binary.html#copy-1">copy/1</a></li>
<li title="copy-2"><a href="binary.html#copy-2">copy/2</a></li>
<li title="decode_unsigned-1"><a href="binary.html#decode_unsigned-1">decode_unsigned/1</a></li>
<li title="decode_unsigned-2"><a href="binary.html#decode_unsigned-2">decode_unsigned/2</a></li>
<li title="encode_unsigned-1"><a href="binary.html#encode_unsigned-1">encode_unsigned/1</a></li>
<li title="encode_unsigned-2"><a href="binary.html#encode_unsigned-2">encode_unsigned/2</a></li>
<li title="first-1"><a href="binary.html#first-1">first/1</a></li>
<li title="last-1"><a href="binary.html#last-1">last/1</a></li>
<li title="list_to_bin-1"><a href="binary.html#list_to_bin-1">list_to_bin/1</a></li>
<li title="longest_common_prefix-1"><a href="binary.html#longest_common_prefix-1">longest_common_prefix/1</a></li>
<li title="longest_common_suffix-1"><a href="binary.html#longest_common_suffix-1">longest_common_suffix/1</a></li>
<li title="match-2"><a href="binary.html#match-2">match/2</a></li>
<li title="match-3"><a href="binary.html#match-3">match/3</a></li>
<li title="matches-2"><a href="binary.html#matches-2">matches/2</a></li>
<li title="matches-3"><a href="binary.html#matches-3">matches/3</a></li>
<li title="part-2"><a href="binary.html#part-2">part/2</a></li>
<li title="part-3"><a href="binary.html#part-3">part/3</a></li>
<li title="referenced_byte_size-1"><a href="binary.html#referenced_byte_size-1">referenced_byte_size/1</a></li>
<li title="replace-3"><a href="binary.html#replace-3">replace/3</a></li>
<li title="replace-4"><a href="binary.html#replace-4">replace/4</a></li>
<li title="split-2"><a href="binary.html#split-2">split/2</a></li>
<li title="split-3"><a href="binary.html#split-3">split/3</a></li>
</ul>
</li>
<li id="no" title="c " expanded="false">c<ul>
<li><a href="c.html">
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<li title="bt-1"><a href="c.html#bt-1">bt/1</a></li>
<li title="c-1"><a href="c.html#c-1">c/1</a></li>
<li title="c-2"><a href="c.html#c-2">c/2</a></li>
<li title="c-3"><a href="c.html#c-3">c/3</a></li>
<li title="cd-1"><a href="c.html#cd-1">cd/1</a></li>
<li title="erlangrc-1"><a href="c.html#erlangrc-1">erlangrc/1</a></li>
<li title="flush-0"><a href="c.html#flush-0">flush/0</a></li>
<li title="help-0"><a href="c.html#help-0">help/0</a></li>
<li title="i-0"><a href="c.html#i-0">i/0</a></li>
<li title="i-3"><a href="c.html#i-3">i/3</a></li>
<li title="l-1"><a href="c.html#l-1">l/1</a></li>
<li title="lc-1"><a href="c.html#lc-1">lc/1</a></li>
<li title="lm-0"><a href="c.html#lm-0">lm/0</a></li>
<li title="ls-0"><a href="c.html#ls-0">ls/0</a></li>
<li title="ls-1"><a href="c.html#ls-1">ls/1</a></li>
<li title="m-0"><a href="c.html#m-0">m/0</a></li>
<li title="m-1"><a href="c.html#m-1">m/1</a></li>
<li title="memory-0"><a href="c.html#memory-0">memory/0</a></li>
<li title="memory-1"><a href="c.html#memory-1">memory/1</a></li>
<li title="mm-0"><a href="c.html#mm-0">mm/0</a></li>
<li title="nc-1"><a href="c.html#nc-1">nc/1</a></li>
<li title="nc-2"><a href="c.html#nc-2">nc/2</a></li>
<li title="ni-0"><a href="c.html#ni-0">ni/0</a></li>
<li title="nl-1"><a href="c.html#nl-1">nl/1</a></li>
<li title="nregs-0"><a href="c.html#nregs-0">nregs/0</a></li>
<li title="pid-3"><a href="c.html#pid-3">pid/3</a></li>
<li title="pwd-0"><a href="c.html#pwd-0">pwd/0</a></li>
<li title="q-0"><a href="c.html#q-0">q/0</a></li>
<li title="regs-0"><a href="c.html#regs-0">regs/0</a></li>
<li title="uptime-0"><a href="c.html#uptime-0">uptime/0</a></li>
<li title="xm-1"><a href="c.html#xm-1">xm/1</a></li>
<li title="y-1"><a href="c.html#y-1">y/1</a></li>
<li title="y-2"><a href="c.html#y-2">y/2</a></li>
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<li id="no" title="calendar " expanded="false">calendar<ul>
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<li title="date_to_gregorian_days-1"><a href="calendar.html#date_to_gregorian_days-1">date_to_gregorian_days/1</a></li>
<li title="date_to_gregorian_days-3"><a href="calendar.html#date_to_gregorian_days-3">date_to_gregorian_days/3</a></li>
<li title="datetime_to_gregorian_seconds-1"><a href="calendar.html#datetime_to_gregorian_seconds-1">datetime_to_gregorian_seconds/1</a></li>
<li title="day_of_the_week-1"><a href="calendar.html#day_of_the_week-1">day_of_the_week/1</a></li>
<li title="day_of_the_week-3"><a href="calendar.html#day_of_the_week-3">day_of_the_week/3</a></li>
<li title="gregorian_days_to_date-1"><a href="calendar.html#gregorian_days_to_date-1">gregorian_days_to_date/1</a></li>
<li title="gregorian_seconds_to_datetime-1"><a href="calendar.html#gregorian_seconds_to_datetime-1">gregorian_seconds_to_datetime/1</a></li>
<li title="is_leap_year-1"><a href="calendar.html#is_leap_year-1">is_leap_year/1</a></li>
<li title="iso_week_number-0"><a href="calendar.html#iso_week_number-0">iso_week_number/0</a></li>
<li title="iso_week_number-1"><a href="calendar.html#iso_week_number-1">iso_week_number/1</a></li>
<li title="last_day_of_the_month-2"><a href="calendar.html#last_day_of_the_month-2">last_day_of_the_month/2</a></li>
<li title="local_time-0"><a href="calendar.html#local_time-0">local_time/0</a></li>
<li title="local_time_to_universal_time-1"><a href="calendar.html#local_time_to_universal_time-1">local_time_to_universal_time/1</a></li>
<li title="local_time_to_universal_time_dst-1"><a href="calendar.html#local_time_to_universal_time_dst-1">local_time_to_universal_time_dst/1</a></li>
<li title="now_to_datetime-1"><a href="calendar.html#now_to_datetime-1">now_to_datetime/1</a></li>
<li title="now_to_local_time-1"><a href="calendar.html#now_to_local_time-1">now_to_local_time/1</a></li>
<li title="now_to_universal_time-1"><a href="calendar.html#now_to_universal_time-1">now_to_universal_time/1</a></li>
<li title="rfc3339_to_system_time-1"><a href="calendar.html#rfc3339_to_system_time-1">rfc3339_to_system_time/1</a></li>
<li title="rfc3339_to_system_time-2"><a href="calendar.html#rfc3339_to_system_time-2">rfc3339_to_system_time/2</a></li>
<li title="seconds_to_daystime-1"><a href="calendar.html#seconds_to_daystime-1">seconds_to_daystime/1</a></li>
<li title="seconds_to_time-1"><a href="calendar.html#seconds_to_time-1">seconds_to_time/1</a></li>
<li title="system_time_to_local_time-2"><a href="calendar.html#system_time_to_local_time-2">system_time_to_local_time/2</a></li>
<li title="system_time_to_rfc3339-1"><a href="calendar.html#system_time_to_rfc3339-1">system_time_to_rfc3339/1</a></li>
<li title="system_time_to_rfc3339-2"><a href="calendar.html#system_time_to_rfc3339-2">system_time_to_rfc3339/2</a></li>
<li title="system_time_to_universal_time-2"><a href="calendar.html#system_time_to_universal_time-2">system_time_to_universal_time/2</a></li>
<li title="time_difference-2"><a href="calendar.html#time_difference-2">time_difference/2</a></li>
<li title="time_to_seconds-1"><a href="calendar.html#time_to_seconds-1">time_to_seconds/1</a></li>
<li title="universal_time-0"><a href="calendar.html#universal_time-0">universal_time/0</a></li>
<li title="universal_time_to_local_time-1"><a href="calendar.html#universal_time_to_local_time-1">universal_time_to_local_time/1</a></li>
<li title="valid_date-1"><a href="calendar.html#valid_date-1">valid_date/1</a></li>
<li title="valid_date-3"><a href="calendar.html#valid_date-3">valid_date/3</a></li>
</ul>
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<li id="no" title="dets " expanded="false">dets<ul>
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<li title="all-0"><a href="dets.html#all-0">all/0</a></li>
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<li title="close-1"><a href="dets.html#close-1">close/1</a></li>
<li title="delete-2"><a href="dets.html#delete-2">delete/2</a></li>
<li title="delete_all_objects-1"><a href="dets.html#delete_all_objects-1">delete_all_objects/1</a></li>
<li title="delete_object-2"><a href="dets.html#delete_object-2">delete_object/2</a></li>
<li title="first-1"><a href="dets.html#first-1">first/1</a></li>
<li title="foldl-3"><a href="dets.html#foldl-3">foldl/3</a></li>
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<li title="from_ets-2"><a href="dets.html#from_ets-2">from_ets/2</a></li>
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<li title="info-2"><a href="dets.html#info-2">info/2</a></li>
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<li title="init_table-3"><a href="dets.html#init_table-3">init_table/3</a></li>
<li title="insert-2"><a href="dets.html#insert-2">insert/2</a></li>
<li title="insert_new-2"><a href="dets.html#insert_new-2">insert_new/2</a></li>
<li title="is_compatible_bchunk_format-2"><a href="dets.html#is_compatible_bchunk_format-2">is_compatible_bchunk_format/2</a></li>
<li title="is_dets_file-1"><a href="dets.html#is_dets_file-1">is_dets_file/1</a></li>
<li title="lookup-2"><a href="dets.html#lookup-2">lookup/2</a></li>
<li title="match-1"><a href="dets.html#match-1">match/1</a></li>
<li title="match-2"><a href="dets.html#match-2">match/2</a></li>
<li title="match-3"><a href="dets.html#match-3">match/3</a></li>
<li title="match_delete-2"><a href="dets.html#match_delete-2">match_delete/2</a></li>
<li title="match_object-1"><a href="dets.html#match_object-1">match_object/1</a></li>
<li title="match_object-2"><a href="dets.html#match_object-2">match_object/2</a></li>
<li title="match_object-3"><a href="dets.html#match_object-3">match_object/3</a></li>
<li title="member-2"><a href="dets.html#member-2">member/2</a></li>
<li title="next-2"><a href="dets.html#next-2">next/2</a></li>
<li title="open_file-1"><a href="dets.html#open_file-1">open_file/1</a></li>
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<li title="repair_continuation-2"><a href="dets.html#repair_continuation-2">repair_continuation/2</a></li>
<li title="safe_fixtable-2"><a href="dets.html#safe_fixtable-2">safe_fixtable/2</a></li>
<li title="select-1"><a href="dets.html#select-1">select/1</a></li>
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<li title="sync-1"><a href="dets.html#sync-1">sync/1</a></li>
<li title="table-1"><a href="dets.html#table-1">table/1</a></li>
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<li title="to_ets-2"><a href="dets.html#to_ets-2">to_ets/2</a></li>
<li title="traverse-2"><a href="dets.html#traverse-2">traverse/2</a></li>
<li title="update_counter-3"><a href="dets.html#update_counter-3">update_counter/3</a></li>
</ul>
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<li id="no" title="dict " expanded="false">dict<ul>
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<li title="append_list-3"><a href="dict.html#append_list-3">append_list/3</a></li>
<li title="erase-2"><a href="dict.html#erase-2">erase/2</a></li>
<li title="fetch-2"><a href="dict.html#fetch-2">fetch/2</a></li>
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</ul>
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<li id="no" title="gb_sets " expanded="false">gb_sets<ul>
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<li title="from_list-1"><a href="gb_sets.html#from_list-1">from_list/1</a></li>
<li title="from_ordset-1"><a href="gb_sets.html#from_ordset-1">from_ordset/1</a></li>
<li title="insert-2"><a href="gb_sets.html#insert-2">insert/2</a></li>
<li title="intersection-1"><a href="gb_sets.html#intersection-1">intersection/1</a></li>
<li title="intersection-2"><a href="gb_sets.html#intersection-2">intersection/2</a></li>
<li title="is_disjoint-2"><a href="gb_sets.html#is_disjoint-2">is_disjoint/2</a></li>
<li title="is_element-2"><a href="gb_sets.html#is_element-2">is_element/2</a></li>
<li title="is_empty-1"><a href="gb_sets.html#is_empty-1">is_empty/1</a></li>
<li title="is_member-2"><a href="gb_sets.html#is_member-2">is_member/2</a></li>
<li title="is_set-1"><a href="gb_sets.html#is_set-1">is_set/1</a></li>
<li title="is_subset-2"><a href="gb_sets.html#is_subset-2">is_subset/2</a></li>
<li title="iterator-1"><a href="gb_sets.html#iterator-1">iterator/1</a></li>
<li title="iterator_from-2"><a href="gb_sets.html#iterator_from-2">iterator_from/2</a></li>
<li title="largest-1"><a href="gb_sets.html#largest-1">largest/1</a></li>
<li title="new-0"><a href="gb_sets.html#new-0">new/0</a></li>
<li title="next-1"><a href="gb_sets.html#next-1">next/1</a></li>
<li title="singleton-1"><a href="gb_sets.html#singleton-1">singleton/1</a></li>
<li title="size-1"><a href="gb_sets.html#size-1">size/1</a></li>
<li title="smallest-1"><a href="gb_sets.html#smallest-1">smallest/1</a></li>
<li title="subtract-2"><a href="gb_sets.html#subtract-2">subtract/2</a></li>
<li title="take_largest-1"><a href="gb_sets.html#take_largest-1">take_largest/1</a></li>
<li title="take_smallest-1"><a href="gb_sets.html#take_smallest-1">take_smallest/1</a></li>
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</ul>
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<li id="no" title="gb_trees " expanded="false">gb_trees<ul>
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<li title="balance-1"><a href="gb_trees.html#balance-1">balance/1</a></li>
<li title="delete-2"><a href="gb_trees.html#delete-2">delete/2</a></li>
<li title="delete_any-2"><a href="gb_trees.html#delete_any-2">delete_any/2</a></li>
<li title="empty-0"><a href="gb_trees.html#empty-0">empty/0</a></li>
<li title="enter-3"><a href="gb_trees.html#enter-3">enter/3</a></li>
<li title="from_orddict-1"><a href="gb_trees.html#from_orddict-1">from_orddict/1</a></li>
<li title="get-2"><a href="gb_trees.html#get-2">get/2</a></li>
<li title="insert-3"><a href="gb_trees.html#insert-3">insert/3</a></li>
<li title="is_defined-2"><a href="gb_trees.html#is_defined-2">is_defined/2</a></li>
<li title="is_empty-1"><a href="gb_trees.html#is_empty-1">is_empty/1</a></li>
<li title="iterator-1"><a href="gb_trees.html#iterator-1">iterator/1</a></li>
<li title="iterator_from-2"><a href="gb_trees.html#iterator_from-2">iterator_from/2</a></li>
<li title="keys-1"><a href="gb_trees.html#keys-1">keys/1</a></li>
<li title="largest-1"><a href="gb_trees.html#largest-1">largest/1</a></li>
<li title="lookup-2"><a href="gb_trees.html#lookup-2">lookup/2</a></li>
<li title="map-2"><a href="gb_trees.html#map-2">map/2</a></li>
<li title="next-1"><a href="gb_trees.html#next-1">next/1</a></li>
<li title="size-1"><a href="gb_trees.html#size-1">size/1</a></li>
<li title="smallest-1"><a href="gb_trees.html#smallest-1">smallest/1</a></li>
<li title="take-2"><a href="gb_trees.html#take-2">take/2</a></li>
<li title="take_any-2"><a href="gb_trees.html#take_any-2">take_any/2</a></li>
<li title="take_largest-1"><a href="gb_trees.html#take_largest-1">take_largest/1</a></li>
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<li title="update-3"><a href="gb_trees.html#update-3">update/3</a></li>
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</ul>
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<li id="no" title="gen_event " expanded="false">gen_event<ul>
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<li title="start-1"><a href="gen_event.html#start-1">start/1</a></li>
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</ul>
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<li id="no" title="gen_server " expanded="false">gen_server<ul>
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<li title="abcast-2"><a href="gen_server.html#abcast-2">abcast/2</a></li>
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<li title="call-2"><a href="gen_server.html#call-2">call/2</a></li>
<li title="call-3"><a href="gen_server.html#call-3">call/3</a></li>
<li title="cast-2"><a href="gen_server.html#cast-2">cast/2</a></li>
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<li title="start-3"><a href="gen_server.html#start-3">start/3</a></li>
<li title="start-4"><a href="gen_server.html#start-4">start/4</a></li>
<li title="start_link-3"><a href="gen_server.html#start_link-3">start_link/3</a></li>
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<li title="Module:code_change-3"><a href="gen_server.html#Module:code_change-3">Module:code_change/3</a></li>
<li title="Module:format_status-2"><a href="gen_server.html#Module:format_status-2">Module:format_status/2</a></li>
<li title="Module:handle_call-3"><a href="gen_server.html#Module:handle_call-3">Module:handle_call/3</a></li>
<li title="Module:handle_cast-2"><a href="gen_server.html#Module:handle_cast-2">Module:handle_cast/2</a></li>
<li title="Module:handle_continue-2"><a href="gen_server.html#Module:handle_continue-2">Module:handle_continue/2</a></li>
<li title="Module:handle_info-2"><a href="gen_server.html#Module:handle_info-2">Module:handle_info/2</a></li>
<li title="Module:init-1"><a href="gen_server.html#Module:init-1">Module:init/1</a></li>
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</ul>
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<li id="no" title="gen_statem " expanded="false">gen_statem<ul>
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<li title="call-2"><a href="gen_statem.html#call-2">call/2</a></li>
<li title="call-3"><a href="gen_statem.html#call-3">call/3</a></li>
<li title="cast-2"><a href="gen_statem.html#cast-2">cast/2</a></li>
<li title="enter_loop-4"><a href="gen_statem.html#enter_loop-4">enter_loop/4</a></li>
<li title="enter_loop-5"><a href="gen_statem.html#enter_loop-5">enter_loop/5</a></li>
<li title="enter_loop-6"><a href="gen_statem.html#enter_loop-6">enter_loop/6</a></li>
<li title="reply-1"><a href="gen_statem.html#reply-1">reply/1</a></li>
<li title="reply-2"><a href="gen_statem.html#reply-2">reply/2</a></li>
<li title="start-3"><a href="gen_statem.html#start-3">start/3</a></li>
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<li title="Module:StateName-3"><a href="gen_statem.html#Module:StateName-3">Module:StateName/3</a></li>
<li title="Module:StateName-3"><a href="gen_statem.html#Module:StateName-3">Module:StateName/3</a></li>
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<!-- refpage --><center><h1>qlc</h1></center>
<h3><span onMouseOver="document.getElementById('ghlink-module-idm281471863492504').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-module-idm281471863492504').style.visibility = 'hidden';"><span id="ghlink-module-idm281471863492504"></span><a class="title_link" name="module" href="#module">Module</a></span></h3>
<div class="REFBODY module-body">qlc</div>
<h3><span onMouseOver="document.getElementById('ghlink-module-summary-idm281471863492056').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-module-summary-idm281471863492056').style.visibility = 'hidden';"><span id="ghlink-module-summary-idm281471863492056"></span><a class="title_link" name="module-summary" href="#module-summary">Module Summary</a></span></h3>
<div class="REFBODY module-summary-body">Query interface to Mnesia, ETS, Dets, and so on.
</div>
<h3><span onMouseOver="document.getElementById('ghlink-description-idm281471863491576').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-description-idm281471863491576').style.visibility = 'hidden';"><span id="ghlink-description-idm281471863491576" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L38" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span><a class="title_link" name="description" href="#description">Description</a></span></h3>
<div class="REFBODY description-body"><p>
<p>This module provides a query interface to
<span class="bold_code bc-18"><a href="javascript:erlhref('../../../../doc/../','mnesia','mnesia.html');">Mnesia</a></span>,
<span class="bold_code bc-19"><a href="ets.html">ETS</a></span>,
<span class="bold_code bc-19"><a href="dets.html">Dets</a></span>,
and other data structures that provide an iterator style
traversal of objects.</p>
</p></div>
<h3><span onMouseOver="document.getElementById('ghlink-overview-idm281471863488120').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-overview-idm281471863488120').style.visibility = 'hidden';"><span id="ghlink-overview-idm281471863488120" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L47" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span><a class="title_link" name="overview" href="#overview">Overview</a></span></h3>
<div class="REFBODY rb-3">
<p>This module provides a query interface to <strong>QLC
tables</strong>. Typical QLC tables are Mnesia, ETS, and
Dets tables. Support is also provided for user-defined tables, see section
<span class="bold_code bc-17"><a href="#implementing_a_qlc_table">Implementing a QLC Table</a></span>.
<a name="query_list_comprehension"></a>
A <strong>query</strong> is expressed using
<strong>Query List Comprehensions</strong> (QLCs). The answers to a
query are determined by data in QLC tables that fulfill the
constraints expressed by the QLCs of the query. QLCs are similar
to ordinary list comprehensions as described in
<span class="bold_code bc-13"><a href="javascript:erlhref('../../../../doc/../','doc/reference_manual','expressions.html#lcs');">Erlang Reference Manual</a></span> and
<span class="bold_code bc-18"><a href="javascript:erlhref('../../../../doc/../','doc/programming_examples','list_comprehensions.html');">Programming Examples</a></span>, except that variables
introduced in patterns cannot be used in list expressions.
In the absence of optimizations and options such as
<span class="code">cache</span> and <span class="code">unique</span> (see section
<span class="bold_code bc-17"><a href="#common_options">Common Options</a></span>, every
QLC free of QLC tables evaluates to the same list of answers as the
identical ordinary list comprehension.</p>
<p>While ordinary list comprehensions evaluate to lists, calling
<span class="bold_code bc-17"><a href="#q-1"><span class="code">q/1,2</span></a></span> returns a
<a name="query_handle"></a><strong>query handle</strong>.
To obtain all the answers to a query, <span class="bold_code bc-17"><a href="#eval-1"><span class="code">eval/1,2</span></a></span> is to be called with the
query handle as first argument. Query handles are essentially
functional objects (funs) created in the module calling <span class="code">q/1,2</span>.
As the funs refer to the module code, be careful not to keep query
handles too long if the module code is to be replaced.
Code replacement is described in section
<span class="bold_code bc-18"><a href="javascript:erlhref('../../../../doc/../','doc/reference_manual','code_loading.html');">Compilation and Code Loading</a></span> in the Erlang Reference Manual.
The list of answers can also be traversed in chunks by use of a
<a name="query_cursor"></a><strong>query cursor</strong>.
Query cursors are created by calling
<span class="bold_code bc-17"><a href="#cursor-1"><span class="code">cursor/1,2</span></a></span> with a query
handle as first argument. Query cursors are essentially Erlang processes.
One answer at a time is sent from the query cursor process to
the process that created the cursor.</p>
</div>
<h3><span onMouseOver="document.getElementById('ghlink-syntax-idm281471863473272').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-syntax-idm281471863473272').style.visibility = 'hidden';"><span id="ghlink-syntax-idm281471863473272" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L92" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span><a class="title_link" name="syntax" href="#syntax">Syntax</a></span></h3>
<div class="REFBODY rb-3">
<p>Syntactically QLCs have the same parts as ordinary list
comprehensions:</p>
<div class="example"><pre>[Expression || Qualifier1, Qualifier2, ...]</pre></div>
<p><span class="code">Expression</span> (the <strong>template</strong>) is any
Erlang expression. Qualifiers are either <strong>filters</strong> or
<strong>generators</strong>. Filters are Erlang expressions returning
<span class="code">boolean()</span>. Generators have the form
<span class="code">Pattern <- ListExpression</span>, where
<span class="code">ListExpression</span> is an expression evaluating to a query
handle or a list. Query handles are returned from
<span class="bold_code bc-17"><a href="#append-1"><span class="code">append/1,2</span></a></span>,
<span class="bold_code bc-17"><a href="#keysort-2"><span class="code">keysort/2,3</span></a></span>,
<span class="bold_code bc-17"><a href="#q-1"><span class="code">q/1,2</span></a></span>,
<span class="bold_code bc-17"><a href="#sort-1"><span class="code">sort/1,2</span></a></span>,
<span class="bold_code bc-17"><a href="#string_to_handle-1"><span class="code">string_to_handle/1,2,3</span></a></span>, and
<span class="bold_code bc-17"><a href="#table-2"><span class="code">table/2</span></a></span>.</p>
</div>
<h3><span onMouseOver="document.getElementById('ghlink-evaluation-idm281471863461960').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-evaluation-idm281471863461960').style.visibility = 'hidden';"><span id="ghlink-evaluation-idm281471863461960" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L116" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span><a class="title_link" name="evaluation" href="#evaluation">Evaluation</a></span></h3>
<div class="REFBODY rb-3">
<p>A query handle is evaluated in the following order:</p>
<ul>
<li>
<p>Inspection of options and the collection of information about
tables. As a result, qualifiers are modified during the optimization
phase.</p>
</li>
<li>
<p>All list expressions are evaluated. If a cursor has been created,
evaluation takes place in the cursor process. For list expressions
that are QLCs, the list expressions of the generators of the QLCs
are evaluated as well. Be careful if list expressions have side
effects, as list expressions are evaluated in unspecified order.</p>
</li>
<li>
<p>The answers are found by evaluating the qualifiers from left to
right, backtracking when some filter returns <span class="code">false</span>, or
collecting the template when all filters return <span class="code">true</span>.</p>
</li>
</ul>
<p>Filters that do not return <span class="code">boolean()</span> but fail are handled
differently depending on their syntax: if the filter is a guard,
it returns <span class="code">false</span>, otherwise the query evaluation fails.
This behavior makes it possible for the <span class="code">qlc</span> module to do
some optimizations without affecting the meaning of a query. For
example, when testing some position of a table and one or more
constants for equality, only
the objects with equal values are candidates for further
evaluation. The other objects are guaranteed to make the filter
return <span class="code">false</span>, but never fail. The (small) set of
candidate objects can often be found by looking up some key
values of the table or by traversing the table using a match
specification. It is necessary to place the guard filters
immediately after the table generator, otherwise the candidate
objects are not restricted to a small set. The reason is
that objects that could make the query evaluation fail must not
be excluded by looking up a key or running a match specification.</p>
</div>
<h3><span onMouseOver="document.getElementById('ghlink-join-idm281471863453032').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-join-idm281471863453032').style.visibility = 'hidden';"><span id="ghlink-join-idm281471863453032" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L159" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span><a class="title_link" name="join" href="#join">Join</a></span></h3>
<div class="REFBODY rb-3">
<p>The <span class="code">qlc</span> module supports fast join of two query handles.
Fast join is possible if some position <span class="code">P1</span> of one query
handler and some position <span class="code">P2</span> of another query handler are
tested for equality. Two fast join methods are provided:</p>
<ul>
<li>
<p><strong>Lookup join</strong> traverses all objects of one query handle
and finds objects of the other handle (a QLC table) such that the
values at <span class="code">P1</span> and <span class="code">P2</span> match or compare equal.
The <span class="code">qlc</span> module does not create
any indexes but looks up values using the key position and
the indexed positions of the QLC table.</p>
</li>
<li>
<p><strong>Merge join</strong> sorts the objects of each query handle if
necessary and filters out objects where the values at
<span class="code">P1</span> and <span class="code">P2</span> do not compare equal. If
many objects with the same value of <span class="code">P2</span> exist, a temporary
file is used for the equivalence classes.</p>
</li>
</ul>
<p>The <span class="code">qlc</span> module warns at compile time if a QLC
combines query handles in such a way that more than one join is
possible. That is, no query planner is provided that can
select a good order between possible join operations. It is up
to the user to order the joins by introducing query handles.</p>
<p>The join is to be expressed as a guard filter. The filter must
be placed immediately after the two joined generators, possibly
after guard filters that use variables from no other generators
but the two joined generators. The <span class="code">qlc</span> module inspects
the operands of
<span class="code">=:=/2</span>, <span class="code">==/2</span>, <span class="code">is_record/2</span>, <span class="code">element/2</span>,
and logical operators (<span class="code">and/2</span>, <span class="code">or/2</span>,
<span class="code">andalso/2</span>, <span class="code">orelse/2</span>, <span class="code">xor/2</span>) when
determining which joins to consider.</p>
</div>
<h3>
<a name="common_options"></a><span onMouseOver="document.getElementById('ghlink-common-options-idm281471863437528').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-common-options-idm281471863437528').style.visibility = 'hidden';"><span id="ghlink-common-options-idm281471863437528" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L199" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span><a class="title_link" name="common-options" href="#common-options">Common Options</a></span>
</h3>
<div class="REFBODY rb-3">
<p>The following options are accepted by
<span class="bold_code bc-17"><a href="#cursor-2"><span class="code">cursor/2</span></a></span>,
<span class="bold_code bc-17"><a href="#eval-2"><span class="code">eval/2</span></a></span>,
<span class="bold_code bc-17"><a href="#fold-4"><span class="code">fold/4</span></a></span>, and
<span class="bold_code bc-17"><a href="#info-2"><span class="code">info/2</span></a></span>:</p>
<ul>
<li>
<p><span class="code">{cache_all, Cache}</span>, where <span class="code">Cache</span> is
equal to <span class="code">ets</span> or <span class="code">list</span> adds a
<span class="code">{cache, Cache}</span> option to every list expression
of the query except tables and lists. Defaults to
<span class="code">{cache_all, no}</span>. Option <span class="code">cache_all</span> is
equivalent to <span class="code">{cache_all, ets}</span>.</p>
</li>
<li>
<p><a name="max_list_size"></a><span class="code">{max_list_size,
MaxListSize}</span>, where <span class="code">MaxListSize</span> is the
size in bytes of terms on the external format. If the
accumulated size of collected objects exceeds
<span class="code">MaxListSize</span>, the objects are written onto a temporary
file. This option is used by option <span class="code">{cache, list}</span>
and by the merge join method. Defaults to 512*1024 bytes.</p>
</li>
<li>
<p><span class="code">{tmpdir_usage, TmpFileUsage}</span> determines the
action taken when <span class="code">qlc</span> is about to create temporary
files on the directory set by option <span class="code">tmpdir</span>. If the
value is <span class="code">not_allowed</span>, an error tuple is returned,
otherwise temporary files are created as needed. Default is
<span class="code">allowed</span>, which means that no further action is taken.
The values <span class="code">info_msg</span>, <span class="code">warning_msg</span>, and
<span class="code">error_msg</span> mean that the function with the corresponding
name in module
<span class="bold_code bc-18"><a href="javascript:erlhref('../../../../doc/../','kernel','error_logger.html');"><span class="code">error_logger</span></a></span>
is called for printing some information (currently the stacktrace).</p>
</li>
<li>
<p><span class="code">{tmpdir, TempDirectory}</span> sets the directory used by
merge join for temporary files and by option
<span class="code">{cache, list}</span>. The option also overrides
option <span class="code">tmpdir</span> of
<span class="bold_code bc-17"><a href="#keysort-3"><span class="code">keysort/3</span></a></span> and
<span class="bold_code bc-17"><a href="#sort-2"><span class="code">sort/2</span></a></span>.
Defaults to <span class="code">""</span>, which means that
the directory returned by <span class="code">file:get_cwd()</span> is used.</p>
</li>
<li>
<p><span class="code">{unique_all, true}</span> adds a
<span class="code">{unique, true}</span> option to every list expression of
the query. Defaults to <span class="code">{unique_all, false}</span>.
Option <span class="code">unique_all</span> is equivalent to
<span class="code">{unique_all, true}</span>.</p>
</li>
</ul>
</div>
<h3>
<a name="getting_started"></a><span onMouseOver="document.getElementById('ghlink-getting-started-idm281471863410984').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-getting-started-idm281471863410984').style.visibility = 'hidden';"><span id="ghlink-getting-started-idm281471863410984" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L254" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span><a class="title_link" name="getting-started" href="#getting-started">Getting Started</a></span>
</h3>
<div class="REFBODY rb-3">
<p>As mentioned earlier,
queries are expressed in the list comprehension syntax as described
in section
<span class="bold_code bc-18"><a href="javascript:erlhref('../../../../doc/../','doc/reference_manual','expressions.html');">Expressions</a></span>
in Erlang Reference Manual. In the following, some familiarity
with list comprehensions is assumed. The examples in section
<span class="bold_code bc-18"><a href="javascript:erlhref('../../../../doc/../','doc/programming_examples','list_comprehensions.html');">List Comprehensions</a></span> in Programming Examples can get you
started. Notice that list comprehensions do not add any computational
power to the language; anything that can be done with list
comprehensions can also be done without them. But they add
syntax for expressing simple search problems, which is compact
and clear once you get used to it.</p>
<p>Many list comprehension expressions can be evaluated by the
<span class="code">qlc</span> module. Exceptions are expressions, such that
variables introduced in patterns (or filters) are used in some
generator later in the list comprehension. As an example,
consider an implementation of <span class="code">lists:append(L)</span>:
<span class="code">[X ||Y <- L, X <- Y]</span>.
<span class="code">Y</span> is introduced in the first generator and used in the second.
The ordinary list comprehension is normally to be preferred when
there is a choice as to which to use. One difference is that
<span class="bold_code bc-17"><a href="#eval-1"><span class="code">eval/1,2</span></a></span>
collects answers in a list that is finally
reversed, while list comprehensions collect answers on the stack
that is finally unwound.</p>
<p>What the <span class="code">qlc</span> module primarily adds to list
comprehensions is that data can be read from QLC tables in small
chunks. A QLC table is created by calling
<span class="bold_code bc-17"><a href="#table-2"><span class="code">qlc:table/2</span></a></span>.
Usually <span class="code">qlc:table/2</span> is not called directly from the query
but through an interface function of some data structure.
Erlang/OTP includes a few examples of such functions:
<span class="bold_code bc-13"><a href="javascript:erlhref('../../../../doc/../','mnesia','mnesia.html#table-1');"><span class="code">mnesia:table/1,2</span></a></span>,
<span class="bold_code bc-15"><a href="ets.html#table-1"><span class="code">ets:table/1,2</span></a></span>, and
<span class="bold_code bc-15"><a href="dets.html#table-1"><span class="code">dets:table/1,2</span></a></span>.
For a given data structure, many functions can create QLC tables, but
common for these functions is that they return a query handle created by
<span class="bold_code bc-17"><a href="#table-2"><span class="code">qlc:table/2</span></a></span>.
Using the QLC tables provided by Erlang/OTP is usually
probably sufficient, but for the more advanced user section
<span class="bold_code bc-17"><a href="#implementing_a_qlc_table">Implementing a QLC Table</a></span> describes the implementation of a function
calling <span class="code">qlc:table/2</span>.</p>
<p>Besides <span class="code">qlc:table/2</span>, other functions
return query handles. They are used more seldom than tables,
but are sometimes useful.
<span class="bold_code bc-17"><a href="#append-1"><span class="code">qlc:append/1,2</span></a></span> traverses
objects from many tables or lists after each other. If, for
example, you want to traverse all answers to a query <span class="code">QH</span> and
then finish off by a term <span class="code">{finished}</span>, you can do that by
calling <span class="code">qlc:append(QH, [{finished}])</span>. <span class="code">append/2</span> first
returns all objects of <span class="code">QH</span>, then <span class="code">{finished}</span>. If a tuple
<span class="code">{finished}</span> exists among the answers to <span class="code">QH</span>, it is
returned twice from <span class="code">append/2</span>.</p>
<p>As another example, consider concatenating the answers to two
queries <span class="code">QH1</span> and <span class="code">QH2</span> while removing all duplicates. This is
accomplished by using option <span class="code">unique</span>:</p>
<div class="example"><pre>qlc:q([X || X <- qlc:append(QH1, QH2)], {unique, true})</pre></div>
<p>The cost is substantial: every returned answer is stored
in an ETS table. Before returning an answer, it is looked up in
the ETS table to check if it has already been returned. Without
the <span class="code">unique</span> option, all answers to <span class="code">QH1</span> would be returned
followed by all answers to <span class="code">QH2</span>. The <span class="code">unique</span> option keeps
the order between the remaining answers.</p>
<p>If the order of the answers is not important, there is an
alternative to the <span class="code">unique</span> option, namely to sort the
answers uniquely:</p>
<div class="example"><pre>qlc:sort(qlc:q([X || X <- qlc:append(QH1, QH2)], {unique, true})).</pre></div>
<p>This query also removes duplicates but the answers are
sorted. If there are many answers, temporary files are used.
Notice that to get the first unique answer, all answers
must be found and sorted. Both alternatives find duplicates by comparing
answers, that is, if <span class="code">A1</span> and <span class="code">A2</span> are answers found in
that order, then <span class="code">A2</span> is a removed if <span class="code">A1 == A2</span>.</p>
<p>To return only a few answers, cursors can be used. The following
code returns no more than five answers using an ETS table for
storing the unique answers:</p>
<div class="example"><pre>C = qlc:cursor(qlc:q([X || X <- qlc:append(QH1, QH2)],{unique,true})),
R = qlc:next_answers(C, 5),
ok = qlc:delete_cursor(C),
R.</pre></div>
<p>QLCs are convenient for stating
constraints on data from two or more tables. The following example
does a natural join on two query handles on position 2:</p>
<div class="example"><pre>qlc:q([{X1,X2,X3,Y1} ||
{X1,X2,X3} <- QH1,
{Y1,Y2} <- QH2,
X2 =:= Y2])</pre></div>
<p>The <span class="code">qlc</span> module evaluates this differently depending on the
query handles <span class="code">QH1</span> and <span class="code">QH2</span>. If, for example, <span class="code">X2</span> is
matched against the key of a QLC table, the lookup join method
traverses the objects of <span class="code">QH2</span> while looking up key
values in the table. However, if not <span class="code">X2</span> or
<span class="code">Y2</span> is matched against the key or an indexed position of a
QLC table, the merge join method ensures that <span class="code">QH1</span>
and <span class="code">QH2</span> are both sorted on position 2 and next do the
join by traversing the objects one by one.</p>
<p>Option <span class="code">join</span> can be used to force the <span class="code">qlc</span> module to use
a certain join method. For the rest of this section it is assumed
that the excessively slow join method called "nested loop" has
been chosen:</p>
<div class="example"><pre>qlc:q([{X1,X2,X3,Y1} ||
{X1,X2,X3} <- QH1,
{Y1,Y2} <- QH2,
X2 =:= Y2],
{join, nested_loop})</pre></div>
<p>In this case the filter is applied to every possible pair
of answers to <span class="code">QH1</span> and <span class="code">QH2</span>, one at a time.
If there are M answers to <span class="code">QH1</span> and N answers to <span class="code">QH2</span>,
the filter is run M*N times.</p>
<p>If <span class="code">QH2</span> is a call to the function for
<span class="bold_code bc-19"><a href="gb_trees.html"><span class="code">gb_trees</span></a></span>, as defined
in section <span class="bold_code bc-17"><a href="#implementing_a_qlc_table">Implementing a QLC Table</a></span>, then <span class="code">gb_table:table/1</span>, the
iterator for the gb-tree is initiated for each answer to
<span class="code">QH1</span>. The objects of the gb-tree are then returned one
by one. This is probably the most efficient way of traversing
the table in that case, as it takes minimal computational
power to get the following object. But if <span class="code">QH2</span> is not a table but
a more complicated QLC, it can be more efficient to use some RAM
memory for collecting the answers in a cache, particularly if
there are only a few answers. It must then be assumed that
evaluating <span class="code">QH2</span> has no side effects so that the meaning of the
query does not change if <span class="code">QH2</span> is evaluated only once. One way of
caching the answers is to evaluate <span class="code">QH2</span> first of all and
substitute the list of answers for <span class="code">QH2</span> in the query. Another way
is to use option <span class="code">cache</span>. It is expressed like this:</p>
<div class="example"><pre>QH2' = qlc:q([X || X <- QH2], {cache, ets})</pre></div>
<p>or only</p>
<div class="example"><pre>QH2' = qlc:q([X || X <- QH2], cache)</pre></div>
<p>The effect of option <span class="code">cache</span> is that when
generator <span class="code">QH2'</span> is run the first time, every answer is stored in
an ETS table. When the next answer of <span class="code">QH1</span> is tried,
answers to <span class="code">QH2'</span>
are copied from the ETS table, which is very fast. As for
option <span class="code">unique</span> the cost is a possibly substantial amount
of RAM memory.</p>
<p>Option <span class="code">{cache, list}</span> offers the
possibility to store the answers in a list on the process heap.
This has the potential of being faster than ETS tables,
as there is no need to copy answers from the table. However, it can
often result in slower evaluation because of more garbage
collections of the process heap and increased RAM memory
consumption because of larger heaps. Another drawback with cache
lists is that if the list size exceeds a limit, a
temporary file is used. Reading the answers from a file is
much slower than copying them from an ETS table. But if the
available RAM memory is scarce, setting the <span class="bold_code bc-17"><a href="#max_list_size">limit</a></span> to some low value is an
alternative.</p>
<p>Option <span class="code">cache_all</span> can be set to
<span class="code">ets</span> or <span class="code">list</span> when evaluating a query. It adds a
<span class="code">cache</span> or <span class="code">{cache, list}</span> option to every list
expression except QLC tables and lists on all levels of the
query. This can be used for testing if caching would improve
efficiency at all. If the answer is yes, further testing is
needed to pinpoint the generators that are to be cached.</p>
</div>
<h3>
<a name="implementing_a_qlc_table"></a><span onMouseOver="document.getElementById('ghlink-implementing-a-qlc-table-idm281471863351352').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-implementing-a-qlc-table-idm281471863351352').style.visibility = 'hidden';"><span id="ghlink-implementing-a-qlc-table-idm281471863351352" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L448" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span><a class="title_link" name="implementing-a-qlc-table" href="#implementing-a-qlc-table">Implementing a QLC Table</a></span>
</h3>
<div class="REFBODY rb-3">
<p>As an example of
how to use function <span class="bold_code bc-17"><a href="#table-2"><span class="code">table/2</span></a></span>,
the implementation of a QLC table for the <span class="bold_code bc-19"><a href="gb_trees.html"><span class="code">gb_trees</span></a></span> module is given:</p>
<div class="example"><pre>-module(gb_table).
-export([table/1]).
table(T) ->
TF = fun() -> qlc_next(gb_trees:next(gb_trees:iterator(T))) end,
InfoFun = fun(num_of_objects) -> gb_trees:size(T);
(keypos) -> 1;
(is_sorted_key) -> true;
(is_unique_objects) -> true;
(_) -> undefined
end,
LookupFun =
fun(1, Ks) ->
lists:flatmap(fun(K) ->
case gb_trees:lookup(K, T) of
{value, V} -> [{K,V}];
none -> []
end
end, Ks)
end,
FormatFun =
fun({all, NElements, ElementFun}) ->
ValsS = io_lib:format("gb_trees:from_orddict(~w)",
[gb_nodes(T, NElements, ElementFun)]),
io_lib:format("gb_table:table(~s)", [ValsS]);
({lookup, 1, KeyValues, _NElements, ElementFun}) ->
ValsS = io_lib:format("gb_trees:from_orddict(~w)",
[gb_nodes(T, infinity, ElementFun)]),
io_lib:format("lists:flatmap(fun(K) -> "
"case gb_trees:lookup(K, ~s) of "
"{value, V} -> [{K,V}];none -> [] end "
"end, ~w)",
[ValsS, [ElementFun(KV) || KV <- KeyValues]])
end,
qlc:table(TF, [{info_fun, InfoFun}, {format_fun, FormatFun},
{lookup_fun, LookupFun},{key_equality,'=='}]).
qlc_next({X, V, S}) ->
[{X,V} | fun() -> qlc_next(gb_trees:next(S)) end];
qlc_next(none) ->
[].
gb_nodes(T, infinity, ElementFun) ->
gb_nodes(T, -1, ElementFun);
gb_nodes(T, NElements, ElementFun) ->
gb_iter(gb_trees:iterator(T), NElements, ElementFun).
gb_iter(_I, 0, _EFun) ->
'...';
gb_iter(I0, N, EFun) ->
case gb_trees:next(I0) of
{X, V, I} ->
[EFun({X,V}) | gb_iter(I, N-1, EFun)];
none ->
[]
end.</pre></div>
<p><span class="code">TF</span> is the traversal function. The <span class="code">qlc</span> module
requires that there is a way of traversing all objects of the
data structure. <span class="code">gb_trees</span> has an iterator function
suitable for that purpose. Notice that for each object returned, a
new fun is created. As long as the list is not terminated by
<span class="code">[]</span>, it is assumed that the tail of the list is a nullary
function and that calling the function returns further objects
(and functions).</p>
<p>The lookup function is optional. It is assumed that the lookup
function always finds values much faster than it would take to
traverse the table. The first argument is the position of the
key. As <span class="code">qlc_next/1</span> returns the objects as <span class="code">{Key, Value}</span>
pairs, the position is 1. Notice that the lookup function is to return
<span class="code">{Key, Value}</span> pairs, as the traversal function does.</p>
<p>The format function is also optional. It is called by
<span class="bold_code bc-17"><a href="#info-1"><span class="code">info/1,2</span></a></span>
to give feedback at runtime of how the query
is to be evaluated. Try to give as good feedback as
possible without showing too much details. In the example, at
most seven objects of the table are shown. The format function
handles two cases: <span class="code">all</span> means that all objects of the
table are traversed; <span class="code">{lookup, 1, KeyValues}</span>
means that the lookup function is used for looking up key
values.</p>
<p>Whether the whole table is traversed or only some keys
looked up depends on how the query is expressed. If the query has
the form</p>
<div class="example"><pre>qlc:q([T || P <- LE, F])</pre></div>
<p>and <span class="code">P</span> is a tuple, the <span class="code">qlc</span> module analyzes
<span class="code">P</span> and <span class="code">F</span> in
compile time to find positions of tuple <span class="code">P</span> that are tested
for equality to constants. If such a position at runtime turns
out to be the key position, the lookup function can be used,
otherwise all objects of the table must be traversed.
The info function <span class="code">InfoFun</span> returns the key position.
There can be indexed positions as well, also returned by the
info function. An index is an extra table that makes lookup on
some position fast. Mnesia maintains indexes upon request,
and introduces so called secondary keys. The <span class="code">qlc</span>
module prefers to look up objects using the key before secondary
keys regardless of the number of constants to look up.</p>
</div>
<h3><span onMouseOver="document.getElementById('ghlink-key-equality-idm281471863331864').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-key-equality-idm281471863331864').style.visibility = 'hidden';"><span id="ghlink-key-equality-idm281471863331864" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L564" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span><a class="title_link" name="key-equality" href="#key-equality">Key Equality</a></span></h3>
<div class="REFBODY rb-3">
<p>Erlang/OTP has two operators for testing term equality: <span class="code">==/2</span>
and <span class="code">=:=/2</span>. The difference is all about the integers that can be
represented by floats. For example, <span class="code">2 == 2.0</span> evaluates to
<span class="code">true</span> while <span class="code">2 =:= 2.0</span> evaluates to <span class="code">false</span>.
Normally this is a minor issue, but the <span class="code">qlc</span> module cannot
ignore the difference, which affects the user's choice of
operators in QLCs.</p>
<p>If the <span class="code">qlc</span> module at compile time can determine that some
constant is free of integers, it does not matter which one of
<span class="code">==/2</span> or <span class="code">=:=/2</span> is used:</p>
<div class="example"><pre>
1> <span class="bold_code bc-12">E1 = ets:new(t, [set]), % uses =:=/2 for key equality</span>
<span class="bold_code bc-12">Q1 = qlc:q([K ||</span>
<span class="bold_code bc-12">{K} <- ets:table(E1),</span>
<span class="bold_code bc-12">K == 2.71 orelse K == a]),</span>
<span class="bold_code bc-12">io:format("~s~n", [qlc:info(Q1)]).</span>
ets:match_spec_run(lists:flatmap(fun(V) ->
ets:lookup(20493, V)
end,
[a,2.71]),
ets:match_spec_compile([{{'$1'},[],['$1']}]))</pre></div>
<p>In the example, operator <span class="code">==/2</span> has been handled
exactly as <span class="code">=:=/2</span> would have been handled. However,
if it cannot be determined at compile time that some
constant is free of integers, and the table uses <span class="code">=:=/2</span>
when comparing keys for equality (see option <span class="bold_code bc-17"><a href="#key_equality">key_equality</a></span>), then the
<span class="code">qlc</span> module does not try to look up the constant. The
reason is that there is in the general case no upper limit on
the number of key values that can compare equal to such a
constant; every combination of integers and floats must be
looked up:</p>
<div class="example"><pre>
2> <span class="bold_code bc-12">E2 = ets:new(t, [set]),</span>
<span class="bold_code bc-12">true = ets:insert(E2, [{{2,2},a},{{2,2.0},b},{{2.0,2},c}]),</span>
<span class="bold_code bc-12">F2 = fun(I) -></span>
<span class="bold_code bc-12">qlc:q([V || {K,V} <- ets:table(E2), K == I])</span>
<span class="bold_code bc-12">end,</span>
<span class="bold_code bc-12">Q2 = F2({2,2}),</span>
<span class="bold_code bc-12">io:format("~s~n", [qlc:info(Q2)]).</span>
ets:table(53264,
[{traverse,
{select,[{{'$1','$2'},[{'==','$1',{const,{2,2}}}],['$2']}]}}])
3> <span class="bold_code bc-12">lists:sort(qlc:e(Q2)).</span>
[a,b,c]</pre></div>
<p>Looking up only <span class="code">{2,2}</span> would not return <span class="code">b</span> and
<span class="code">c</span>.</p>
<p>If the table uses <span class="code">==/2</span> when comparing keys for equality,
the <span class="code">qlc</span> module looks up the constant regardless of
which operator is used in the QLC. However, <span class="code">==/2</span> is to
be preferred:</p>
<div class="example"><pre>
4> <span class="bold_code bc-12">E3 = ets:new(t, [ordered_set]), % uses ==/2 for key equality</span>
<span class="bold_code bc-12">true = ets:insert(E3, [{{2,2.0},b}]),</span>
<span class="bold_code bc-12">F3 = fun(I) -></span>
<span class="bold_code bc-12">qlc:q([V || {K,V} <- ets:table(E3), K == I])</span>
<span class="bold_code bc-12">end,</span>
<span class="bold_code bc-12">Q3 = F3({2,2}),</span>
<span class="bold_code bc-12">io:format("~s~n", [qlc:info(Q3)]).</span>
ets:match_spec_run(ets:lookup(86033, {2,2}),
ets:match_spec_compile([{{'$1','$2'},[],['$2']}]))
5> <span class="bold_code bc-12">qlc:e(Q3).</span>
[b]</pre></div>
<p>Lookup join is handled analogously to lookup of constants in a
table: if the join operator is <span class="code">==/2</span>, and the table where
constants are to be looked up uses <span class="code">=:=/2</span> when testing
keys for equality, then the <span class="code">qlc</span> module does not consider
lookup join for that table.</p>
</div>
<h3><span onMouseOver="document.getElementById('ghlink-data-types-idm281471863304392').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-data-types-idm281471863304392').style.visibility = 'hidden';"><span id="ghlink-data-types-idm281471863304392"></span><a class="title_link" name="data-types" href="#data-types">Data Types</a></span></h3>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-abstract_expr').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-abstract_expr').style.visibility = 'hidden';">
<span id="ghlink-type-abstract_expr" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L645" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-abstract_expr">abstract_expr()</a> = <span class="bold_code bc-15"><a href="erl_parse.html#type-abstract_expr">erl_parse:abstract_expr()</a></span></span><br>
</div>
<div class="data-type-desc"><div class="REFBODY rb-7"><p><p>Parse trees for Erlang expression, see section <span class="bold_code bc-18"><a href="javascript:erlhref('../../../../doc/../','erts','absform.html');">The Abstract Format</a></span>
in the ERTS User's Guide.</p></p></div></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-answer').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-answer').style.visibility = 'hidden';">
<span id="ghlink-type-answer" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L651" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-answer">answer()</a> = term()</span><br>
</div>
<div class="data-type-desc"></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-answers').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-answers').style.visibility = 'hidden';">
<span id="ghlink-type-answers" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L654" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-answers">answers()</a> = [<span class="bold_code bc-17"><a href="#type-answer">answer()</a></span>]</span><br>
</div>
<div class="data-type-desc"></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-cache').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-cache').style.visibility = 'hidden';">
<span id="ghlink-type-cache" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L657" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-cache">cache()</a> = ets | list | no</span><br>
</div>
<div class="data-type-desc"></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-match_expression').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-match_expression').style.visibility = 'hidden';">
<span id="ghlink-type-match_expression" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L660" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-match_expression">match_expression()</a> = <span class="bold_code bc-15"><a href="ets.html#type-match_spec">ets:match_spec()</a></span></span><br>
</div>
<div class="data-type-desc"><div class="REFBODY rb-7"><p><p>Match specification, see section <span class="bold_code bc-18"><a href="javascript:erlhref('../../../../doc/../','erts','match_spec.html');">Match Specifications in Erlang</a></span>
in the ERTS User's Guide and <span class="bold_code bc-19"><a href="ms_transform.html"><span class="code">ms_transform(3)</span></a></span>.</p></p></div></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-no_files').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-no_files').style.visibility = 'hidden';">
<span id="ghlink-type-no_files" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L667" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-no_files">no_files()</a> = integer() >= 1</span><br>
</div>
<div class="data-type-desc"><div class="REFBODY rb-7"><p><p>An integer > 1.</p></p></div></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-key_pos').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-key_pos').style.visibility = 'hidden';">
<span id="ghlink-type-key_pos" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L671" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-key_pos">key_pos()</a> = integer() >= 1 | [integer() >= 1]</span><br>
</div>
<div class="data-type-desc"></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-max_list_size').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-max_list_size').style.visibility = 'hidden';">
<span id="ghlink-type-max_list_size" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L674" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-max_list_size">max_list_size()</a> = integer() >= 0</span><br>
</div>
<div class="data-type-desc"></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-order').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-order').style.visibility = 'hidden';">
<span id="ghlink-type-order" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L677" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-order">order()</a> = ascending | descending | <span class="bold_code bc-17"><a href="#type-order_fun">order_fun()</a></span></span><br>
</div>
<div class="data-type-desc"></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-order_fun').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-order_fun').style.visibility = 'hidden';">
<span id="ghlink-type-order_fun" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L680" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-order_fun">order_fun()</a> = fun((term(), term()) -> boolean())</span><br>
</div>
<div class="data-type-desc"></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-query_cursor').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-query_cursor').style.visibility = 'hidden';">
<span id="ghlink-type-query_cursor" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L683" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-query_cursor">query_cursor()</a></span><br>
</div>
<div class="data-type-desc"><div class="REFBODY rb-7"><p><p>A <span class="bold_code bc-17"><a href="#query_cursor">query cursor</a></span>.</p>
</p></div></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-query_handle').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-query_handle').style.visibility = 'hidden';">
<span id="ghlink-type-query_handle" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L688" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-query_handle">query_handle()</a></span><br>
</div>
<div class="data-type-desc"><div class="REFBODY rb-7"><p><p>A <span class="bold_code bc-17"><a href="#query_handle">query handle</a></span>.</p>
</p></div></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-query_handle_or_list').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-query_handle_or_list').style.visibility = 'hidden';">
<span id="ghlink-type-query_handle_or_list" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L693" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-query_handle_or_list">query_handle_or_list()</a> = <span class="bold_code bc-17"><a href="#type-query_handle">query_handle()</a></span> | list()</span><br>
</div>
<div class="data-type-desc"></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-query_list_comprehension').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-query_list_comprehension').style.visibility = 'hidden';">
<span id="ghlink-type-query_list_comprehension" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L696" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-query_list_comprehension">query_list_comprehension()</a> = term()</span><br>
</div>
<div class="data-type-desc"><div class="REFBODY rb-7"><p><p>A literal
<span class="bold_code bc-17"><a href="#query_list_comprehension">query list comprehension</a></span>.</p></p></div></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-spawn_options').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-spawn_options').style.visibility = 'hidden';">
<span id="ghlink-type-spawn_options" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L702" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-spawn_options">spawn_options()</a> = default | [<span class="bold_code bc-15"><a href="proc_lib.html#type-spawn_option">proc_lib:spawn_option()</a></span>]</span><br>
</div>
<div class="data-type-desc"></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-sort_options').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-sort_options').style.visibility = 'hidden';">
<span id="ghlink-type-sort_options" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L705" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-sort_options">sort_options()</a> = [<span class="bold_code bc-17"><a href="#type-sort_option">sort_option()</a></span>] | <span class="bold_code bc-17"><a href="#type-sort_option">sort_option()</a></span></span><br>
</div>
<div class="data-type-desc"></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-sort_option').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-sort_option').style.visibility = 'hidden';">
<span id="ghlink-type-sort_option" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L708" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-sort_option">sort_option()</a> = <br> {compressed, boolean()} |<br> {no_files, <span class="bold_code bc-17"><a href="#type-no_files">no_files()</a></span>} |<br> {order, <span class="bold_code bc-17"><a href="#type-order">order()</a></span>} |<br> {size, integer() >= 1} |<br> {tmpdir, <span class="bold_code bc-17"><a href="#type-tmp_directory">tmp_directory()</a></span>} |<br> {unique, boolean()}</span><br>
</div>
<div class="data-type-desc"><div class="REFBODY rb-7"><p><p>See <span class="bold_code bc-19"><a href="file_sorter.html"><span class="code">file_sorter(3)</span></a></span>.</p></p></div></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-tmp_directory').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-tmp_directory').style.visibility = 'hidden';">
<span id="ghlink-type-tmp_directory" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L713" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-tmp_directory">tmp_directory()</a> = [] | <span class="bold_code bc-14"><a href="javascript:erlhref('../../../../doc/../','kernel','file.html#type-name');">file:name()</a></span></span><br>
</div>
<div class="data-type-desc"></div>
</div>
<div class="data-types-body">
<div class="data-type-name" onMouseOver="document.getElementById('ghlink-type-tmp_file_usage').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-type-tmp_file_usage').style.visibility = 'hidden';">
<span id="ghlink-type-tmp_file_usage" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L716" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>
<span class="bold_code bc-4"><a name="type-tmp_file_usage">tmp_file_usage()</a> = <br> allowed | not_allowed | info_msg | warning_msg | error_msg</span><br>
</div>
<div class="data-type-desc"></div>
</div>
<h3><span onMouseOver="document.getElementById('ghlink-exports-idm281471863269512').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-exports-idm281471863269512').style.visibility = 'hidden';"><span id="ghlink-exports-idm281471863269512"></span><a class="title_link" name="exports" href="#exports">Exports</a></span></h3>
<div class="exports-body">
<p><a name="append-1"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-append-1-idm281471851970184').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-append-1-idm281471851970184').style.visibility = 'hidden';">
<span id="ghlink-append-1-idm281471851970184" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L722" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>append(QHL) -> QH</div><div class="REFBODY fun-types">
<h3 class="func-types-title">Types</h3>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QHL = [<span class="bold_code bc-17"><a href="#type-query_handle_or_list">query_handle_or_list()</a></span>]</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QH = <span class="bold_code bc-17"><a href="#type-query_handle">query_handle()</a></span></span></div>
</div></p>
<div class="REFBODY rb-7"><p>
<p>Returns a query handle. When evaluating query handle
<span class="code">QH</span>, all answers to the first query handle in
<span class="code">QHL</span> are returned, followed by all answers
to the remaining query handles in <span class="code">QHL</span>.</p>
</p></div>
<p><a name="append-2"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-append-2-idm281471850807928').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-append-2-idm281471850807928').style.visibility = 'hidden';">
<span id="ghlink-append-2-idm281471850807928" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L733" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>append(QH1, QH2) -> QH3</div><div class="REFBODY fun-types">
<h3 class="func-types-title">Types</h3>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QH1 = QH2 = <span class="bold_code bc-17"><a href="#type-query_handle_or_list">query_handle_or_list()</a></span></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2"></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QH3 = <span class="bold_code bc-17"><a href="#type-query_handle">query_handle()</a></span></span></div>
</div></p>
<div class="REFBODY rb-7"><p>
<p>Returns a query handle. When evaluating query handle
<span class="code">QH3</span>, all answers to
<span class="code">QH1</span> are returned, followed by all answers
to <span class="code">QH2</span>.</p>
<p><span class="code">append(QH1, QH2)</span> is equivalent to
<span class="code">append([QH1, QH2])</span>.</p>
</p></div>
<p><a name="cursor-1"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-cursor-1-idm281471851699384').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-cursor-1-idm281471851699384').style.visibility = 'hidden';">
<span id="ghlink-cursor-1-idm281471851699384" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L746" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>cursor(QH) -> Cursor</div><a name="cursor-2"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-cursor-2-idm281471852138248').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-cursor-2-idm281471852138248').style.visibility = 'hidden';">
<span id="ghlink-cursor-2-idm281471852138248" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L746" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>cursor(QH, Options) -> Cursor</div><div class="REFBODY fun-types">
<h3 class="func-types-title">Types</h3>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QH = <span class="bold_code bc-17"><a href="#type-query_handle_or_list">query_handle_or_list()</a></span></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Options = [Option] | Option</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Option = <br> {cache_all, <span class="bold_code bc-17"><a href="#type-cache">cache()</a></span>} |<br> cache_all |<br> {max_list_size, <span class="bold_code bc-17"><a href="#type-max_list_size">max_list_size()</a></span>} |<br> {spawn_options, <span class="bold_code bc-17"><a href="#type-spawn_options">spawn_options()</a></span>} |<br> {tmpdir_usage, <span class="bold_code bc-17"><a href="#type-tmp_file_usage">tmp_file_usage()</a></span>} |<br> {tmpdir, <span class="bold_code bc-17"><a href="#type-tmp_directory">tmp_directory()</a></span>} |<br> {unique_all, boolean()} |<br> unique_all</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Cursor = <span class="bold_code bc-17"><a href="#type-query_cursor">query_cursor()</a></span></span></div>
</div></p>
<div class="REFBODY rb-7"><p>
<p>Creates a query cursor and
makes the calling process the owner of the cursor. The
cursor is to be used as argument to
<span class="bold_code bc-17"><a href="#next_answers-1"><span class="code">next_answers/1,2</span></a></span> and (eventually)
<span class="bold_code bc-17"><a href="#delete_cursor-1"><span class="code">delete_cursor/1</span></a></span>.
Calls <span class="bold_code bc-13"><a href="javascript:erlhref('../../../../doc/../','erts','erlang.html#spawn_opt-2');"><span class="code">erlang:spawn_opt/2</span></a></span> to spawn and link to
a process that evaluates the query handle. The value of option
<span class="code">spawn_options</span> is used as last argument when calling
<span class="code">spawn_opt/2</span>. Defaults to <span class="code">[link]</span>.</p>
<p><strong>Example:</strong></p>
<div class="example"><pre>
1> <span class="bold_code bc-12">QH = qlc:q([{X,Y} || X <- [a,b], Y <- [1,2]]),</span>
<span class="bold_code bc-12">QC = qlc:cursor(QH),</span>
<span class="bold_code bc-12">qlc:next_answers(QC, 1).</span>
[{a,1}]
2> <span class="bold_code bc-12">qlc:next_answers(QC, 1).</span>
[{a,2}]
3> <span class="bold_code bc-12">qlc:next_answers(QC, all_remaining).</span>
[{b,1},{b,2}]
4> <span class="bold_code bc-12">qlc:delete_cursor(QC).</span>
ok</pre></div>
<p><span class="code">cursor(QH)</span> is equivalent to
<span class="code">cursor(QH, [])</span>.</p>
</p></div>
<p><a name="delete_cursor-1"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-delete_cursor-1-idm281471852085528').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-delete_cursor-1-idm281471852085528').style.visibility = 'hidden';">
<span id="ghlink-delete_cursor-1-idm281471852085528" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L779" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>delete_cursor(QueryCursor) -> ok</div><div class="REFBODY fun-types">
<h3 class="func-types-title">Types</h3>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QueryCursor = <span class="bold_code bc-17"><a href="#type-query_cursor">query_cursor()</a></span></span></div>
</div></p>
<div class="REFBODY rb-7"><p>
<p>Deletes a query cursor. Only the owner of the cursor can
delete the cursor.</p>
</p></div>
<p><a name="e-1"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-e-1-idm281471852083992').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-e-1-idm281471852083992').style.visibility = 'hidden';">
<span id="ghlink-e-1-idm281471852083992" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L788" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>e(QH) -> Answers | Error</div><a name="e-2"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-e-2-idm281471850819272').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-e-2-idm281471850819272').style.visibility = 'hidden';">
<span id="ghlink-e-2-idm281471850819272" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L788" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>e(QH, Options) -> Answers | Error</div><a name="eval-1"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-eval-1-idm281471852815720').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-eval-1-idm281471852815720').style.visibility = 'hidden';">
<span id="ghlink-eval-1-idm281471852815720" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L788" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>eval(QH) -> Answers | Error</div><a name="eval-2"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-eval-2-idm281471850822872').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-eval-2-idm281471850822872').style.visibility = 'hidden';">
<span id="ghlink-eval-2-idm281471850822872" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L788" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>eval(QH, Options) -> Answers | Error</div><div class="REFBODY fun-types">
<h3 class="func-types-title">Types</h3>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QH = <span class="bold_code bc-17"><a href="#type-query_handle_or_list">query_handle_or_list()</a></span></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Answers = <span class="bold_code bc-17"><a href="#type-answers">answers()</a></span></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Options = [Option] | Option</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Option = <br> {cache_all, <span class="bold_code bc-17"><a href="#type-cache">cache()</a></span>} |<br> cache_all |<br> {max_list_size, <span class="bold_code bc-17"><a href="#type-max_list_size">max_list_size()</a></span>} |<br> {tmpdir_usage, <span class="bold_code bc-17"><a href="#type-tmp_file_usage">tmp_file_usage()</a></span>} |<br> {tmpdir, <span class="bold_code bc-17"><a href="#type-tmp_directory">tmp_directory()</a></span>} |<br> {unique_all, boolean()} |<br> unique_all</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Error = {error, module(), Reason}</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Reason = <span class="bold_code bc-15"><a href="file_sorter.html#type-reason">file_sorter:reason()</a></span></span></div>
</div></p>
<div class="REFBODY rb-7"><p>
<p>Evaluates a query handle in the
calling process and collects all answers in a list.</p>
<p><strong>Example:</strong></p>
<div class="example"><pre>
1> <span class="bold_code bc-12">QH = qlc:q([{X,Y} || X <- [a,b], Y <- [1,2]]),</span>
<span class="bold_code bc-12">qlc:eval(QH).</span>
[{a,1},{a,2},{b,1},{b,2}]</pre></div>
<p><span class="code">eval(QH)</span> is equivalent to
<span class="code">eval(QH, [])</span>.</p>
</p></div>
<p><a name="fold-3"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-fold-3-idm281471850809128').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-fold-3-idm281471850809128').style.visibility = 'hidden';">
<span id="ghlink-fold-3-idm281471850809128" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L807" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>fold(Function, Acc0, QH) -> Acc1 | Error</div><a name="fold-4"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-fold-4-idm281471851421560').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-fold-4-idm281471851421560').style.visibility = 'hidden';">
<span id="ghlink-fold-4-idm281471851421560" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L807" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>fold(Function, Acc0, QH, Options) -> Acc1 | Error</div><div class="REFBODY fun-types">
<h3 class="func-types-title">Types</h3>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QH = <span class="bold_code bc-17"><a href="#type-query_handle_or_list">query_handle_or_list()</a></span></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Function = fun((<span class="bold_code bc-17"><a href="#type-answer">answer()</a></span>, AccIn) -> AccOut)</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Acc0 = Acc1 = AccIn = AccOut = term()</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2"></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2"></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2"></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Options = [Option] | Option</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Option = <br> {cache_all, <span class="bold_code bc-17"><a href="#type-cache">cache()</a></span>} |<br> cache_all |<br> {max_list_size, <span class="bold_code bc-17"><a href="#type-max_list_size">max_list_size()</a></span>} |<br> {tmpdir_usage, <span class="bold_code bc-17"><a href="#type-tmp_file_usage">tmp_file_usage()</a></span>} |<br> {tmpdir, <span class="bold_code bc-17"><a href="#type-tmp_directory">tmp_directory()</a></span>} |<br> {unique_all, boolean()} |<br> unique_all</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Error = {error, module(), Reason}</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Reason = <span class="bold_code bc-15"><a href="file_sorter.html#type-reason">file_sorter:reason()</a></span></span></div>
</div></p>
<div class="REFBODY rb-7"><p>
<p>Calls <span class="code">Function</span> on successive answers to
the query handle together with an extra argument
<span class="code">AccIn</span>. The query handle and the function
are evaluated in the calling process.
<span class="code">Function</span> must return a new accumulator,
which is passed to the next call.
<span class="code">Acc0</span> is returned if there are no answers
to the query handle.</p>
<p><strong>Example:</strong></p>
<div class="example"><pre>
1> <span class="bold_code bc-12">QH = [1,2,3,4,5,6],</span>
<span class="bold_code bc-12">qlc:fold(fun(X, Sum) -> X + Sum end, 0, QH).</span>
21</pre></div>
<p><span class="code">fold(Function, Acc0,
QH)</span> is equivalent to
<span class="code">fold(Function, Acc0,
QH, [])</span>.</p>
</p></div>
<p><a name="format_error-1"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-format_error-1-idm281471851413496').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-format_error-1-idm281471851413496').style.visibility = 'hidden';">
<span id="ghlink-format_error-1-idm281471851413496" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L832" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>format_error(Error) -> Chars</div><div class="REFBODY fun-types">
<h3 class="func-types-title">Types</h3>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Error = {error, module(), term()}</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Chars = <span class="bold_code bc-15"><a href="io_lib.html#type-chars">io_lib:chars()</a></span></span></div>
</div></p>
<div class="REFBODY rb-7"><p>
<p>Returns a descriptive string in English of an error tuple
returned by some of the functions of the <span class="code">qlc</span> module
or the parse transform. This function is mainly used by the
compiler invoking the parse transform.</p>
</p></div>
<p><a name="info-1"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-info-1-idm281471852401016').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-info-1-idm281471852401016').style.visibility = 'hidden';">
<span id="ghlink-info-1-idm281471852401016" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L843" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>info(QH) -> Info</div><a name="info-2"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-info-2-idm281471852522216').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-info-2-idm281471852522216').style.visibility = 'hidden';">
<span id="ghlink-info-2-idm281471852522216" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L843" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>info(QH, Options) -> Info</div><div class="REFBODY fun-types">
<h3 class="func-types-title">Types</h3>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QH = <span class="bold_code bc-17"><a href="#type-query_handle_or_list">query_handle_or_list()</a></span></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Options = [Option] | Option</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Option = EvalOption | ReturnOption</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">EvalOption = <br> {cache_all, <span class="bold_code bc-17"><a href="#type-cache">cache()</a></span>} |<br> cache_all |<br> {max_list_size, <span class="bold_code bc-17"><a href="#type-max_list_size">max_list_size()</a></span>} |<br> {tmpdir_usage, <span class="bold_code bc-17"><a href="#type-tmp_file_usage">tmp_file_usage()</a></span>} |<br> {tmpdir, <span class="bold_code bc-17"><a href="#type-tmp_directory">tmp_directory()</a></span>} |<br> {unique_all, boolean()} |<br> unique_all</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">ReturnOption = <br> {depth, Depth} |<br> {flat, boolean()} |<br> {format, Format} |<br> {n_elements, NElements}</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Depth = infinity | integer() >= 0</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Format = abstract_code | string</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">NElements = infinity | integer() >= 1</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Info = <span class="bold_code bc-17"><a href="#type-abstract_expr">abstract_expr()</a></span> | string()</span></div>
</div></p>
<div class="REFBODY rb-7"><p>
<p>Returns information about a
query handle. The information describes the simplifications
and optimizations that are the results of preparing the
query for evaluation. This function is probably mainly useful
during debugging.</p>
<p>The information has the form of an Erlang expression where
QLCs most likely occur. Depending on the format functions of
mentioned QLC tables, it is not certain that the information
is absolutely accurate.</p>
<p>Options:</p>
<ul>
<li>
<p>The default is to return a sequence of QLCs in a block, but
if option <span class="code">{flat, false}</span> is specified, one single
QLC is returned.</p>
</li>
<li>
<p>The default is to return a string, but if
option <span class="code">{format, abstract_code}</span> is specified,
abstract code is returned instead. In the abstract code,
port identifiers, references, and pids are represented by
strings.</p>
</li>
<li>
<p>The default is to return all elements in lists, but if
option <span class="code">{n_elements, NElements}</span> is specified, only
a limited number of elements are returned.</p>
</li>
<li>
<p>The default is to show all parts of
objects and match specifications,
but if option <span class="code">{depth, Depth}</span> is specified, parts
of terms below a certain depth are replaced by <span class="code">'...'</span>.</p>
</li>
</ul>
<p><span class="code">info(QH)</span> is equivalent to
<span class="code">info(QH, [])</span>.</p>
<p><strong>Examples:</strong></p>
<p>In the following example two simple QLCs are inserted only to
hold option <span class="code">{unique, true}</span>:</p>
<div class="example"><pre>
1> <span class="bold_code bc-12">QH = qlc:q([{X,Y} || X <- [x,y], Y <- [a,b]]),</span>
<span class="bold_code bc-12">io:format("~s~n", [qlc:info(QH, unique_all)]).</span>
begin
V1 =
qlc:q([
SQV ||
SQV <- [x,y]
],
[{unique,true}]),
V2 =
qlc:q([
SQV ||
SQV <- [a,b]
],
[{unique,true}]),
qlc:q([
{X,Y} ||
X <- V1,
Y <- V2
],
[{unique,true}])
end</pre></div>
<p>In the following example QLC <span class="code">V2</span> has
been inserted to show the joined generators and the join
method chosen. A convention is used for lookup join: the
first generator (<span class="code">G2</span>) is the one traversed, the second
(<span class="code">G1</span>) is the table where constants are looked up.</p>
<div class="example"><pre>
1> <span class="bold_code bc-12">E1 = ets:new(e1, []),</span>
<span class="bold_code bc-12">E2 = ets:new(e2, []),</span>
<span class="bold_code bc-12">true = ets:insert(E1, [{1,a},{2,b}]),</span>
<span class="bold_code bc-12">true = ets:insert(E2, [{a,1},{b,2}]),</span>
<span class="bold_code bc-12">Q = qlc:q([{X,Z,W} ||</span>
<span class="bold_code bc-12">{X, Z} <- ets:table(E1),</span>
<span class="bold_code bc-12">{W, Y} <- ets:table(E2),</span>
<span class="bold_code bc-12">X =:= Y]),</span>
<span class="bold_code bc-12">io:format("~s~n", [qlc:info(Q)]).</span>
begin
V1 =
qlc:q([
P0 ||
P0 = {W,Y} <- ets:table(17)
]),
V2 =
qlc:q([
[G1|G2] ||
G2 <- V1,
G1 <- ets:table(16),
element(2, G1) =:= element(1, G2)
],
[{join,lookup}]),
qlc:q([
{X,Z,W} ||
[{X,Z}|{W,Y}] <- V2
])
end</pre></div>
</p></div>
<p><a name="keysort-2"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-keysort-2-idm281471851432200').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-keysort-2-idm281471851432200').style.visibility = 'hidden';">
<span id="ghlink-keysort-2-idm281471851432200" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L948" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>keysort(KeyPos, QH1) -> QH2</div><a name="keysort-3"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-keysort-3-idm281471851051048').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-keysort-3-idm281471851051048').style.visibility = 'hidden';">
<span id="ghlink-keysort-3-idm281471851051048" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L948" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>keysort(KeyPos, QH1, SortOptions) -> QH2</div><div class="REFBODY fun-types">
<h3 class="func-types-title">Types</h3>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">KeyPos = <span class="bold_code bc-17"><a href="#type-key_pos">key_pos()</a></span></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">SortOptions = <span class="bold_code bc-17"><a href="#type-sort_options">sort_options()</a></span></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QH1 = <span class="bold_code bc-17"><a href="#type-query_handle_or_list">query_handle_or_list()</a></span></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QH2 = <span class="bold_code bc-17"><a href="#type-query_handle">query_handle()</a></span></span></div>
</div></p>
<div class="REFBODY rb-7"><p>
<p>Returns a query handle. When evaluating query handle
<span class="code">QH2</span>, the answers to query handle
<span class="code">QH1</span> are sorted by <span class="bold_code bc-15"><a href="file_sorter.html#keysort-4"><span class="code">file_sorter:keysort/4</span></a></span>
according to the options.</p>
<p>The sorter uses temporary files only if
<span class="code">QH1</span> does not evaluate to a list and the
size of the binary representation of the answers exceeds
<span class="code">Size</span> bytes, where <span class="code">Size</span> is the value of option
<span class="code">size</span>.</p>
<p><span class="code">keysort(KeyPos, QH1)</span>
is equivalent to
<span class="code">keysort(KeyPos, QH1, [])</span>.</p>
</p></div>
<p><a name="next_answers-1"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-next_answers-1-idm281471851859144').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-next_answers-1-idm281471851859144').style.visibility = 'hidden';">
<span id="ghlink-next_answers-1-idm281471851859144" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L969" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>next_answers(QueryCursor) -> Answers | Error</div><a name="next_answers-2"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-next_answers-2-idm281471851852200').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-next_answers-2-idm281471851852200').style.visibility = 'hidden';">
<span id="ghlink-next_answers-2-idm281471851852200" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L969" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>next_answers(QueryCursor, NumberOfAnswers) -> Answers | Error</div><div class="REFBODY fun-types">
<h3 class="func-types-title">Types</h3>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QueryCursor = <span class="bold_code bc-17"><a href="#type-query_cursor">query_cursor()</a></span></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Answers = <span class="bold_code bc-17"><a href="#type-answers">answers()</a></span></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">NumberOfAnswers = all_remaining | integer() >= 1</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Error = {error, module(), Reason}</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Reason = <span class="bold_code bc-15"><a href="file_sorter.html#type-reason">file_sorter:reason()</a></span></span></div>
</div></p>
<div class="REFBODY rb-7"><p>
<p>Returns some or all of the remaining answers to a query
cursor. Only the owner of <span class="code">QueryCursor</span> can
retrieve answers.</p>
<p>Optional argument <span class="code">NumberOfAnswers</span> determines the
maximum number of answers returned. Defaults to
<span class="code">10</span>. If less than the requested number of answers is
returned, subsequent calls to <span class="code">next_answers</span>
return <span class="code">[]</span>.</p>
</p></div>
<p><a name="q-1"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-q-1-idm281471852311656').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-q-1-idm281471852311656').style.visibility = 'hidden';">
<span id="ghlink-q-1-idm281471852311656" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L985" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>q(QLC) -> QH</div><a name="q-2"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-q-2-idm281471851858392').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-q-2-idm281471851858392').style.visibility = 'hidden';">
<span id="ghlink-q-2-idm281471851858392" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L985" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>q(QLC, Options) -> QH</div><div class="REFBODY fun-types">
<h3 class="func-types-title">Types</h3>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QH = <span class="bold_code bc-17"><a href="#type-query_handle">query_handle()</a></span></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Options = [Option] | Option</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Option = <br> {max_lookup, MaxLookup} |<br> {cache, <span class="bold_code bc-17"><a href="#type-cache">cache()</a></span>} |<br> cache |<br> {join, Join} |<br> {lookup, Lookup} |<br> {unique, boolean()} |<br> unique</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">MaxLookup = integer() >= 0 | infinity</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Join = any | lookup | merge | nested_loop</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Lookup = boolean() | any</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QLC = <span class="bold_code bc-17"><a href="#type-query_list_comprehension">query_list_comprehension()</a></span></span></div>
</div></p>
<div class="REFBODY rb-7"><p>
<p>Returns a query handle for a QLC.
The QLC must be the first argument to this function, otherwise
it is evaluated as an ordinary list comprehension. It is also
necessary to add the following line to the source code:</p>
<div class="example"><pre>-include_lib("stdlib/include/qlc.hrl").</pre></div>
<p>This causes a parse transform to substitute a fun for the QLC. The
(compiled) fun is called when the query handle is evaluated.</p>
<p>When calling <span class="code">qlc:q/1,2</span> from the Erlang shell, the
parse transform is automatically called. When this occurs, the fun
substituted for the QLC is not compiled but is evaluated by
<span class="bold_code bc-19"><a href="erl_eval.html"><span class="code">erl_eval(3)</span></a></span>. This
is also true when expressions are evaluated by
<span class="code">file:eval/1,2</span> or in the debugger.</p>
<p>To be explicit, this does not work:</p>
<div class="example"><pre>
...
A = [X || {X} <- [{1},{2}]],
QH = qlc:q(A),
...</pre></div>
<p>Variable <span class="code">A</span> is bound to the evaluated value
of the list comprehension (<span class="code">[1,2]</span>). The compiler
complains with an error message ("argument is not a query
list comprehension"); the shell process stops with a
<span class="code">badarg</span> reason.</p>
<p><span class="code">q(QLC)</span> is equivalent to
<span class="code">q(QLC, [])</span>.</p>
<p>Options:</p>
<ul>
<li>
<p>Option <span class="code">{cache, ets}</span> can be used to cache
the answers to a QLC. The answers are stored in one ETS
table for each cached QLC. When a cached QLC is
evaluated again, answers are fetched from the table without
any further computations. Therefore, when all answers to a
cached QLC have been found, the ETS tables used for
caching answers to the qualifiers of the QLC can be emptied.
Option <span class="code">cache</span> is equivalent to <span class="code">{cache, ets}</span>.</p>
</li>
<li>
<p>Option <span class="code">{cache, list}</span> can be used to cache
the answers to a QLC like
<span class="code">{cache, ets}</span>. The difference is that the answers
are kept in a list (on the process heap). If the answers
would occupy more than a certain amount of RAM memory, a
temporary file is used for storing the answers. Option
<span class="code">max_list_size</span> sets the limit in bytes and the temporary
file is put on the directory set by option <span class="code">tmpdir</span>.</p>
<p>Option <span class="code">cache</span> has no effect if it is known that
the QLC is to be evaluated at most once.
This is always true for the top-most QLC
and also for the list expression of the first
generator in a list of qualifiers. Notice that in the presence
of side effects in filters or callback functions, the answers
to QLCs can be affected by option <span class="code">cache</span>.</p>
</li>
<li>
<p>Option <span class="code">{unique, true}</span> can be used to remove
duplicate answers to a QLC. The unique
answers are stored in one ETS table for each QLC.
The table is emptied every time it is known
that there are no more answers to the QLC.
Option <span class="code">unique</span> is equivalent to
<span class="code">{unique, true}</span>. If option <span class="code">unique</span> is
combined with option <span class="code">{cache, ets}</span>, two ETS
tables are used, but the full answers are stored in one
table only. If option <span class="code">unique</span> is combined with option
<span class="code">{cache, list}</span>, the answers are sorted
twice using
<span class="bold_code bc-17"><a href="#keysort-3"><span class="code">keysort/3</span></a></span>;
once to remove duplicates and once to restore the order.</p>
</li>
</ul>
<p>Options <span class="code">cache</span> and <span class="code">unique</span> apply not only
to the QLC itself but also to the results of looking up constants,
running match specifications, and joining handles.</p>
<p><strong>Example:</strong></p>
<p>In the following example the cached results of the merge join are
traversed for each value of <span class="code">A</span>. Notice that without option
<span class="code">cache</span> the join would have been carried out
three times, once for each value of <span class="code">A</span>.</p>
<div class="example"><pre>
1> <span class="bold_code bc-12">Q = qlc:q([{A,X,Z,W} ||</span>
<span class="bold_code bc-12">A <- [a,b,c],</span>
<span class="bold_code bc-12">{X,Z} <- [{a,1},{b,4},{c,6}],</span>
<span class="bold_code bc-12">{W,Y} <- [{2,a},{3,b},{4,c}],</span>
<span class="bold_code bc-12">X =:= Y],</span>
<span class="bold_code bc-12">{cache, list}),</span>
<span class="bold_code bc-12">io:format("~s~n", [qlc:info(Q)]).</span>
begin
V1 =
qlc:q([
P0 ||
P0 = {X,Z} <-
qlc:keysort(1, [{a,1},{b,4},{c,6}], [])
]),
V2 =
qlc:q([
P0 ||
P0 = {W,Y} <-
qlc:keysort(2, [{2,a},{3,b},{4,c}], [])
]),
V3 =
qlc:q([
[G1|G2] ||
G1 <- V1,
G2 <- V2,
element(1, G1) == element(2, G2)
],
[{join,merge},{cache,list}]),
qlc:q([
{A,X,Z,W} ||
A <- [a,b,c],
[{X,Z}|{W,Y}] <- V3,
X =:= Y
])
end</pre></div>
<p><span class="bold_code bc-17"><a href="#sort-1"><span class="code">sort/1,2</span></a></span> and
<span class="bold_code bc-17"><a href="#keysort-2"><span class="code">keysort/2,3</span></a></span>
can also be used for
caching answers and for removing duplicates. When sorting
answers are cached in a list, possibly stored on a temporary
file, and no ETS tables are used.</p>
<p>Sometimes (see <span class="bold_code bc-17"><a href="#table-2"><span class="code">table/2</span></a></span>) traversal
of tables can be done by looking up key values, which is
assumed to be fast. Under certain (rare) circumstances
there can be too many key values to look up.
<a name="max_lookup"></a>
Option <span class="code">{max_lookup, MaxLookup}</span> can then be used
to limit the number of lookups: if more than
<span class="code">MaxLookup</span> lookups would be required, no lookups are
done but the table is traversed instead. Defaults to
<span class="code">infinity</span>, which means that there is no limit on the
number of keys to look up.</p>
<p><strong>Example:</strong></p>
<p>In the following example, using the <span class="code">gb_table</span> module from
section <span class="bold_code bc-17"><a href="#implementing_a_qlc_table">Implementing a QLC Table</a></span>, there are six keys to look up:
<span class="code">{1,a}</span>, <span class="code">{1,b}</span>, <span class="code">{1,c}</span>, <span class="code">{2,a}</span>,
<span class="code">{2,b}</span>, and <span class="code">{2,c}</span>. The reason is that the two
elements of key <span class="code">{X, Y}</span> are compared separately.</p>
<div class="example"><pre>
1> <span class="bold_code bc-12">T = gb_trees:empty(),</span>
<span class="bold_code bc-12">QH = qlc:q([X || {{X,Y},_} <- gb_table:table(T),</span>
<span class="bold_code bc-12">((X == 1) or (X == 2)) andalso</span>
<span class="bold_code bc-12">((Y == a) or (Y == b) or (Y == c))]),</span>
<span class="bold_code bc-12">io:format("~s~n", [qlc:info(QH)]).</span>
ets:match_spec_run(
lists:flatmap(fun(K) ->
case
gb_trees:lookup(K,
gb_trees:from_orddict([]))
of
{value,V} ->
[{K,V}];
none ->
[]
end
end,
[{1,a},{1,b},{1,c},{2,a},{2,b},{2,c}]),
ets:match_spec_compile([{{{'$1','$2'},'_'},[],['$1']}]))</pre></div>
<p>Options:</p>
<ul>
<li>
<p>Option <span class="code">{lookup, true}</span> can be used to ensure
that the <span class="code">qlc</span> module looks up constants in some
QLC table. If there are more than one QLC table among the
list expressions of the generators,
constants must be looked up in at least one
of the tables. The evaluation of the query fails if there
are no constants to look up. This option is useful
when it would be unacceptable to traverse all
objects in some table. Setting option <span class="code">lookup</span> to
<span class="code">false</span> ensures that no constants are looked up
(<span class="code">{max_lookup, 0}</span> has the same effect).
Defaults to <span class="code">any</span>, which means that constants are
looked up whenever possible.</p>
</li>
<li>
<p>Option <span class="code">{join, Join}</span> can be used to ensure
that a certain join method is used:</p>
<ul>
<li>
<span class="code">{join, lookup}</span> invokes the lookup join
method.</li>
<li>
<span class="code">{join, merge}</span> invokes the merge join
method.</li>
<li>
<span class="code">{join, nested_loop}</span> invokes the method of
matching every pair of objects from two handles. This
method is mostly very slow.</li>
</ul>
<p>The evaluation of the query fails if the <span class="code">qlc</span> module
cannot carry out the chosen join method. Defaults to
<span class="code">any</span>, which means that some fast join
method is used if possible.</p>
</li>
</ul>
</p></div>
<p><a name="sort-1"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-sort-1-idm281471850896472').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-sort-1-idm281471850896472').style.visibility = 'hidden';">
<span id="ghlink-sort-1-idm281471850896472" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L1190" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>sort(QH1) -> QH2</div><a name="sort-2"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-sort-2-idm281471850892024').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-sort-2-idm281471850892024').style.visibility = 'hidden';">
<span id="ghlink-sort-2-idm281471850892024" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L1190" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>sort(QH1, SortOptions) -> QH2</div><div class="REFBODY fun-types">
<h3 class="func-types-title">Types</h3>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">SortOptions = <span class="bold_code bc-17"><a href="#type-sort_options">sort_options()</a></span></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QH1 = <span class="bold_code bc-17"><a href="#type-query_handle_or_list">query_handle_or_list()</a></span></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QH2 = <span class="bold_code bc-17"><a href="#type-query_handle">query_handle()</a></span></span></div>
</div></p>
<div class="REFBODY rb-7"><p>
<p>Returns a query handle. When evaluating query handle
<span class="code">QH2</span>, the answers to query handle
<span class="code">QH1</span> are sorted by <span class="bold_code bc-15"><a href="file_sorter.html#sort-3"><span class="code">file_sorter:sort/3</span></a></span>
according to the options.</p>
<p>The sorter uses temporary files only if
<span class="code">QH1</span> does not evaluate to a list and the
size of the binary representation of the answers exceeds
<span class="code">Size</span> bytes, where <span class="code">Size</span> is the value of option
<span class="code">size</span>.</p>
<p><span class="code">sort(QH1)</span> is equivalent to
<span class="code">sort(QH1, [])</span>.</p>
</p></div>
<p><a name="string_to_handle-1"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-string_to_handle-1-idm281471851187928').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-string_to_handle-1-idm281471851187928').style.visibility = 'hidden';">
<span id="ghlink-string_to_handle-1-idm281471851187928" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L1210" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>string_to_handle(QueryString) -> QH | Error</div><a name="string_to_handle-2"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-string_to_handle-2-idm281471851180776').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-string_to_handle-2-idm281471851180776').style.visibility = 'hidden';">
<span id="ghlink-string_to_handle-2-idm281471851180776" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L1210" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>string_to_handle(QueryString, Options) -> QH | Error</div><a name="string_to_handle-3"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-string_to_handle-3-idm281471851454296').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-string_to_handle-3-idm281471851454296').style.visibility = 'hidden';">
<span id="ghlink-string_to_handle-3-idm281471851454296" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L1210" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>string_to_handle(QueryString, Options, Bindings) -> QH | Error</div><div class="REFBODY fun-types">
<h3 class="func-types-title">Types</h3>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QueryString = string()</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Options = [Option] | Option</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Option = <br> {max_lookup, MaxLookup} |<br> {cache, <span class="bold_code bc-17"><a href="#type-cache">cache()</a></span>} |<br> cache |<br> {join, Join} |<br> {lookup, Lookup} |<br> {unique, boolean()} |<br> unique</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">MaxLookup = integer() >= 0 | infinity</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Join = any | lookup | merge | nested_loop</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Lookup = boolean() | any</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Bindings = <span class="bold_code bc-15"><a href="erl_eval.html#type-binding_struct">erl_eval:binding_struct()</a></span></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QH = <span class="bold_code bc-17"><a href="#type-query_handle">query_handle()</a></span></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Error = {error, module(), Reason}</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Reason = <span class="bold_code bc-15"><a href="erl_parse.html#type-error_info">erl_parse:error_info()</a></span> | <span class="bold_code bc-15"><a href="erl_scan.html#type-error_info">erl_scan:error_info()</a></span></span></div>
</div></p>
<div class="REFBODY rb-7"><p>
<p>A string version of <span class="bold_code bc-17"><a href="#q-1"><span class="code">q/1,2</span></a></span>.
When the query handle is evaluated, the fun created by the parse
transform is interpreted by
<span class="bold_code bc-19"><a href="erl_eval.html"><span class="code">erl_eval(3)</span></a></span>.
The query string is to be one single QLC terminated by a period.</p>
<p><strong>Example:</strong></p>
<div class="example"><pre>
1> <span class="bold_code bc-12">L = [1,2,3],</span>
<span class="bold_code bc-12">Bs = erl_eval:add_binding('L', L, erl_eval:new_bindings()),</span>
<span class="bold_code bc-12">QH = qlc:string_to_handle("[X+1 || X <- L].", [], Bs),</span>
<span class="bold_code bc-12">qlc:eval(QH).</span>
[2,3,4]</pre></div>
<p><span class="code">string_to_handle(QueryString)</span>
is equivalent to
<span class="code">string_to_handle(QueryString, [])</span>.</p>
<p><span class="code">string_to_handle(QueryString,
Options)</span> is equivalent to
<span class="code">string_to_handle(QueryString,
Options, erl_eval:new_bindings())</span>.</p>
<p>This function is probably mainly useful when called from
outside of Erlang, for example from a driver written in C.</p>
</p></div>
<p><a name="table-2"></a><div class="bold_code func-head" onMouseOver="document.getElementById('ghlink-table-2-idm281471851491016').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-table-2-idm281471851491016').style.visibility = 'hidden';">
<span id="ghlink-table-2-idm281471851491016" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L1240" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span>table(TraverseFun, Options) -> QH</div><div class="REFBODY fun-types">
<h3 class="func-types-title">Types</h3>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">TraverseFun = TraverseFun0 | TraverseFun1</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">TraverseFun0 = fun(() -> TraverseResult)</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">TraverseFun1 = fun((<span class="bold_code bc-17"><a href="#type-match_expression">match_expression()</a></span>) -> TraverseResult)</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">TraverseResult = Objects | term()</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Objects = [] | [term() | ObjectList]</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">ObjectList = TraverseFun0 | Objects</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Options = [Option] | Option</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Option = <br> {format_fun, FormatFun} |<br> {info_fun, InfoFun} |<br> {lookup_fun, LookupFun} |<br> {parent_fun, ParentFun} |<br> {post_fun, PostFun} |<br> {pre_fun, PreFun} |<br> {key_equality, KeyComparison}</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">FormatFun = undefined | fun((SelectedObjects) -> FormatedTable)</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">SelectedObjects = <br> all |<br> {all, NElements, DepthFun} |<br> {match_spec, <span class="bold_code bc-17"><a href="#type-match_expression">match_expression()</a></span>} |<br> {lookup, Position, Keys} |<br> {lookup, Position, Keys, NElements, DepthFun}</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">NElements = infinity | integer() >= 1</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">DepthFun = fun((term()) -> term())</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">FormatedTable = {Mod, Fun, Args} | <span class="bold_code bc-17"><a href="#type-abstract_expr">abstract_expr()</a></span> | string()</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">InfoFun = undefined | fun((InfoTag) -> InfoValue)</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">InfoTag = indices | is_unique_objects | keypos | num_of_objects</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">InfoValue = undefined | term()</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">LookupFun = undefined | fun((Position, Keys) -> LookupResult)</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">LookupResult = [term()] | term()</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">ParentFun = undefined | fun(() -> ParentFunValue)</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">PostFun = undefined | fun(() -> term())</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">PreFun = undefined | fun((PreArgs) -> term())</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">PreArgs = [PreArg]</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">PreArg = {parent_value, ParentFunValue} | {stop_fun, StopFun}</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">ParentFunValue = undefined | term()</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">StopFun = undefined | fun(() -> term())</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">KeyComparison = '=:=' | '=='</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Position = integer() >= 1</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Keys = [term()]</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Mod = Fun = atom()</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2"></span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">Args = [term()]</span></div>
<div class="REFTYPES rt-1"><span class="bold_code bc-2">QH = <span class="bold_code bc-17"><a href="#type-query_handle">query_handle()</a></span></span></div>
</div></p>
<div class="REFBODY rb-7"><p>
<p>Returns a query handle for a QLC table.
In Erlang/OTP there is support for ETS, Dets, and
Mnesia tables, but many other data structures can be turned
into QLC tables. This is accomplished by letting function(s) in the
module implementing the data structure create a query handle by
calling <span class="code">qlc:table/2</span>. The different ways to traverse the table
and properties of the table are handled by callback
functions provided as options to <span class="code">qlc:table/2</span>.</p>
<ul>
<li>
<p>Callback function <span class="code">TraverseFun</span> is
used for traversing the table. It is to return a list of
objects terminated by either <span class="code">[]</span> or a nullary fun to
be used for traversing the not yet traversed objects of the
table. Any other return value is immediately returned as
value of the query evaluation. Unary
<span class="code">TraverseFun</span>s are to accept a match
specification as argument. The match specification is
created by the parse transform by analyzing the pattern of
the generator calling <span class="code">qlc:table/2</span> and filters using
variables introduced in the pattern. If the parse transform
cannot find a match specification equivalent to the pattern
and filters, <span class="code">TraverseFun</span> is called
with a match specification returning every object.</p>
<ul>
<li>
<p>Modules that can use match specifications for optimized
traversal of tables are to call <span class="code">qlc:table/2</span> with an unary
<span class="code">TraverseFun</span>. An example is
<span class="bold_code bc-15"><a href="ets.html#table-2"><span class="code">ets:table/2</span></a></span>.</p>
</li>
<li>
<p>Other modules can provide a nullary
<span class="code">TraverseFun</span>. An example is
<span class="code">gb_table:table/1</span> in section
<span class="bold_code bc-17"><a href="#implementing_a_qlc_table">Implementing a QLC Table</a></span>.</p>
</li>
</ul>
</li>
<li>
<p>Unary callback function <span class="code">PreFun</span> is
called once before the table is read for the first time.
If the call fails, the query evaluation fails.</p>
<p>Argument <span class="code">PreArgs</span> is a list of tagged values.
There are two tags, <span class="code">parent_value</span> and <span class="code">stop_fun</span>, used
by Mnesia for managing transactions.</p>
<ul>
<li>
<p>The value of <span class="code">parent_value</span> is
the value returned by <span class="code">ParentFun</span>, or
<span class="code">undefined</span> if there is no <span class="code">ParentFun</span>.
<span class="code">ParentFun</span> is called once just before the
call of <span class="code">PreFun</span> in the context of the
process calling
<span class="bold_code bc-17"><a href="#eval-1"><span class="code">eval/1,2</span></a></span>,
<span class="bold_code bc-17"><a href="#fold-3"><span class="code">fold/3,4</span></a></span>, or
<span class="bold_code bc-17"><a href="#cursor-1"><span class="code">cursor/1,2</span></a></span>.
</p>
</li>
<li>
<p>The value of <span class="code">stop_fun</span> is a nullary fun
that deletes the cursor if called from the parent, or
<span class="code">undefined</span> if there is no cursor.</p>
</li>
</ul>
</li>
<li>
<p>Nullary callback function
<span class="code">PostFun</span> is called once after the table
was last read. The return value, which is caught, is ignored.
If <span class="code">PreFun</span> has been called for a table,
<span class="code">PostFun</span> is guaranteed to be called for
that table, even if the evaluation of the query fails for
some reason.</p>
<p>The pre (post) functions for different tables are evaluated in
unspecified order.</p>
<p>Other table access than reading, such as calling
<span class="code">InfoFun</span>, is assumed to be OK at any time.</p>
</li>
<li>
<p><a name="lookup_fun"></a>Binary callback
function <span class="code">LookupFun</span> is used for looking
up objects in the table. The first argument
<span class="code">Position</span> is the key position or an
indexed position and the second argument
<span class="code">Keys</span> is a sorted list of unique values.
The return value is to be a list of all objects (tuples),
such that the element at <span class="code">Position</span> is a member of
<span class="code">Keys</span>. Any other return value is
immediately returned as value of the query evaluation.
<span class="code">LookupFun</span> is called instead of
traversing the table if the parse transform at compile time
can determine that the filters match and compare the element
at <span class="code">Position</span> in such a way that only
<span class="code">Keys</span> need to be looked up to
find all potential answers.</p>
<p>The key position is obtained by calling
<span class="code">InfoFun(keypos)</span> and the indexed
positions by calling
<span class="code">InfoFun(indices)</span>. If the key position
can be used for lookup, it is always chosen, otherwise the
indexed position requiring the least number of lookups is
chosen. If there is a tie between two indexed positions, the
one occurring first in the list returned by
<span class="code">InfoFun</span> is chosen. Positions requiring
more than <span class="bold_code bc-17"><a href="#max_lookup">max_lookup</a></span>
lookups are ignored.</p>
</li>
<li>
<p>Unary callback function <span class="code">InfoFun</span> is
to return information about the table. <span class="code">undefined</span>
is to be returned if the value of some tag is unknown:</p>
<dl>
<dt><strong><span class="code">indices</span></strong></dt>
<dd>Returns a list of indexed positions, a list of positive
integers.</dd>
<dt><strong><span class="code">is_unique_objects</span></strong></dt>
<dd>Returns <span class="code">true</span> if the objects returned by
<span class="code">TraverseFun</span> are unique.
</dd>
<dt><strong><span class="code">keypos</span></strong></dt>
<dd>Returns the position of the table key, a positive integer.
</dd>
<dt><strong><span class="code">is_sorted_key</span></strong></dt>
<dd>Returns <span class="code">true</span> if the objects returned by
<span class="code">TraverseFun</span> are sorted on the key.
</dd>
<dt><strong><span class="code">num_of_objects</span></strong></dt>
<dd>Returns the number of objects in the table, a non-negative
integer.
</dd>
</dl>
</li>
<li>
<p>Unary callback function <span class="code">FormatFun</span>
is used by <span class="bold_code bc-17"><a href="#info-1"><span class="code">info/1,2</span></a></span>
for displaying the call that created the query handle of the
table. Defaults to <span class="code">undefined</span>, which means that
<span class="code">info/1,2</span> displays a call to <span class="code">'$MOD':'$FUN'/0</span>.
It is up to <span class="code">FormatFun</span> to present the
selected objects of the table in a suitable way. However, if
a character list is chosen for presentation, it must be an
Erlang expression that can be scanned and parsed (a trailing
dot is added by <span class="code">info/1,2</span> though).</p>
<p><span class="code">FormatFun</span> is called with an argument
that describes the selected objects based on optimizations
done as a result of analyzing the filters of the QLC where
the call to <span class="code">qlc:table/2</span> occurs. The argument can have the
following values:</p>
<dl>
<dt><strong><span class="code">{lookup, Position, Keys, NElements, DepthFun}</span>.</strong></dt>
<dd>
<p><span class="code">LookupFun</span> is used for looking up objects in the
table.</p>
</dd>
<dt><strong><span class="code">{match_spec, MatchExpression}</span></strong></dt>
<dd>
<p>No way of finding all possible answers by looking up keys
was found, but the filters could be transformed into a
match specification. All answers are found by calling
<span class="code">TraverseFun(MatchExpression)</span>.</p>
</dd>
<dt><strong><span class="code">{all, NElements, DepthFun}</span></strong></dt>
<dd>
<p>No optimization was found. A match specification matching
all objects is used if <span class="code">TraverseFun</span> is unary.</p>
<p><span class="code">NElements</span> is the value of the <span class="code">info/1,2</span> option
<span class="code">n_elements</span>.</p>
<p><span class="code">DepthFun</span> is a function that can be used for
limiting the size of terms; calling
<span class="code">DepthFun(Term)</span> substitutes <span class="code">'...'</span> for
parts of <span class="code">Term</span> below the depth specified by the
<span class="code">info/1,2</span> option <span class="code">depth</span>.</p>
<p>If calling <span class="code">FormatFun</span> with an
argument including <span class="code">NElements</span> and
<span class="code">DepthFun</span> fails, <span class="code">FormatFun</span>
is called once again with an argument excluding
<span class="code">NElements</span> and <span class="code">DepthFun</span>
(<span class="code">{lookup, Position, Keys}</span> or
<span class="code">all</span>).</p>
</dd>
</dl>
</li>
<li>
<p><a name="key_equality"></a>The value of option
<span class="code">key_equality</span> is to be <span class="code">'=:='</span> if the table
considers two keys equal if they match, and to be
<span class="code">'=='</span> if two keys are equal if they compare equal.
Defaults to <span class="code">'=:='</span>.</p>
</li>
</ul>
<p>For the various options recognized by <span class="code">table/1,2</span>
in respective module, see
<span class="bold_code bc-15"><a href="ets.html#table-1"><span class="code">ets(3)</span></a></span>,
<span class="bold_code bc-15"><a href="dets.html#table-1"><span class="code">dets(3)</span></a></span>, and
<span class="bold_code bc-13"><a href="javascript:erlhref('../../../../doc/../','mnesia','mnesia.html#table-1');"><span class="code">mnesia(3)</span></a></span>.
</p>
</p></div>
</div>
<h3><span onMouseOver="document.getElementById('ghlink-see-also-idm281471863006424').style.visibility = 'visible';" onMouseOut="document.getElementById('ghlink-see-also-idm281471863006424').style.visibility = 'hidden';"><span id="ghlink-see-also-idm281471863006424" class="ghlink"><a href="https://github.com/erlang/otp/edit/maint/lib/stdlib/doc/src/qlc.xml#L1446" title="Found an issue with the documentation? Fix it by clicking here!"><span class="pencil"></span></a></span><a class="title_link" name="see-also" href="#see-also">See Also</a></span></h3>
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<p><span class="bold_code bc-19"><a href="dets.html"><span class="code">dets(3)</span></a></span>,
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