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<img alt="Erlang logo" src="../../../../doc/erlang-logo.png"><br><small><a href="index.html">Reference Manual</a><br><a href="release_notes.html">Release Notes</a><br><a href="../pdf/parsetools-2.0.7.pdf">PDF</a><br><a href="../../../../doc/index.html">Top</a></small><p><strong>Parse Tools</strong><br><strong>Reference Manual</strong><br><small>Version 2.0.7</small></p>
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<!-- refpage --><center><h1>yecc</h1></center>
<h3>MODULE</h3>
<div class="REFBODY">yecc</div>
<h3>MODULE SUMMARY</h3>
<div class="REFBODY">LALR-1 Parser Generator</div>
<h3>DESCRIPTION</h3>
<div class="REFBODY"><p>
<p>An LALR-1 parser generator for Erlang, similar to <span class="code">yacc</span>.
Takes a BNF grammar definition as input, and produces Erlang code
for a parser. </p>
<p>To understand this text, you also have to
look at the <span class="code">yacc</span> documentation in the UNIX(TM) manual. This
is most probably necessary in order to understand the idea of a
parser generator, and the principle and problems of LALR parsing
with finite look-ahead.</p>
</p></div>
<h3>EXPORTS</h3>
<p><a name="file-1"><span class="bold_code">file(Grammarfile [, Options]) -> YeccRet</span></a><br></p>
<div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES">
<span class="bold_code">Grammarfile = filename()</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Options = Option | [Option]</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Option = - see below -</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">YeccRet = {ok, Parserfile} | {ok, Parserfile, Warnings} | error | {error, Warnings, Errors}</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Parserfile = filename()</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Warnings = Errors = [{filename(), [ErrorInfo]}]</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">ErrorInfo = {ErrorLine, module(), Reason}</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">ErrorLine = integer()</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Reason = - formatable by format_error/1 -</span><br>
</div>
</div>
<div class="REFBODY"><p>
<p><span class="code">Grammarfile</span> is the file of declarations and grammar
rules. Returns <span class="code">ok</span> upon success, or <span class="code">error</span> if
there are errors. An Erlang file containing the parser is
created if there are no errors. The options are:
</p>
<dl>
<dt><strong><span class="code">{parserfile, Parserfile}</span>.</strong></dt>
<dd>
<span class="code">Parserfile</span> is the name of the file that will
contain the Erlang parser code that is generated. The
default (<span class="code">""</span>) is to add the extension <span class="code">.erl</span>
to <span class="code">Grammarfile</span> stripped of the <span class="code">.yrl</span>
extension.
</dd>
<dt><strong><span class="code">{includefile, Includefile}</span>.</strong></dt>
<dd>Indicates a customized prologue file which the user
may want to use instead of the default file
<span class="code">lib/parsetools/include/yeccpre.hrl</span> which is
otherwise included at the beginning of the resulting
parser file. <strong>N.B.</strong> The <span class="code">Includefile</span> is
included 'as is' in the parser file, so it must not have a
module declaration of its own, and it should not be
compiled. It must, however, contain the necessary export
declarations. The default is indicated by <span class="code">""</span>.
</dd>
<dt><strong><span class="code">{report_errors, bool()}</span>.</strong></dt>
<dd>Causes errors to be printed as they occur. Default is
<span class="code">true</span>.
</dd>
<dt><strong><span class="code">{report_warnings, bool()}</span>.</strong></dt>
<dd>Causes warnings to be printed as they occur. Default is
<span class="code">true</span>.
</dd>
<dt><strong><span class="code">{report, bool()}</span>.</strong></dt>
<dd>This is a short form for both <span class="code">report_errors</span> and
<span class="code">report_warnings</span>.
</dd>
<dt><strong><span class="code">warnings_as_errors</span></strong></dt>
<dd>
<p>Causes warnings to be treated as errors.</p>
</dd>
<dt><strong><span class="code">{return_errors, bool()}</span>.</strong></dt>
<dd>If this flag is set, <span class="code">{error, Errors, Warnings}</span>
is returned when there are errors. Default is
<span class="code">false</span>.
</dd>
<dt><strong><span class="code">{return_warnings, bool()}</span>.</strong></dt>
<dd>If this flag is set, an extra field containing
<span class="code">Warnings</span> is added to the tuple returned upon
success. Default is <span class="code">false</span>.
</dd>
<dt><strong><span class="code">{return, bool()}</span>.</strong></dt>
<dd>This is a short form for both <span class="code">return_errors</span> and
<span class="code">return_warnings</span>.
</dd>
<dt><strong><span class="code">{verbose, bool()}</span>. </strong></dt>
<dd>Determines whether the parser generator should give
full information about resolved and unresolved parse
action conflicts (<span class="code">true</span>), or only about those
conflicts that prevent a parser from being generated from
the input grammar (<span class="code">false</span>, the default).
</dd>
</dl>
<p>Any of the Boolean options can be set to <span class="code">true</span> by
stating the name of the option. For example, <span class="code">verbose</span>
is equivalent to <span class="code">{verbose, true}</span>.
</p>
<p>The value of the <span class="code">Parserfile</span> option stripped of the
<span class="code">.erl</span> extension is used by Yecc as the module name of
the generated parser file.</p>
<p>Yecc will add the extension <span class="code">.yrl</span> to the
<span class="code">Grammarfile</span> name, the extension <span class="code">.hrl</span> to the
<span class="code">Includefile</span> name, and the extension <span class="code">.erl</span> to
the <span class="code">Parserfile</span> name, unless the extension is already
there.</p>
</p></div>
<p><a name="format_error-1"><span class="bold_code">format_error(Reason) -> Chars</span></a><br></p>
<div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES">
<span class="bold_code">Reason = - as returned by yecc:file/1,2 -</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Chars = [char() | Chars]</span><br>
</div>
</div>
<div class="REFBODY"><p>
<p>Returns a descriptive string in English of an error tuple
returned by <span class="code">yecc:file/1,2</span>. This function is mainly
used by the compiler invoking Yecc.</p>
</p></div>
<h3><a name="id60823">Pre-Processing</a></h3>
<div class="REFBODY">
<p>A <span class="code">scanner</span> to pre-process the text (program, etc.) to be
parsed is not provided in the <span class="code">yecc</span> module. The scanner
serves as a kind of lexicon look-up routine. It is possible to
write a grammar that uses only character tokens as terminal
symbols, thereby eliminating the need for a scanner, but this
would make the parser larger and slower.</p>
<p>The user should implement a scanner that segments the input
text, and turns it into one or more lists of tokens. Each token
should be a tuple containing information about syntactic
category, position in the text (e.g. line number), and the
actual terminal symbol found in the text: <span class="code">{Category, LineNumber, Symbol}</span>.</p>
<p>If a terminal symbol is the only member of a category, and the
symbol name is identical to the category name, the token format
may be <span class="code">{Symbol, LineNumber}</span>.</p>
<p>A list of tokens produced by the scanner should end with a
special <span class="code">end_of_input</span> tuple which the parser is looking
for. The format of this tuple should be <span class="code">{Endsymbol, LastLineNumber}</span>, where <span class="code">Endsymbol</span> is an identifier
that is distinguished from all the terminal and non-terminal
categories of the syntax rules. The <span class="code">Endsymbol</span> may be
declared in the grammar file (see below).</p>
<p>The simplest case is to segment the input string into a list of
identifiers (atoms) and use those atoms both as categories and
values of the tokens. For example, the input string <span class="code">aaa bbb 777, X</span> may be scanned (tokenized) as:</p>
<div class="example"><pre>
[{aaa, 1}, {bbb, 1}, {777, 1}, {',' , 1}, {'X', 1},
{'$end', 1}]. </pre></div>
<p>This assumes that this is the first line of the input text, and
that <span class="code">'$end'</span> is the distinguished <span class="code">end_of_input</span>
symbol.</p>
<p>The Erlang scanner in the <span class="code">io</span> module can be used as a
starting point when writing a new scanner. Study
<span class="code">yeccscan.erl</span> in order to see how a filter can be added on
top of <span class="code">io:scan_erl_form/3</span> to provide a scanner for
Yecc that tokenizes grammar files before parsing them
with the Yecc parser. A more general approach to scanner
implementation is to use a scanner generator. A scanner
generator in Erlang called <span class="code">leex</span> is under development.</p>
</div>
<h3><a name="id60976">Grammar Definition Format</a></h3>
<div class="REFBODY">
<p>Erlang style <span class="code">comments</span>, starting with a <span class="code">'%'</span>, are
allowed in grammar files.</p>
<p>Each <span class="code">declaration</span> or <span class="code">rule</span> ends with a dot (the
character <span class="code">'.'</span>).</p>
<p>The grammar starts with an optional <span class="code">header</span> section. The
header is put first in the generated file, before the module
declaration. The purpose of the header is to provide a means to
make the documentation generated by <span class="code">EDoc</span> look nicer. Each
header line should be enclosed in double quotes, and newlines
will be inserted between the lines. For example:</p>
<div class="example"><pre>
Header "%% Copyright (C)"
"%% @private"
"%% @Author John"</pre></div>
<p>Next comes a declaration of the <span class="code">nonterminal categories</span>
to be used in the rules. For example:</p>
<div class="example"><pre>
Nonterminals sentence nounphrase verbphrase. </pre></div>
<p>A non-terminal category can be used at the left hand side (=
<span class="code">lhs</span>, or <span class="code">head</span>) of a grammar rule. It can also
appear at the right hand side of rules.</p>
<p>Next comes a declaration of the <span class="code">terminal categories</span>,
which are the categories of tokens produced by the scanner. For
example:</p>
<div class="example"><pre>
Terminals article adjective noun verb. </pre></div>
<p>Terminal categories may only appear in the right hand sides (=
<span class="code">rhs</span>) of grammar rules.</p>
<p>Next comes a declaration of the <span class="code">rootsymbol</span>, or start
category of the grammar. For example:</p>
<div class="example"><pre>
Rootsymbol sentence. </pre></div>
<p>This symbol should appear in the lhs of at least one grammar
rule. This is the most general syntactic category which the
parser ultimately will parse every input string into.</p>
<p>After the rootsymbol declaration comes an optional declaration
of the <span class="code">end_of_input</span> symbol that your scanner is expected
to use. For example:</p>
<div class="example"><pre>
Endsymbol '$end'. </pre></div>
<p>Next comes one or more declarations of <span class="code">operator precedences</span>, if needed. These are used to resolve
shift/reduce conflicts (see <span class="code">yacc</span> documentation).</p>
<p>Examples of operator declarations:</p>
<div class="example"><pre>
Right 100 '='.
Nonassoc 200 '==' '=/='.
Left 300 '+'.
Left 400 '*'.
Unary 500 '-'. </pre></div>
<p>These declarations mean that <span class="code">'='</span> is defined as a
<span class="code">right associative binary</span> operator with precedence 100,
<span class="code">'=='</span> and <span class="code">'=/='</span> are operators with <span class="code">no associativity</span>, <span class="code">'+'</span> and <span class="code">'*'</span> are <span class="code">left associative binary</span> operators, where <span class="code">'*'</span> takes
precedence over <span class="code">'+'</span> (the normal case), and <span class="code">'-'</span> is
a <span class="code">unary</span> operator of higher precedence than <span class="code">'*'</span>.
The fact that '==' has no associativity means that an expression
like <span class="code">a == b == c</span> is considered a syntax error.</p>
<p>Certain rules are assigned precedence: each rule gets its
precedence from the last terminal symbol mentioned in the right
hand side of the rule. It is also possible to declare precedence
for non-terminals, "one level up". This is practical when an
operator is overloaded (see also example 3 below).</p>
<p>Next come the <span class="code">grammar rules</span>. Each rule has the general
form</p>
<div class="example"><pre>
Left_hand_side -> Right_hand_side : Associated_code. </pre></div>
<p>The left hand side is a non-terminal category. The right hand
side is a sequence of one or more non-terminal or terminal
symbols with spaces between. The associated code is a sequence
of zero or more Erlang expressions (with commas <span class="code">','</span> as
separators). If the associated code is empty, the separating
colon <span class="code">':'</span> is also omitted. A final dot marks the end of
the rule.</p>
<p>Symbols such as <span class="code">'{'</span>, <span class="code">'.'</span>, etc., have to be
enclosed in single quotes when used as terminal or non-terminal
symbols in grammar rules. The use of the symbols
<span class="code">'$empty'</span>, <span class="code">'$end'</span>, and <span class="code">'$undefined'</span> should
be avoided.</p>
<p>The last part of the grammar file is an optional section with
Erlang code (= function definitions) which is included 'as is'
in the resulting parser file. This section must start with the
pseudo declaration, or key words</p>
<div class="example"><pre>
Erlang code. </pre></div>
<p>No syntax rule definitions or other declarations may follow
this section. To avoid conflicts with internal variables, do not
use variable names beginning with two underscore characters
('__') in the Erlang code in this section, or in the code
associated with the individual syntax rules.</p>
<p>The optional <span class="code">expect</span> declaration can be placed anywhere
before the last optional section with Erlang code. It is used
for suppressing the warning about conflicts that is ordinarily
given if the grammar is ambiguous. An example:</p>
<div class="example"><pre>
Expect 2. </pre></div>
<p>The warning is given if the number of shift/reduce conflicts
differs from 2, or if there are reduce/reduce conflicts.
</p>
</div>
<h3><a name="id62438">Examples</a></h3>
<div class="REFBODY">
<p>A grammar to parse list expressions (with empty associated
code):</p>
<div class="example"><pre>
Nonterminals list elements element.
Terminals atom '(' ')'.
Rootsymbol list.
list -> '(' ')'.
list -> '(' elements ')'.
elements -> element.
elements -> element elements.
element -> atom.
element -> list. </pre></div>
<p>This grammar can be used to generate a parser which parses list
expressions, such as <span class="code">(), (a), (peter charles), (a (b c) d (())), ...</span> provided that your scanner tokenizes, for
example, the input <span class="code">(peter charles)</span> as follows:</p>
<div class="example"><pre>
[{'(', 1} , {atom, 1, peter}, {atom, 1, charles}, {')', 1},
{'$end', 1}] </pre></div>
<p>When a grammar rule is used by the parser to parse (part of)
the input string as a grammatical phrase, the associated code is
evaluated, and the value of the last expression becomes the
value of the parsed phrase. This value may be used by the parser
later to build structures that are values of higher phrases of
which the current phrase is a part. The values initially
associated with terminal category phrases, i.e. input tokens,
are the token tuples themselves.</p>
<p>Below is an example of the grammar above with structure
building code added:</p>
<div class="example"><pre>
list -> '(' ')' : nil.
list -> '(' elements ')' : '$2'.
elements -> element : {cons, '$1', nil}.
elements -> element elements : {cons, '$1', '$2'}.
element -> atom : '$1'.
element -> list : '$1'. </pre></div>
<p>With this code added to the grammar rules, the parser produces
the following value (structure) when parsing the input string
<span class="code">(a b c).</span>. This still assumes that this was the first
input line that the scanner tokenized:</p>
<div class="example"><pre>
{cons, {atom, 1, a,} {cons, {atom, 1, b},
{cons, {atom, 1, c}, nil}}} </pre></div>
<p>The associated code contains <span class="code">pseudo variables</span> <span class="code">'$1'</span>, <span class="code">'$2'</span>, <span class="code">'$3'</span>, etc. which refer to (are
bound to) the values associated previously by the parser with
the symbols of the right hand side of the rule. When these
symbols are terminal categories, the values are token tuples of
the input string (see above).</p>
<p>The associated code may not only be used to build structures
associated with phrases, but may also be used for syntactic and
semantic tests, printout actions (for example for tracing), etc.
during the parsing process. Since tokens contain positional
(line number) information, it is possible to produce error
messages which contain line numbers. If there is no associated
code after the right hand side of the rule, the value
<span class="code">'$undefined'</span> is associated with the phrase.</p>
<p>The right hand side of a grammar rule may be empty. This is
indicated by using the special symbol <span class="code">'$empty'</span> as rhs.
Then the list grammar above may be simplified to:</p>
<div class="example"><pre>
list -> '(' elements ')' : '$2'.
elements -> element elements : {cons, '$1', '$2'}.
elements -> '$empty' : nil.
element -> atom : '$1'.
element -> list : '$1'. </pre></div>
</div>
<h3><a name="id62048">Generating a Parser</a></h3>
<div class="REFBODY">
<p>To call the parser generator, use the following command:</p>
<div class="example"><pre>
yecc:file(Grammarfile). </pre></div>
<p>An error message from Yecc will be shown if the grammar
is not of the LALR type (for example too ambiguous).
Shift/reduce conflicts are resolved in favor of shifting if
there are no operator precedence declarations. Refer to the
<span class="code">yacc</span> documentation on the use of operator precedence.</p>
<p>The output file contains Erlang source code for a parser module
with module name equal to the <span class="code">Parserfile</span> parameter. After
compilation, the parser can be called as follows (the module
name is assumed to be <span class="code">myparser</span>):</p>
<div class="example"><pre>
myparser:parse(myscanner:scan(Inport)) </pre></div>
<p>The call format may be different if a customized prologue file
has been included when generating the parser instead of the
default file <span class="code">lib/parsetools/include/yeccpre.hrl</span>.</p>
<p>With the standard prologue, this call will return either
<span class="code">{ok, Result}</span>, where <span class="code">Result</span> is a structure that the
Erlang code of the grammar file has built, or <span class="code">{error, {Line_number, Module, Message}}</span> if there was a syntax error
in the input.</p>
<p><span class="code">Message</span> is something which may be converted into a
string by calling <span class="code">Module:format_error(Message)</span>
and printed with <span class="code">io:format/3</span>.</p>
<div class="note">
<div class="label">Note</div>
<div class="content"><p>
<p>By default, the parser that was generated will not print out
error messages to the screen. The user will have to do this
either by printing the returned error messages, or by
inserting tests and print instructions in the Erlang code
associated with the syntax rules of the grammar file.</p>
</p></div>
</div>
<p>It is also possible to make the parser ask for more input
tokens when needed if the following call format is used:</p>
<div class="example"><pre>
myparser:parse_and_scan({Function, Args})
myparser:parse_and_scan({Mod, Tokenizer, Args}) </pre></div>
<p>The tokenizer <span class="code">Function</span> is either a fun or a tuple
<span class="code">{Mod, Tokenizer}</span>. The call <span class="code">apply(Function, Args)</span>
or <span class="code">apply({Mod, Tokenizer}, Args)</span> is executed whenever a
new token is needed. This, for example, makes it possible to
parse from a file, token by token.</p>
<p>The tokenizer used above has to be implemented so as to return
one of the following:</p>
<div class="example"><pre>
{ok, Tokens, Endline}
{eof, Endline}
{error, Error_description, Endline} </pre></div>
<p>This conforms to the format used by the scanner in the Erlang
<span class="code">io</span> library module.</p>
<p>If <span class="code">{eof, Endline}</span> is returned immediately, the call to
<span class="code">parse_and_scan/1</span> returns <span class="code">{ok, eof}</span>. If <span class="code">{eof, Endline}</span> is returned before the parser expects end of input,
<span class="code">parse_and_scan/1</span> will, of course, return an error message
(see above). Otherwise <span class="code">{ok, Result}</span> is returned.</p>
</div>
<h3><a name="id61037">More Examples</a></h3>
<div class="REFBODY">
<p>1. A grammar for parsing infix arithmetic expressions into
prefix notation, without operator precedence:</p>
<div class="example"><pre>
Nonterminals E T F.
Terminals '+' '*' '(' ')' number.
Rootsymbol E.
E -> E '+' T: ['$2', '$1', '$3'].
E -> T : '$1'.
T -> T '*' F: ['$2', '$1', '$3'].
T -> F : '$1'.
F -> '(' E ')' : '$2'.
F -> number : '$1'. </pre></div>
<p>2. The same with operator precedence becomes simpler:</p>
<div class="example"><pre>
Nonterminals E.
Terminals '+' '*' '(' ')' number.
Rootsymbol E.
Left 100 '+'.
Left 200 '*'.
E -> E '+' E : ['$2', '$1', '$3'].
E -> E '*' E : ['$2', '$1', '$3'].
E -> '(' E ')' : '$2'.
E -> number : '$1'. </pre></div>
<p>3. An overloaded minus operator:</p>
<div class="example"><pre>
Nonterminals E uminus.
Terminals '*' '-' number.
Rootsymbol E.
Left 100 '-'.
Left 200 '*'.
Unary 300 uminus.
E -> E '-' E.
E -> E '*' E.
E -> uminus.
E -> number.
uminus -> '-' E. </pre></div>
<p>4. The Yecc grammar that is used for parsing grammar
files, including itself:</p>
<div class="example"><pre>
Nonterminals
grammar declaration rule head symbol symbols attached_code
token tokens.
Terminals
atom float integer reserved_symbol reserved_word string char var
'->' ':' dot.
Rootsymbol grammar.
Endsymbol '$end'.
grammar -> declaration : '$1'.
grammar -> rule : '$1'.
declaration -> symbol symbols dot: {'$1', '$2'}.
rule -> head '->' symbols attached_code dot: {rule, ['$1' | '$3'],
'$4'}.
head -> symbol : '$1'.
symbols -> symbol : ['$1'].
symbols -> symbol symbols : ['$1' | '$2'].
attached_code -> ':' tokens : {erlang_code, '$2'}.
attached_code -> '$empty' : {erlang_code,
[{atom, 0, '$undefined'}]}.
tokens -> token : ['$1'].
tokens -> token tokens : ['$1' | '$2'].
symbol -> var : value_of('$1').
symbol -> atom : value_of('$1').
symbol -> integer : value_of('$1').
symbol -> reserved_word : value_of('$1').
token -> var : '$1'.
token -> atom : '$1'.
token -> float : '$1'.
token -> integer : '$1'.
token -> string : '$1'.
token -> char : '$1'.
token -> reserved_symbol : {value_of('$1'), line_of('$1')}.
token -> reserved_word : {value_of('$1'), line_of('$1')}.
token -> '->' : {'->', line_of('$1')}.
token -> ':' : {':', line_of('$1')}.
Erlang code.
value_of(Token) ->
element(3, Token).
line_of(Token) ->
element(2, Token). </pre></div>
<div class="note">
<div class="label">Note</div>
<div class="content"><p>
<p>The symbols <span class="code">'->'</span>, and <span class="code">':'</span> have to be treated in
a special way, as they are meta symbols of the grammar
notation, as well as terminal symbols of the Yecc
grammar.</p>
</p></div>
</div>
<p>5. The file <span class="code">erl_parse.yrl</span> in the <span class="code">lib/stdlib/src</span>
directory contains the grammar for Erlang.</p>
<div class="note">
<div class="label">Note</div>
<div class="content"><p>
<p>Syntactic tests are used in the code associated with some
rules, and an error is thrown (and caught by the generated
parser to produce an error message) when a test fails. The
same effect can be achieved with a call to
<span class="code">return_error(Error_line, Message_string)</span>, which is
defined in the <span class="code">yeccpre.hrl</span> default header file.</p>
</p></div>
</div>
</div>
<h3><a name="id61730">Files</a></h3>
<div class="REFBODY">
<div class="example"><pre>
lib/parsetools/include/yeccpre.hrl </pre></div>
</div>
<h3><a name="id61744">See Also</a></h3>
<div class="REFBODY">
<p>Aho & Johnson: 'LR Parsing', ACM Computing Surveys, vol. 6:2, 1974.</p>
</div>
</div>
<div class="footer">
<hr>
<p>Copyright © 1997-2012 Ericsson AB. All Rights Reserved.</p>
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