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><DIV
CLASS="CHAPTER"
><H1
><A
NAME="OPTIONS"
></A
>Chapter 3. Changing the behaviour of Hugs</H1
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="OPTIONS-LIST"
>3.1. Hugs options</A
></H1
><P
>The behaviour of Hugs may be modified by options.
These are initially set to default values,
and then read from the following sources in order:</P
><P
></P
><OL
TYPE="1"
><LI
><P
>(Windows only) the registry under the <TT
CLASS="LITERAL"
>HKEY_LOCAL_MACHINE</TT
> key.</P
></LI
><LI
><P
>(Windows only) the registry under the <TT
CLASS="LITERAL"
>HKEY_CURRENT_USER</TT
> key.
This is where options set using the <B
CLASS="COMMAND"
>:set</B
> command are saved
(see <A
HREF="using-hugs.html#BASIC-COMMANDS"
>Section 2.1</A
>).
This step is skipped if the environment variable
<CODE
CLASS="ENVAR"
>IGNORE_USER_REGISTRY</CODE
> is set,
providing an emergency workaround if the settings are invalid.</P
></LI
><LI
><P
>(Hugs for Windows only)
the GUI settings.</P
></LI
><LI
><P
>(Mac OS prior to Mac OS X)
the preferences file <SPAN
CLASS="QUOTE"
>"<TT
CLASS="FILENAME"
>Hugs Preferences</TT
>"</SPAN
>.</P
></LI
><LI
><P
>The environment variable <CODE
CLASS="ENVAR"
>HUGSFLAGS</CODE
>, if set.</P
></LI
><LI
><P
>The Hugs command line.</P
></LI
></OL
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Parsing of option strings: </B
>In each of the above cases except command line arguments,
the string should consist of zero or more options separated by whitespace.
To include a literal space in an option, either precede it with a backslash
or wrap the whole option in double quotes.
Double quoted options may also contain the escape sequences
<SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>\ </TT
>"</SPAN
>,
<SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>\"</TT
>"</SPAN
> and
<SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>\\</TT
>"</SPAN
>.
Other backslashes are interpreted literally.</P
></BLOCKQUOTE
></DIV
><P
>Most options can be changed within Hugs using the <B
CLASS="COMMAND"
>:set</B
>
command (see <A
HREF="using-hugs.html#BASIC-COMMANDS"
>Section 2.1</A
>).</P
><P
>Hugs takes two kinds of options:
<P
></P
><UL
><LI
><P
>Toggles like <CODE
CLASS="OPTION"
>+t</CODE
> or <CODE
CLASS="OPTION"
>-t</CODE
>,
which start with <TT
CLASS="LITERAL"
>+</TT
> or <TT
CLASS="LITERAL"
>-</TT
>
to turn them on or off, respectively.</P
></LI
><LI
><P
>Options that set a parameter value, like
<CODE
CLASS="OPTION"
>-P</CODE
><TT
CLASS="REPLACEABLE"
><I
>str</I
></TT
>,
in which <TT
CLASS="LITERAL"
>-</TT
> could be replaced by <TT
CLASS="LITERAL"
>+</TT
>,
the choice making no difference.</P
></LI
></UL
></P
><P
>A special option is provided to allow several options in a single argument:</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="OPTION-SPLIT-OPTIONS"
></A
><CODE
CLASS="OPTION"
>-X</CODE
><TT
CLASS="REPLACEABLE"
><I
>str</I
></TT
></DT
><DD
><P
>The string <TT
CLASS="REPLACEABLE"
><I
>str</I
></TT
> is interpreted as an option string,
in the manner described above.
This is useful, for example, in some uses of <B
CLASS="COMMAND"
>runhugs</B
>
(see <A
HREF="others.html#RUNHUGS"
>Section 4.1</A
>).</P
></DD
></DL
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="OPTIONS-LANGUAGE"
>3.1.1. Language options</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="OPTION-98"
></A
><CODE
CLASS="OPTION"
>+98</CODE
></DT
><DD
><P
>Accept only Haskell 98.
This is on by default, and cannot be changed within Hugs.
Turning this off enables several special Hugs extensions,
which are described in <A
HREF="hugs-ghc.html"
>Chapter 6</A
>, <A
HREF="hugs-only.html"
>Chapter 7</A
>
and the <I
CLASS="CITETITLE"
>Hugs 98 User Manual</I
>.</P
></DD
><DT
><A
NAME="OPTION-CONSTRAINT-CUTOFF"
></A
><CODE
CLASS="OPTION"
>-c</CODE
><TT
CLASS="REPLACEABLE"
><I
>num</I
></TT
></DT
><DD
><P
>Set the constraint cutoff limit in the type checker to
<TT
CLASS="REPLACEABLE"
><I
>num</I
></TT
> (default: 40).
See <A
HREF="class-extensions.html#FLEXIBLE-INSTANCES"
>Section 6.2.2</A
>.</P
></DD
><DT
><A
NAME="OPTION-OVERLAPPING"
></A
><CODE
CLASS="OPTION"
>+o</CODE
></DT
><DD
><P
>Allow certain overlapping instances (a Hugs extension; default: off).
See <A
HREF="class-extensions.html#OVERLAPPING-INSTANCES"
>Section 6.2.3</A
> for details.</P
></DD
><DT
><A
NAME="OPTION-UNSAFE-OVERLAPPING"
></A
><CODE
CLASS="OPTION"
>+O</CODE
></DT
><DD
><P
>Allow certain overlapping instances (a Hugs extension; default: off).
These are the same overlapping instances as accepted by <CODE
CLASS="OPTION"
>+o</CODE
>,
but <CODE
CLASS="OPTION"
>+O</CODE
> also accepts ambiguous uses of these instances,
even though this is unsafe (see <A
HREF="class-extensions.html#OVERLAPPING-INSTANCES"
>Section 6.2.3</A
>).</P
></DD
><DT
><A
NAME="OPTION-HERE-DOCUMENTS"
></A
><CODE
CLASS="OPTION"
>+H</CODE
></DT
><DD
><P
>Support <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>here documents</I
></SPAN
>
(named after similar things in Unix shells),
another way of writing large string literals
(see <A
HREF="here-documents.html"
>Section 7.3</A
>).
This extension is turned off by default.</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="OPTIONS-LOADING"
>3.1.2. Module loading options</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="OPTION-LITERATE"
></A
><CODE
CLASS="OPTION"
>+l</CODE
></DT
><DD
><P
>Literate scripts as default (default: off).</P
><P
>Files with names ending in <SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>.hs</TT
>"</SPAN
>
are always treated as ordinary Haskell,
while those ending in <SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>.lhs</TT
>"</SPAN
>
are always treated as literate scripts.
This option determines whether other files are literate scripts or not.
(See <A
HREF="others.html#RUNHUGS"
>Section 4.1</A
> for an example.)</P
></DD
><DT
><A
NAME="OPTION-DOTS"
></A
><CODE
CLASS="OPTION"
>+.</CODE
></DT
><DD
><P
>Print dots to show progress while loading modules (default: off).</P
></DD
><DT
><A
NAME="OPTION-QUIET"
></A
><CODE
CLASS="OPTION"
>+q</CODE
></DT
><DD
><P
>Print nothing to show progress while loading modules (default: on).</P
></DD
><DT
><A
NAME="OPTION-SHOW-LOADED"
></A
><CODE
CLASS="OPTION"
>+w</CODE
></DT
><DD
><P
>Always show which files were loaded (default: off).</P
></DD
><DT
><A
NAME="OPTION-PREPROCESSOR"
></A
><CODE
CLASS="OPTION"
>-F</CODE
><TT
CLASS="REPLACEABLE"
><I
>cmd</I
></TT
></DT
><DD
><P
>Preprocess source files before loading.
Instead of reading a source file directly, Hugs will
execute <TT
CLASS="REPLACEABLE"
><I
>cmd</I
></TT
> with the source file name as argument,
and read the standard output.</P
><P
>This is handy for preprocessing source files with the C preprocessor,
or some preprocessor implementing a language extension.
However it is slower.
In particular (because of the way Hugs handles imports),
the preprocessor will be run twice on files that import modules
that have not been loaded yet.</P
></DD
><DT
><A
NAME="OPTION-SEARCH-PATH"
></A
><CODE
CLASS="OPTION"
>-P</CODE
><TT
CLASS="REPLACEABLE"
><I
>str</I
></TT
></DT
><DD
><P
>Set search path for source files to <TT
CLASS="REPLACEABLE"
><I
>str</I
></TT
>,
which should be a list of directories separated by colons
(semicolons on Windows, DOS or Macs).
A null entry in this list will be replaced by the previous search path;
a null <TT
CLASS="REPLACEABLE"
><I
>str</I
></TT
> means the default path.
Any occurrences of <TT
CLASS="LITERAL"
>{Hugs}</TT
> in this string will be replaced
by the Hugs library directory.
Similarly, <TT
CLASS="LITERAL"
>{Home}</TT
> is expanded to your home directory.
An entry of the form
<SPAN
CLASS="QUOTE"
>"<TT
CLASS="REPLACEABLE"
><I
>directory</I
></TT
><TT
CLASS="LITERAL"
>/*</TT
>"</SPAN
>
means all the immediate subdirectories of <TT
CLASS="REPLACEABLE"
><I
>directory</I
></TT
>.
(See <A
HREF="packages.html"
>Section 3.4</A
> for a way of using this.)
On Unix, the default value is
<PRE
CLASS="SCREEN"
>.:{Home}/lib/hugs/packages/*:/usr/local/lib/hugs/packages/*:{Hugs}/packages/*</PRE
></P
><P
>The interpreter won't let you change the search path if that would
prevent it from reading the <TT
CLASS="LITERAL"
>Prelude</TT
>.
If an entry occurs more than once in the search path, the extra copies
will be removed, as searching them would be a waste of time.</P
></DD
><DT
><A
NAME="OPTION-SUFFIXES"
></A
><CODE
CLASS="OPTION"
>-S</CODE
><TT
CLASS="REPLACEABLE"
><I
>str</I
></TT
></DT
><DD
><P
>Set list of filename suffixes.</P
><P
>Normally, when you import a module <TT
CLASS="REPLACEABLE"
><I
>M</I
></TT
>,
Hugs looks for files <TT
CLASS="FILENAME"
><TT
CLASS="REPLACEABLE"
><I
>M</I
></TT
>.hs</TT
>
and <TT
CLASS="FILENAME"
><TT
CLASS="REPLACEABLE"
><I
>M</I
></TT
>.lhs</TT
>
in each directory in you search path.
With this option, you can change this list,
in a similar way to the <CODE
CLASS="OPTION"
>-P</CODE
> option for the search path.
By default, the suffix list is <SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>.hs:.lhs</TT
>"</SPAN
>,
which gives the behaviour just described.
(NB: the <SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>:</TT
>"</SPAN
> is the Unix separator.
Windows or Macs use <SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>;</TT
>"</SPAN
> instead.)
If you use <TT
CLASS="LITERAL"
>-S:.xhs</TT
> then the suffix
list becomes <SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>.hs:.lhs:.xhs</TT
>"</SPAN
>,
so Hugs will look for
<TT
CLASS="FILENAME"
><TT
CLASS="REPLACEABLE"
><I
>M</I
></TT
>.hs</TT
>,
<TT
CLASS="FILENAME"
><TT
CLASS="REPLACEABLE"
><I
>M</I
></TT
>.lhs</TT
>
and <TT
CLASS="FILENAME"
><TT
CLASS="REPLACEABLE"
><I
>M</I
></TT
>.xhs</TT
>.</P
><P
>A null entry in this list will be replaced by the previous suffix list;
a null <TT
CLASS="REPLACEABLE"
><I
>str</I
></TT
> means the default list.</P
><P
>The interpreter won't let you change the suffix list if that would
prevent it from reading the <TT
CLASS="LITERAL"
>Prelude</TT
>,
i.e. you must include <SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>.hs</TT
>"</SPAN
>.
Note also that the interpreter knows that files ending in
<SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>.lhs</TT
>"</SPAN
> are literate scripts;
no other suffix is treated that way.</P
><P
>This option can be useful in conjunction with the preprocessor option
(<CODE
CLASS="OPTION"
>-F</CODE
>).
The preprocessor can examine the filename to decide what to do with the file.</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="OPTIONS-EDITOR"
>3.1.3. Specifying a source file editor</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="OPTION-EDITOR"
></A
><CODE
CLASS="OPTION"
>-E</CODE
><TT
CLASS="REPLACEABLE"
><I
>str</I
></TT
></DT
><DD
><P
>Specify the editor used by the <TT
CLASS="LITERAL"
>:edit</TT
> command
(see <A
HREF="module-commands.html"
>Section 2.2</A
>).
For example, to have Hugs invoke <TT
CLASS="LITERAL"
>vi</TT
> to edit your files,
use
<PRE
CLASS="SCREEN"
>-Evi</PRE
>
The argument string is actually a template string that gets expanded
by Hugs, via the following rules:
<P
></P
><UL
><LI
><P
>all occurrences of <TT
CLASS="LITERAL"
>%d</TT
> are replaced by the line number
of where the last error occurred (if any).
Please consult your editor's documentation for ways of specifying the
line number.</P
></LI
><LI
><P
>all occurrences of <TT
CLASS="LITERAL"
>%s</TT
> are replaced by the name of the file.
If an occurrence of <TT
CLASS="LITERAL"
>%s</TT
> is both preceded by and followed
by space, the filename is enclosed in double-quotes.</P
></LI
><LI
><P
>all occurrences of <TT
CLASS="LITERAL"
>%f</TT
> are replaced by the absolute filename
(provided your platform lets you find the absolute path to a file.)
Most of the time, <TT
CLASS="LITERAL"
>%s</TT
> will be just fine, but in case your
editor doesn't handle relative filenames correctly, try
using <TT
CLASS="LITERAL"
>%f</TT
>.</P
></LI
><LI
><P
>all occurrences of <TT
CLASS="LITERAL"
>%%</TT
> are replaced by <TT
CLASS="LITERAL"
>%</TT
>.</P
></LI
><LI
><P
>(Win32 only): if the <CODE
CLASS="OPTION"
>-E</CODE
> string is prefixed with
the character <SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>&</TT
>"</SPAN
>,
then the invocation is asynchronous, that is, the editor process
is created, but Hugs won't wait for the editor to terminate.</P
></LI
><LI
><P
>(Win32 only): if the <CODE
CLASS="OPTION"
>-E</CODE
> string is prefixed with
the character <SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>!</TT
>"</SPAN
>,
then the invocation will be asynchronous and use the underlying
command processor/shell to execute the command.</P
></LI
></UL
>
If neither <TT
CLASS="LITERAL"
>%s</TT
> nor <TT
CLASS="LITERAL"
>%f</TT
> occurs within
the <CODE
CLASS="OPTION"
>-E</CODE
> string,
then the filename is appended before invoking the editor.</P
><P
>Here are some example editor configurations:
<P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
>TextPad</DT
><DD
><PRE
CLASS="SCREEN"
>-E"c:/Program Files/TextPad 4/textpad \"%s\"(%d)"</PRE
></DD
><DT
>vi and clones</DT
><DD
><PRE
CLASS="SCREEN"
>-E"vi +%d %s"</PRE
></DD
><DT
>gnuclient (for use with (X)Emacs)</DT
><DD
><PRE
CLASS="SCREEN"
>-E"gnuclient +%d %s"</PRE
></DD
></DL
></DIV
> </P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="OPTIONS-EVALUATION"
>3.1.4. Evaluation and printing options</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="OPTION-PROMPT"
></A
><CODE
CLASS="OPTION"
>-p</CODE
><TT
CLASS="REPLACEABLE"
><I
>str</I
></TT
></DT
><DD
><P
>Set prompt string to <TT
CLASS="REPLACEABLE"
><I
>str</I
></TT
>
(default: <SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>%s> </TT
>"</SPAN
>).
Any <TT
CLASS="LITERAL"
>%s</TT
> in <TT
CLASS="REPLACEABLE"
><I
>str</I
></TT
>
will be replaced by the current module name.</P
></DD
><DT
><A
NAME="OPTION-REPEAT-STRING"
></A
><CODE
CLASS="OPTION"
>-r</CODE
><TT
CLASS="REPLACEABLE"
><I
>str</I
></TT
></DT
><DD
><P
>Set the string denoting the last expression to <TT
CLASS="REPLACEABLE"
><I
>str</I
></TT
>
(default: <SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>$$</TT
>"</SPAN
>).</P
></DD
><DT
><A
NAME="OPTION-FULL-KIND-ERRORS"
></A
><CODE
CLASS="OPTION"
>+k</CODE
></DT
><DD
><P
>Show kind errors in full (default: off).</P
><P
>In Haskell, each type expression has a <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>kind</I
></SPAN
>.
These kinds do not appear in the source language,
but they are checked for consistency.
By default, Hugs reports such errors as an <TT
CLASS="LITERAL"
>Illegal type</TT
>.
For example, the declaration
<PRE
CLASS="PROGRAMLISTING"
>instance Monad Int</PRE
>
gives rise to the error
<PRE
CLASS="SCREEN"
>ERROR "Test.hs":4 - Illegal type in class constraint</PRE
>
However if <CODE
CLASS="OPTION"
>+k</CODE
> is given,
the error message is identified as a <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Kind error</I
></SPAN
>,
and is expanded to include the conflicting kinds:
<PRE
CLASS="SCREEN"
>ERROR "Test.hs":4 - Kind error in class constraint
*** constructor : Int
*** kind : *
*** does not match : * -> *</PRE
>
Also, when <CODE
CLASS="OPTION"
>+k</CODE
> is given,
the output of the <B
CLASS="COMMAND"
>:info</B
> will include kind information
for classes and type constructors:
<PRE
CLASS="SCREEN"
>Hugs> :info Monad
-- constructor class with arity * -> *
...
Hugs> :info Int
-- type constructor with kind *</PRE
></P
></DD
><DT
><A
NAME="OPTION-DEFAULT-TYPES"
></A
><CODE
CLASS="OPTION"
>+T</CODE
></DT
><DD
><P
>Apply defaulting rules to types before printing (default: off).</P
><P
>When printing out types, the interpreter will normally not try to
simplify types by applying defaulting rules, e.g.,
<PRE
CLASS="SCREEN"
>Hugs> :t 1
1 :: Num a => a
Hugs></PRE
>
With the <CODE
CLASS="OPTION"
>+T</CODE
> option,
the interpreter attempts to <SPAN
CLASS="QUOTE"
>"default"</SPAN
> types first,
using the same rules as for expressions (see <A
HREF="using-hugs.html#BASIC-COMMANDS"
>Section 2.1</A
>):
<PRE
CLASS="SCREEN"
>Hugs> :set +T
Hugs> :t 1
1 :: Integer
Hugs></PRE
></P
></DD
><DT
><A
NAME="OPTION-QUALIFY-NAMES"
></A
><CODE
CLASS="OPTION"
>+Q</CODE
></DT
><DD
><P
>Qualify names when printing (default: off).</P
><P
>By default, the interpreter will print out names without qualifying
them with their defining modules.
Most of the time that's exactly what you want,
but can become confusing if you re-define types and functions;
the error messages not pinning down what entity it is referring to.
To have the interpreter qualify the names, use <CODE
CLASS="OPTION"
>+Q</CODE
>.
Typically, you use <CODE
CLASS="OPTION"
>+Q</CODE
> when resolving errors,
but turn it back off again afterwards.</P
></DD
><DT
><A
NAME="OPTION-PRINT-TYPE"
></A
><CODE
CLASS="OPTION"
>+t</CODE
></DT
><DD
><P
>Print the type of each expression evaluated (default: off).</P
><P
>Normally Hugs merely prints the value of each expression evaluated:
<PRE
CLASS="SCREEN"
>Hugs> 1+2
3</PRE
>
With the <CODE
CLASS="OPTION"
>+t</CODE
> option, it also adds the type of the expression:
<PRE
CLASS="SCREEN"
>Hugs> :set +t
Hugs> 1+2
3 :: Integer</PRE
>
Note that defaulting has been applied to the type of the expression in order
to evaluate it, so the type differs from that reported by the
<B
CLASS="COMMAND"
>:type</B
> command (assuming that the
<A
HREF="options.html#OPTION-DEFAULT-TYPES"
><CODE
CLASS="OPTION"
>+T</CODE
></A
> option
is not used):
<PRE
CLASS="SCREEN"
>Hugs> :type 1+2
1 + 2 :: Num a => a</PRE
></P
></DD
><DT
><A
NAME="OPTION-USE-SHOW"
></A
><CODE
CLASS="OPTION"
>+u</CODE
></DT
><DD
><P
>Use <CODE
CLASS="FUNCTION"
>show</CODE
> to display results (default: on).</P
><P
>By default, the values of expressions typed at the prompt are printed using
the <CODE
CLASS="FUNCTION"
>show</CODE
> member of the <TT
CLASS="LITERAL"
>Show</TT
> class:
<PRE
CLASS="SCREEN"
>Hugs> [Just (2+3), Nothing]
[Just 5,Nothing]</PRE
>
You can define this function as desired for any new datatype.
If the type of the expression is not an instance of the
<TT
CLASS="LITERAL"
>Show</TT
> class, an error results:
<PRE
CLASS="SCREEN"
>Hugs> id
ERROR - Cannot find "show" function for:
*** Expression : id
*** Of type : a -> a</PRE
>
With the <CODE
CLASS="OPTION"
>-u</CODE
> option, a built-in printer is used instead,
and this works for any type:
<PRE
CLASS="SCREEN"
>Hugs> :set -u
Hugs> id
id
Hugs> \x -> x
v1497
Hugs> [Just (2+3), Nothing]
[Just 5,Nothing]</PRE
>
Another feature of the built-in printer is its treatment of failures
(or exceptions).
Normally, an exception causes immediate failure of the expression:
<PRE
CLASS="SCREEN"
>Hugs> :set +u
Hugs> 1 + 2/0
Program error: divide by zero
Hugs> [1, 2 + error "foo", 3]
[1,
Program error: foo</PRE
>
However the built-in printer prints the whole value, with embedded exceptions:
<PRE
CLASS="SCREEN"
>Hugs> :set -u
Hugs> [1, 2 + error "foo", 3]
[1,{error "foo"},3]</PRE
>
Sometimes a component could produce one of two or more exceptions,
but the built-in printer shows only one of them:
<PRE
CLASS="SCREEN"
>Hugs> 1 + error "foo" + error "bar"
{error "foo"}</PRE
></P
></DD
><DT
><A
NAME="OPTION-DISPLAY-IO"
></A
><CODE
CLASS="OPTION"
>+I</CODE
></DT
><DD
><P
>Display results of IO programs (default: off).</P
><P
>By default, an expression of <TT
CLASS="LITERAL"
>IO</TT
> type typed at the prompt
is executed for effect, but the final value it produces is discarded.
When <CODE
CLASS="OPTION"
>+I</CODE
> is used, such an expression is evaluated,
and then its result is printed with <CODE
CLASS="FUNCTION"
>Prelude.print</CODE
>:
<PRE
CLASS="SCREEN"
>Hugs> :set +I
Hugs> (return 'a' :: IO Char)
'a'
Hugs></PRE
>
i.e., evaluating an IO action <TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
>
with <CODE
CLASS="OPTION"
>+I</CODE
> in effect is equivalent to evaluating
<SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>do { x <- m ; print x }</TT
>"</SPAN
>
with <CODE
CLASS="OPTION"
>-I</CODE
>.</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="OPTIONS-RESOURCE"
>3.1.5. Resource usage options</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="OPTION-HEAP-SIZE"
></A
><CODE
CLASS="OPTION"
>-h</CODE
><TT
CLASS="REPLACEABLE"
><I
>num</I
></TT
></DT
><DD
><P
>Set the maximum size in the Hugs heap (default: <TT
CLASS="LITERAL"
>250k</TT
>).
The argument should be a decimal number, and may by suffixed with
<SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>k</TT
>"</SPAN
> (thousands),
<SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>M</TT
>"</SPAN
> (millions) or
<SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>G</TT
>"</SPAN
>
(billions, if your machine has that much memory).
Case is not significant.
The heap size is measured in cells,
each of which usually comprises two <TT
CLASS="LITERAL"
>int</TT
>s
(taking up 8 bytes on most common architectures).</P
><P
>Setting this option with <B
CLASS="COMMAND"
>:set</B
> does not change the heap
size for the current execution of Hugs.
On Win32, however, all options are saved to the registry,
so it will take effect the next time Hugs is run.</P
></DD
><DT
><A
NAME="OPTION-PRINT-REDUCTIONS"
></A
><CODE
CLASS="OPTION"
>+s</CODE
></DT
><DD
><P
>Print statistics after each evaluation (default: off).</P
><P
>For each evaluation, this option shows
<P
></P
><UL
><LI
><P
>the number of reductions performed (a crude measure of the amount of
work performed by the interpreter),</P
></LI
><LI
><P
>the total number of cells allocated during evaluation, and</P
></LI
><LI
><P
>the number of garbage collections that occurred during evaluation (if any).</P
></LI
></UL
>
Note that even the most trivial evaluation involves several reductions and
cells, because Hugs wraps the expression in code to print the value and
catch exceptions:
<PRE
CLASS="SCREEN"
>Hugs> True
True
(25 reductions, 46 cells)</PRE
>
Note that the cell count measures the total amount of allocation,
rather than the number of cells in use at any time
(the <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>residency</I
></SPAN
>).
For that, the <A
HREF="options.html#OPTION-PRINT-GC"
><CODE
CLASS="OPTION"
>+g</CODE
></A
>
option may be more useful.
In general these statistics cannot be safely used for much more than
spotting general trends.</P
></DD
><DT
><A
NAME="OPTION-PRINT-GC"
></A
><CODE
CLASS="OPTION"
>+g</CODE
></DT
><DD
><P
>Print the number of cells recovered after each garbage collection
(default: off).
This can be useful for analysing the <I
CLASS="FIRSTTERM"
>residency</I
> of an
algorithm, the amount of memory it is actually using at each point in time.
For example,
<PRE
CLASS="SCREEN"
>Hugs> :set +g
Hugs> length [1..60000]
{{Gc:237618}}{{Gc:237617}}{{Gc:237616}}{{Gc:237623}}{{Gc:237621}}
{{Gc:237628}}{{Gc:237623}}{{Gc:237618}}60000</PRE
>
We see that the computation creates a lot of cells, but the number recovered
on each garbage collection is roughly the same, so its residency is constant.
In contrast, with
<PRE
CLASS="SCREEN"
>Hugs> let xs = [1..60000] in sum xs `div` length xs
{{Gc:237510}}{{Gc:213862}}{{Gc:190948}}{{Gc:170500}}{{Gc:152225}}
{{Gc:135925}}{{Gc:121350}}{{Gc:108350}}{{Gc:96750}}{{Gc:86375}}
{{Gc:77125}}{{Gc:68860}}{{Gc:61490}}{{Gc:72948}}{{Gc:97265}}
{{Gc:129688}}{{Gc:172916}}{{Gc:230551}}30000</PRE
>
we see that the amount reclaimed by each garbage collection is steadily
falling until a certain point (because the original list is retained).
These examples use the default heap size of 250000 cells;
this may be changed with the
<A
HREF="options.html#OPTION-HEAP-SIZE"
><CODE
CLASS="OPTION"
>-h</CODE
></A
> option.</P
><P
>Since these garbage collection messages will be unpredictably interleaved
with the desired output, you would usually only turn <CODE
CLASS="OPTION"
>+g</CODE
>
on to analyse memory problems, and then turn it off afterwards.</P
></DD
><DT
><A
NAME="OPTION-ROOT-OPTIMISATION"
></A
><CODE
CLASS="OPTION"
>+R</CODE
></DT
><DD
><P
>Enable root optimisation (default: on).</P
><P
>This usually gives a small gain in speed, but you might want to turn it off
if you're using the observation-based debugger
(see <A
HREF="observe.html#OBSERVE-AND-ROOT-OPTIMISATION"
>Section 7.4.2.3</A
>).</P
></DD
></DL
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