Options can be specified in three ways:</para>
<sect2>
- <title>command-line arguments</title>
+ <title>Command-line arguments</title>
<indexterm><primary>structure, command-line</primary></indexterm>
<indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
ghc [argument...]
</screen>
- <para>command-line arguments are either options or file names.</para>
+ <para>Command-line arguments are either options or file names.</para>
- <para>command-line options begin with <literal>-</literal>.
+ <para>Command-line options begin with <literal>-</literal>.
They may <emphasis>not</emphasis> be grouped:
<option>-vO</option> is different from <option>-v -O</option>.
Options need not precede filenames: e.g., <literal>ghc *.o -o
</sect2>
<sect2 id="source-file-options">
- <title>command line options in source files</title>
+ <title>Command line options in source files</title>
<indexterm><primary>source-file options</primary></indexterm>
<para>Sometimes it is useful to make the connection between a
source file and the command-line options it requires quite
- tight. For instance, if a Haskell source file uses GHC
- extensions, it will always need to be compiled with the
- <option>-fglasgow-exts</option> option. Rather than maintaining
+ tight. For instance, if a Haskell source file deliberately
+ uses name shadowing, it should be compiled with the
+ <option>-fno-warn-name-shadowing</option> option. Rather than maintaining
the list of per-file options in a <filename>Makefile</filename>,
it is possible to do this directly in the source file using the
<literal>OPTIONS_GHC</literal> pragma <indexterm><primary>OPTIONS_GHC
pragma</primary></indexterm>:</para>
<programlisting>
-{-# OPTIONS_GHC -fglasgow-exts #-}
+{-# OPTIONS_GHC -fno-warn-name-shadowing #-}
module X where
...
</programlisting>
- <para><literal>OPTIONS_GHC</literal> pragmas are only looked for at
- the top of your source files, upto the first
- (non-literate,non-empty) line not containing
- <literal>OPTIONS_GHC</literal>. Multiple <literal>OPTIONS_GHC</literal>
- pragmas are recognised. Do not put comments before, or on the same line
- as, the <literal>OPTIONS_GHC</literal> pragma.</para>
+ <para><literal>OPTIONS_GHC</literal> is a <emphasis>file-header pragma</emphasis>
+ (see <xref linkend="pragmas"/>).</para>
+
+ <para>Only <emphasis>dynamic</emphasis> flags can be used in an <literal>OPTIONS_GHC</literal> pragma
+ (see <xref linkend="static-dynamic-flags"/>).</para>
<para>Note that your command shell does not
get to the source file options, they are just included literally
maintains internally, so you'll be desperately disappointed if
you try to glob etc. inside <literal>OPTIONS_GHC</literal>.</para>
- <para>NOTE: the contents of OPTIONS_GHC are prepended to the
- command-line options, so you <emphasis>do</emphasis> have the
- ability to override OPTIONS_GHC settings via the command
- line.</para>
+ <para>NOTE: the contents of OPTIONS_GHC are appended to the
+ command-line options, so options given in the source file
+ override those given on the command-line.</para>
<para>It is not recommended to move all the contents of your
Makefiles into your source files, but in some circumstances, the
<literal>OPTIONS_GHC</literal> pragma is the Right Thing. (If you
- use <option>-keep-hc-file-too</option> and have OPTION flags in
+ use <option>-keep-hc-file</option> and have OPTION flags in
your module, the OPTIONS_GHC will get put into the generated .hc
file).</para>
</sect2>
<indexterm><primary>mode</primary><secondary>options</secondary>
</indexterm>
- <para>Each of GHC's command line options is classified as either
- <firstterm>static</firstterm> or <firstterm>dynamic</firstterm> or
+ <para>Each of GHC's command line options is classified as
+ <firstterm>static</firstterm>, <firstterm>dynamic</firstterm> or
<firstterm>mode</firstterm>:</para>
<variablelist>
<term>Mode flags</term>
<listitem>
<para>For example, <option>--make</option> or <option>-E</option>.
- There may be only a single mode flag on the command line. The
+ There may only be a single mode flag on the command line. The
available modes are listed in <xref linkend="modes"/>.</para>
</listitem>
</varlistentry>
<listitem>
<para>Most non-mode flags fall into this category. A dynamic flag
may be used on the command line, in a
- <literal>GHC_OPTIONS</literal> pragma in a source file, or set
+ <literal>OPTIONS_GHC</literal> pragma in a source file, or set
using <literal>:set</literal> in GHCi.</para>
</listitem>
</varlistentry>
<para>The flag reference tables (<xref
linkend="flag-reference"/>) lists the status of each flag.</para>
+
+ <para>There are a few flags that are static except that they can
+ also be used with GHCi's <literal>:set</literal> command; these
+ are listed as “static/<literal>:set</literal>” in the
+ table.</para>
</sect1>
<sect1 id="file-suffixes">
<variablelist>
<varlistentry>
<term>
- <cmdsynopsis><command>ghc</command>
- <arg choice='plain'>––interactive</arg>
+ <cmdsynopsis><command>ghc --interactive</command>
</cmdsynopsis>
<indexterm><primary>interactive mode</primary></indexterm>
<indexterm><primary>ghci</primary></indexterm>
<varlistentry>
<term>
- <cmdsynopsis><command>ghc</command>
- <arg choice='plain'>––make</arg>
+ <cmdsynopsis><command>ghc --make</command>
</cmdsynopsis>
<indexterm><primary>make mode</primary></indexterm>
<indexterm><primary><option>––make</option></primary></indexterm>
<varlistentry>
<term>
- <cmdsynopsis><command>ghc</command>
- <arg choice='plain'>–e</arg> <arg choice='plain'><replaceable>expr</replaceable></arg>
+ <cmdsynopsis><command>ghc -e</command>
+ <arg choice='plain'><replaceable>expr</replaceable></arg>
</cmdsynopsis>
<indexterm><primary>eval mode</primary></indexterm>
</term>
<varlistentry>
<term>
<cmdsynopsis>
- <command>ghc</command>
- <group>
- <arg>-E</arg>
- <arg>-C</arg>
- <arg>-S</arg>
- <arg>-c</arg>
- </group>
+ <command>ghc -E</command>
+ <command>ghc -c</command>
+ <command>ghc -S</command>
+ <command>ghc -c</command>
</cmdsynopsis>
<indexterm><primary><option>-E</option></primary></indexterm>
<indexterm><primary><option>-C</option></primary></indexterm>
<varlistentry>
<term>
<cmdsynopsis>
- <command>ghc</command>
- <arg choice='plain'>–M</arg>
+ <command>ghc -M</command>
</cmdsynopsis>
<indexterm><primary>dependency-generation mode</primary></indexterm>
</term>
<para>Dependency-generation mode. In this mode, GHC can be
used to generate dependency information suitable for use in
a <literal>Makefile</literal>. See <xref
- linkend="sec-makefile-dependencies"/>.</para>
+ linkend="makefile-dependencies"/>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<cmdsynopsis>
- <command>ghc</command>
- <arg choice='plain'>––mk-dll</arg>
+ <command>ghc --mk-dll</command>
</cmdsynopsis>
- <indexterm><primary>dependency-generation mode</primary></indexterm>
+ <indexterm><primary>DLL-creation mode</primary></indexterm>
</term>
<listitem>
<para>DLL-creation mode (Windows only). See <xref
linkend="win32-dlls-create"/>.</para>
</listitem>
</varlistentry>
+
+ <varlistentry>
+ <term>
+ <cmdsynopsis>
+ <command>ghc --help</command> <command>ghc -?</command>
+ </cmdsynopsis>
+ <indexterm><primary><option>––help</option></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Cause GHC to spew a long usage message to standard
+ output and then exit.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
+ <cmdsynopsis>
+ <command>ghc --show-iface <replaceable>file</replaceable></command>
+ </cmdsynopsis>
+ <indexterm><primary><option>––--show-iface</option></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Read the interface in
+ <replaceable>file</replaceable> and dump it as text to
+ <literal>stdout</literal>. For example <literal>ghc --show-iface M.hi</literal>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
+ <cmdsynopsis>
+ <command>ghc --supported-languages</command>
+ </cmdsynopsis>
+ <indexterm><primary><option>––supported-languages</option></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Print the supported language extensions.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
+ <cmdsynopsis>
+ <command>ghc --info</command>
+ </cmdsynopsis>
+ <indexterm><primary><option>––info</option></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Print information about the compiler.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
+ <cmdsynopsis>
+ <command>ghc --version</command>
+ <command>ghc -V</command>
+ </cmdsynopsis>
+ <indexterm><primary><option>-V</option></primary></indexterm>
+ <indexterm><primary><option>––version</option></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Print a one-line string including GHC's version number.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
+ <cmdsynopsis>
+ <command>ghc --numeric-version</command>
+ </cmdsynopsis>
+ <indexterm><primary><option>––numeric-version</option></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Print GHC's numeric version number only.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
+ <cmdsynopsis>
+ <command>ghc --print-libdir</command>
+ </cmdsynopsis>
+ <indexterm><primary><option>––print-libdir</option></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Print the path to GHC's library directory. This is
+ the top of the directory tree containing GHC's libraries,
+ interfaces, and include files (usually something like
+ <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
+ is the value of
+ <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary></indexterm>
+ in the package configuration file
+ (see <xref linkend="packages"/>).</para>
+ </listitem>
+ </varlistentry>
+
</variablelist>
<sect2 id="make-mode">
<para>When given the <option>––make</option> option,
GHC will build a multi-module Haskell program by following
- dependencies from a single root module (usually
+ dependencies from one or more root modules (usually just
<literal>Main</literal>). For example, if your
<literal>Main</literal> module is in a file called
<filename>Main.hs</filename>, you could compile and link the
names or module names; GHC will figure out all the modules in
the program by following the imports from these initial modules.
It will then attempt to compile each module which is out of
- date, and finally if there is a <literal>Main</literal> module,
+ date, and finally, if there is a <literal>Main</literal> module,
the program will also be linked into an executable.</para>
<para>The main advantages to using <literal>ghc
<para>The first phase to run is determined by each input-file
suffix, and the last phase is determined by a flag. If no
- relevant flag is present, then go all the way through linking.
+ relevant flag is present, then go all the way through to linking.
This table summarises:</para>
<informaltable>
<para>Note: The option <option>-E</option><indexterm><primary>-E
option</primary></indexterm> runs just the pre-processing passes
- of the compiler, dumping the result in a file. Note that this
- differs from the previous behaviour of dumping the file to
- standard output.</para>
+ of the compiler, dumping the result in a file.</para>
<sect3 id="overriding-suffixes">
<title>Overriding the default behaviour for a file</title>
<para>As described above, the way in which a file is processed by GHC
- depends on its suffix. This behaviour can be overriden using the
+ depends on its suffix. This behaviour can be overridden using the
<option>-x</option> option:</para>
<variablelist>
<indexterm><primary>help options</primary></indexterm>
<indexterm><primary>verbosity options</primary></indexterm>
+ <para>See also the <option>--help</option>, <option>--version</option>, <option>--numeric-version</option>,
+ and <option>--print-libdir</option> modes in <xref linkend="modes"/>.</para>
<variablelist>
<varlistentry>
<term>
- <option>––help</option>
- <indexterm><primary><option>––help</option></primary></indexterm>
- </term>
- <term>
- <option>-?</option>
- <indexterm><primary><option>-?</option></primary></indexterm>
+ <option>-n</option>
+ <indexterm><primary><option>-n</option></primary></indexterm>
</term>
<listitem>
- <para>Cause GHC to spew a long usage message to standard
- output and then exit.</para>
+ <para>Does a dry-run, i.e. GHC goes through all the motions
+ of compiling as normal, but does not actually run any
+ external commands.</para>
</listitem>
</varlistentry>
</varlistentry>
<varlistentry>
- <term>
- <option>-V</option>
- <indexterm><primary><option>-V</option></primary></indexterm>
- </term>
- <term>
- <option>––version</option>
- <indexterm><primary><option>––version</option></primary></indexterm>
- </term>
- <listitem>
- <para>Print a one-line string including GHC's version number.</para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term>
- <option>––numeric-version</option>
- <indexterm><primary><option>––numeric-version</option></primary></indexterm>
- </term>
- <listitem>
- <para>Print GHC's numeric version number only.</para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term>
- <option>––print-libdir</option>
- <indexterm><primary><option>––print-libdir</option></primary></indexterm>
- </term>
- <listitem>
- <para>Print the path to GHC's library directory. This is
- the top of the directory tree containing GHC's libraries,
- interfaces, and include files (usually something like
- <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
- is the value of
- <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary>
- </indexterm>in the package configuration file (see <xref
- linkend="packages"/>).</para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
<term><option>-ferror-spans</option>
<indexterm><primary><option>-ferror-spans</option></primary>
</indexterm>
</varlistentry>
<varlistentry>
+ <term><option>-H</option><replaceable>size</replaceable>
+ <indexterm><primary><option>-H</option></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Set the minimum size of the heap to
+ <replaceable>size</replaceable>.
+ This option is equivalent to
+ <literal>+RTS -H<replaceable>size</replaceable></literal>,
+ see <xref linkend="rts-options-gc" />.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
<term><option>-Rghc-timing</option>
<indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
</term>
of warnings which are generally likely to indicate bugs in your
program. These are:
<option>-fwarn-overlapping-patterns</option>,
- <option>-fwarn-deprecations</option>,
+ <option>-fwarn-warnings-deprecations</option>,
+ <option>-fwarn-deprecated-flags</option>,
<option>-fwarn-duplicate-exports</option>,
- <option>-fwarn-missing-fields</option>, and
- <option>-fwarn-missing-methods</option>. The following flags are
+ <option>-fwarn-missing-fields</option>,
+ <option>-fwarn-missing-methods</option>,
+ <option>-fwarn-lazy-unlifted-bindings</option>,
+ <option>-fwarn-wrong-do-bind</option>, and
+ <option>-fwarn-dodgy-foreign-imports</option>. The following
+ flags are
simple ways to select standard “packages” of warnings:
</para>
<indexterm><primary>-W option</primary></indexterm>
<para>Provides the standard warnings plus
<option>-fwarn-incomplete-patterns</option>,
+ <option>-fwarn-dodgy-imports</option>,
<option>-fwarn-unused-matches</option>,
<option>-fwarn-unused-imports</option>, and
<option>-fwarn-unused-binds</option>.</para>
</varlistentry>
<varlistentry>
- <term><option>-w</option>:</term>
+ <term><option>-Wall</option>:</term>
<listitem>
- <indexterm><primary><option>-w</option></primary></indexterm>
- <para>Turns off all warnings, including the standard ones.</para>
+ <indexterm><primary><option>-Wall</option></primary></indexterm>
+ <para>Turns on all warning options that indicate potentially
+ suspicious code. The warnings that are
+ <emphasis>not</emphasis> enabled by <option>-Wall</option>
+ are
+ <option>-fwarn-simple-patterns</option>,
+ <option>-fwarn-tabs</option>,
+ <option>-fwarn-incomplete-record-updates</option>,
+ <option>-fwarn-monomorphism-restriction</option>,
+ <option>-fwarn-unused-do-bind</option>, and
+ <option>-fwarn-implicit-prelude</option>.</para>
</listitem>
</varlistentry>
<varlistentry>
- <term><option>-Wall</option>:</term>
+ <term><option>-w</option>:</term>
<listitem>
- <indexterm><primary><option>-Wall</option></primary></indexterm>
- <para>Turns on all warning options.</para>
+ <indexterm><primary><option>-w</option></primary></indexterm>
+ <para>Turns off all warnings, including the standard ones and
+ those that <literal>-Wall</literal> doesn't enable.</para>
</listitem>
</varlistentry>
</listitem>
</varlistentry>
+ <varlistentry>
+ <term><option>-Wwarn</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-Wwarn</option></primary></indexterm>
+ <para>Warnings are treated only as warnings, not as errors. This is
+ the default, but can be useful to negate a
+ <option>-Werror</option> flag.</para>
+ </listitem>
+ </varlistentry>
+
</variablelist>
<para>The full set of warning options is described below. To turn
<variablelist>
<varlistentry>
- <term><option>-fwarn-deprecations</option>:</term>
+ <term><option>-fwarn-unrecognised-pragmas</option>:</term>
<listitem>
- <indexterm><primary><option>-fwarn-deprecations</option></primary>
+ <indexterm><primary><option>-fwarn-unrecognised-pragmas</option></primary>
</indexterm>
+ <indexterm><primary>warnings</primary></indexterm>
+ <indexterm><primary>pragmas</primary></indexterm>
+ <para>Causes a warning to be emitted when a
+ pragma that GHC doesn't recognise is used. As well as pragmas
+ that GHC itself uses, GHC also recognises pragmas known to be used
+ by other tools, e.g. <literal>OPTIONS_HUGS</literal> and
+ <literal>DERIVE</literal>.</para>
+
+ <para>This option is on by default.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-warnings-deprecations</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-warnings-deprecations</option></primary>
+ </indexterm>
+ <indexterm><primary>warnings</primary></indexterm>
<indexterm><primary>deprecations</primary></indexterm>
+ <para>Causes a warning to be emitted when a
+ module, function or type with a WARNING or DEPRECATED pragma
+ is used. See <xref linkend="warning-deprecated-pragma"/> for more
+ details on the pragmas.</para>
+
+ <para>This option is on by default.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-deprecated-flags</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-deprecated-flags</option></primary>
+ </indexterm>
+ <indexterm><primary>deprecated-flags</primary></indexterm>
<para>Causes a warning to be emitted when a deprecated
- function or type is used. Entities can be marked as
- deprecated using a pragma, see <xref
- linkend="deprecated-pragma"/>.</para>
+ commandline flag is used.</para>
+
+ <para>This option is on by default.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-dodgy-foreign-imports</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-dodgy-foreign-imports</option></primary>
+ </indexterm>
+ <para>Causes a warning to be emitted for foreign imports of
+ the following form:</para>
+<programlisting>
+foreign import "f" f :: FunPtr t
+</programlisting>
+ <para>on the grounds that it probably should be</para>
+<programlisting>
+foreign import "&f" f :: FunPtr t
+</programlisting>
+ <para>The first form declares that `f` is a (pure) C
+ function that takes no arguments and returns a pointer to a
+ C function with type `t`, whereas the second form declares
+ that `f` itself is a C function with type `t`. The first
+ declaration is usually a mistake, and one that is hard to
+ debug because it results in a crash, hence this
+ warning.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-dodgy-imports</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
+ </indexterm>
+ <para>Causes a warning to be emitted when a datatype
+ <literal>T</literal> is imported
+ with all constructors, i.e. <literal>T(..)</literal>, but has been
+ exported abstractly, i.e. <literal>T</literal>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-lazy-unlifted-bindings</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-lazy-unlifted-bindings</option></primary>
+ </indexterm>
+ <para>Causes a warning to be emitted when an unlifted type
+ is bound in a way that looks lazy, e.g.
+ <literal>where (I# x) = ...</literal>. Use
+ <literal>where !(I# x) = ...</literal> instead. This will be an
+ error, rather than a warning, in GHC 6.14.
+ </para>
</listitem>
</varlistentry>
</varlistentry>
<varlistentry>
+ <term><option>-fwarn-implicit-prelude</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
+ <indexterm><primary>implicit prelude, warning</primary></indexterm>
+ <para>Have the compiler warn if the Prelude is implicitly
+ imported. This happens unless either the Prelude module is
+ explicitly imported with an <literal>import ... Prelude ...</literal>
+ line, or this implicit import is disabled (either by
+ <option>-XNoImplicitPrelude</option> or a
+ <literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>
+
+ <para>Note that no warning is given for syntax that implicitly
+ refers to the Prelude, even if <option>-XNoImplicitPrelude</option>
+ would change whether it refers to the Prelude.
+ For example, no warning is given when
+ <literal>368</literal> means
+ <literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
+ (where <literal>Prelude</literal> refers to the actual Prelude module,
+ regardless of the imports of the module being compiled).</para>
+
+ <para>This warning is off by default.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
<term><option>-fwarn-incomplete-patterns</option>:</term>
<listitem>
<indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
g [] = 2
</programlisting>
- <para>This option isn't enabled be default because it can be
+ <para>This option isn't enabled by default because it can be
a bit noisy, and it doesn't always indicate a bug in the
program. However, it's generally considered good practice
to cover all the cases in your functions.</para>
f foo = foo { x = 6 }
</programlisting>
- <para>This option isn't enabled be default because it can be
+ <para>This option isn't enabled by default because it can be
very noisy, and it often doesn't indicate a bug in the
program.</para>
</listitem>
<para>If you would like GHC to check that every top-level
function/value has a type signature, use the
- <option>-fwarn-missing-signatures</option> option. This
+ <option>-fwarn-missing-signatures</option> option. As part of
+ the warning GHC also reports the inferred type. The
option is off by default.</para>
</listitem>
</varlistentry>
inner-scope value has the same name as an outer-scope value,
i.e. the inner value shadows the outer one. This can catch
typographical errors that turn into hard-to-find bugs, e.g.,
- in the inadvertent cyclic definition <literal>let x = ... x
- ... in</literal>.</para>
-
- <para>Consequently, this option does
- <emphasis>will</emphasis> complain about cyclic recursive
- definitions.</para>
+ in the inadvertent capture of what would be a recursive call in
+ <literal>f = ... let f = id in ... f ...</literal>.</para>
+ <para>The warning is suppressed for names beginning with an underscore. For example
+ <programlisting>
+ f x = do { _ignore <- this; _ignore <- that; return (the other) }
+ </programlisting>
+ </para>
</listitem>
</varlistentry>
<para>This option causes a warning to be emitted whenever the
module contains an "orphan" instance declaration or rewrite rule.
- An instance declartion is an orphan if it appears in a module in
+ An instance declaration is an orphan if it appears in a module in
which neither the class nor the type being instanced are declared
in the same module. A rule is an orphan if it is a rule for a
function declared in another module. A module containing any
<para>The trouble with orphans is that GHC must pro-actively read the interface
files for all orphan modules, just in case their instances or rules
play a role, whether or not the module's interface would otherwise
- be of any use. Other things being equal, avoid orphan modules.</para>
+ be of any use. See <xref linkend="orphan-modules"/> for details.
+ </para>
</listitem>
</varlistentry>
</term>
<listitem>
<para>By default, the compiler will warn you if a set of
- patterns are overlapping, i.e.,</para>
+ patterns are overlapping, e.g.,</para>
<programlisting>
f :: String -> Int
patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
Normally, these aren't treated as incomplete patterns by
<option>-fwarn-incomplete-patterns</option>.</para>
- <para>``Lambda-bound patterns'' includes all places where there is a single pattern,
+ <para>“Lambda-bound patterns” includes all places where there is a single pattern,
including list comprehensions and do-notation. In these cases, a pattern-match
failure is quite legitimate, and triggers filtering (list comprehensions) or
the monad <literal>fail</literal> operation (monads). For example:
</programlisting>
Switching on <option>-fwarn-simple-patterns</option> will elicit warnings about
these probably-innocent cases, which is why the flag is off by default. </para>
- <para> The <literal>deriving( Read )</literal> mechanism produces monadic code with
- pattern matches, so you will also get misleading warnings about the compiler-generated
- code. (This is arguably a Bad Thing, but it's awkward to fix.)</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-tabs</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
+ <indexterm><primary>tabs, warning</primary></indexterm>
+ <para>Have the compiler warn if there are tabs in your source
+ file.</para>
+ <para>This warning is off by default.</para>
</listitem>
</varlistentry>
the Haskell defaulting mechanism for numeric types kicks
in. This is useful information when converting code from a
context that assumed one default into one with another,
- e.g., the `default default' for Haskell 1.4 caused the
+ e.g., the ‘default default’ for Haskell 1.4 caused the
otherwise unconstrained value <constant>1</constant> to be
given the type <literal>Int</literal>, whereas Haskell 98
defaults it to <literal>Integer</literal>. This may lead to
</varlistentry>
<varlistentry>
+ <term><option>-fwarn-monomorphism-restriction</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
+ <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
+ <para>Have the compiler warn/inform you where in your source
+ the Haskell Monomorphism Restriction is applied. If applied silently
+ the MR can give rise to unexpected behaviour, so it can be helpful
+ to have an explicit warning that it is being applied.</para>
+
+ <para>This warning is off by default.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
<term><option>-fwarn-unused-binds</option>:</term>
<listitem>
<indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
</listitem>
</varlistentry>
+ <varlistentry>
+ <term><option>-fwarn-unused-do-bind</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-unused-do-bind</option></primary></indexterm>
+ <indexterm><primary>unused do binding, warning</primary></indexterm>
+ <indexterm><primary>do binding, unused</primary></indexterm>
+
+ <para>Report expressions occuring in <literal>do</literal> and <literal>mdo</literal> blocks
+ that appear to silently throw information away.
+ For instance <literal>do { mapM popInt xs ; return 10 }</literal> would report
+ the first statement in the <literal>do</literal> block as suspicious,
+ as it has the type <literal>StackM [Int]</literal> and not <literal>StackM ()</literal>, but that
+ <literal>[Int]</literal> value is not bound to anything. The warning is suppressed by
+ explicitly mentioning in the source code that your program is throwing something away:
+ <programlisting>
+ do { _ <- mapM popInt xs ; return 10 }
+ </programlisting>
+ Of course, in this particular situation you can do even better:
+ <programlisting>
+ do { mapM_ popInt xs ; return 10 }
+ </programlisting>
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-wrong-do-bind</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-wrong-do-bind</option></primary></indexterm>
+ <indexterm><primary>apparently erroneous do binding, warning</primary></indexterm>
+ <indexterm><primary>do binding, apparently erroneous</primary></indexterm>
+
+ <para>Report expressions occuring in <literal>do</literal> and <literal>mdo</literal> blocks
+ that appear to lack a binding.
+ For instance <literal>do { return (popInt 10) ; return 10 }</literal> would report
+ the first statement in the <literal>do</literal> block as suspicious,
+ as it has the type <literal>StackM (StackM Int)</literal> (which consists of two nested applications
+ of the same monad constructor), but which is not then "unpacked" by binding the result.
+ The warning is suppressed by explicitly mentioning in the source code that your program is throwing something away:
+ <programlisting>
+ do { _ <- return (popInt 10) ; return 10 }
+ </programlisting>
+ For almost all sensible programs this will indicate a bug, and you probably intended to write:
+ <programlisting>
+ do { popInt 10 ; return 10 }
+ </programlisting>
+ </para>
+ </listitem>
+ </varlistentry>
+
</variablelist>
<para>If you're feeling really paranoid, the
<para>Note that higher optimisation levels cause more
cross-module optimisation to be performed, which can have an
impact on how much of your program needs to be recompiled when
- you change something. This is one reaosn to stick to
+ you change something. This is one reason to stick to
no-optimisation when developing code.</para>
<variablelist>
<para>Means: “Generate good-quality code without
taking too long about it.” Thus, for example:
<command>ghc -c -O Main.lhs</command></para>
-
- <para><option>-O</option> currently also implies
- <option>-fvia-C</option>. This may change in the
- future.</para>
</listitem>
</varlistentry>
<literal>Exception.assert</literal> in source code (in
other words, rewriting <literal>Exception.assert p
e</literal> to <literal>e</literal> (see <xref
- linkend="sec-assertions"/>). This flag is turned on by
+ linkend="assertions"/>). This flag is turned on by
<option>-O</option>.
</para>
</listitem>
<varlistentry>
<term>
+ <option>-fspec-constr</option>
+ <indexterm><primary><option>-fspec-constr</option></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Turn on call-pattern specialisation.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
+ <option>-fliberate-case</option>
+ <indexterm><primary><option>-fliberate-case</option></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Turn on the liberate-case transformation.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
+ <option>-fstatic-argument-transformation</option>
+ <indexterm><primary><option>-fstatic-argument-transformation</option></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Turn on the static argument transformation.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
<option>-fno-state-hack</option>
<indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
</term>
<varlistentry>
<term>
+ <option>-fomit-interface-pragmas</option>
+ <indexterm><primary><option>-fomit-interface-pragmas</option></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Tells GHC to omit all inessential information from the interface file
+ generated for the module being compiled (say M). This means that a module
+ importing M will see only the <emphasis>types</emphasis> of the functions that M exports, but not
+ their unfoldings, strictness info, etc. Hence, for example,
+ no function exported by M will be inlined
+ into an importing module. The benefit is that modules that import M will
+ need to be recompiled less often (only when M's exports change their type,
+ not when they change their implementation).
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
+ <option>-fignore-interface-pragmas</option>
+ <indexterm><primary><option>-fignore-interface-pragmas</option></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Tells GHC to ignore all inessential information when reading interface files.
+ That is, even if <filename>M.hi</filename> contains unfolding or strictness information
+ for a function, GHC will ignore that information.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
<option>-funbox-strict-fields</option>:
<indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
<indexterm><primary>strict constructor fields</primary></indexterm>
<varlistentry>
<term>
- <option>-funfolding-update-in-place<n></option>
- <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
- </term>
- <listitem>
- <para>Switches on an experimental "optimisation".
- Switching it on makes the compiler a little keener to
- inline a function that returns a constructor, if the
- context is that of a thunk.
-<programlisting>
- x = plusInt a b
-</programlisting>
- If we inlined plusInt we might get an opportunity to use
- update-in-place for the thunk 'x'.</para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term>
- <option>-funfolding-creation-threshold<n></option>:
+ <option>-funfolding-creation-threshold=<replaceable>n</replaceable></option>:
<indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
<indexterm><primary>inlining, controlling</primary></indexterm>
<indexterm><primary>unfolding, controlling</primary></indexterm>
</varlistentry>
<varlistentry>
- <term><option>-funfolding-use-threshold<n></option>:</term>
+ <term><option>-funfolding-use-threshold=<replaceable>n</replaceable></option></term>
<listitem>
<indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
<indexterm><primary>inlining, controlling</primary></indexterm>
&phases;
- <sect1 id="sec-using-concurrent">
+ <sect1 id="using-concurrent">
<title>Using Concurrent Haskell</title>
<indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
</variablelist>
</sect1>
- <sect1 id="sec-using-smp">
+ <sect1 id="using-smp">
<title>Using SMP parallelism</title>
<indexterm><primary>parallelism</primary>
</indexterm>
is also possible to obtain performance improvements with parallelism
on programs that do not use concurrency. This section describes how to
use GHC to compile and run parallel programs, in <xref
- linkend="lang-parallel" /> we desribe the language features that affect
+ linkend="lang-parallel" /> we describe the language features that affect
parallelism.</para>
- <sect2 id="parallel-options">
- <title>Options to enable SMP parallelism</title>
+ <sect2 id="parallel-compile-options">
+ <title>Compile-time options for SMP parallelism</title>
<para>In order to make use of multiple CPUs, your program must be
linked with the <option>-threaded</option> option (see <xref
- linkend="options-linker" />). Then, to run a program on multiple
- CPUs, use the RTS <option>-N</option> option:</para>
+ linkend="options-linker" />). Additionally, the following
+ compiler options affect parallelism:</para>
<variablelist>
+ <varlistentry>
+ <term><option>-feager-blackholing</option></term>
+ <indexterm><primary><option>-feager-blackholing</option></primary></indexterm>
+ <listitem>
+ <para>
+ Blackholing is the act of marking a thunk (lazy
+ computuation) as being under evaluation. It is useful for
+ three reasons: firstly it lets us detect certain kinds of
+ infinite loop (the <literal>NonTermination</literal>
+ exception), secondly it avoids certain kinds of space
+ leak, and thirdly it avoids repeating a computation in a
+ parallel program, because we can tell when a computation
+ is already in progress.</para>
+
+ <para>
+ The option <option>-feager-blackholing</option> causes
+ each thunk to be blackholed as soon as evaluation begins.
+ The default is "lazy blackholing", whereby thunks are only
+ marked as being under evaluation when a thread is paused
+ for some reason. Lazy blackholing is typically more
+ efficient (by 1-2% or so), because most thunks don't
+ need to be blackholed. However, eager blackholing can
+ avoid more repeated computation in a parallel program, and
+ this often turns out to be important for parallelism.
+ </para>
+
+ <para>
+ We recommend compiling any code that is intended to be run
+ in parallel with the <option>-feager-blackholing</option>
+ flag.
+ </para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </sect2>
+
+ <sect2 id="parallel-options">
+ <title>RTS options for SMP parallelism</title>
+
+ <para>To run a program on multiple CPUs, use the
+ RTS <option>-N</option> option:</para>
+
+ <variablelist>
<varlistentry>
- <term><option>-N<replaceable>x</replaceable></option></term>
+ <term><option>-N<optional><replaceable>x</replaceable></optional></option></term>
<listitem>
<para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
Use <replaceable>x</replaceable> simultaneous threads when
running the program. Normally <replaceable>x</replaceable>
- should be chosen to match the number of CPU cores on the machine.
- There is no means (currently) by which this value may vary after
- the program has started.</para>
-
- <para>For example, on a dual-core machine we would probably use
+ should be chosen to match the number of CPU cores on the
+ machine<footnote><para>Whether hyperthreading cores should be counted or not is an
+ open question; please feel free to experiment and let us know what
+ results you find.</para></footnote>. For example,
+ on a dual-core machine we would probably use
<literal>+RTS -N2 -RTS</literal>.</para>
- <para>Whether hyperthreading cores should be counted or not is an
- open question; please feel free to experiment and let us know what
- results you find.</para>
+ <para>Omitting <replaceable>x</replaceable>,
+ i.e. <literal>+RTS -N -RTS</literal>, lets the runtime
+ choose the value of <replaceable>x</replaceable> itself
+ based on how many processors are in your machine.</para>
+
+ <para>Be careful when using all the processors in your
+ machine: if some of your processors are in use by other
+ programs, this can actually harm performance rather than
+ improve it.</para>
+
+ <para>Setting <option>-N</option> also has the effect of
+ enabling the parallel garbage collector (see
+ <xref linkend="rts-options-gc" />).</para>
+
+ <para>There is no means (currently) by which this value
+ may vary after the program has started.</para>
</listitem>
</varlistentry>
</variablelist>
+
+ <para>The following options affect the way the runtime schedules
+ threads on CPUs:</para>
+
+ <variablelist>
+ <varlistentry>
+ <term><option>-qm</option></term>
+ <indexterm><primary><option>-qm</option></primary><secondary>RTS
+ option</secondary></indexterm>
+ <listitem>
+ <para>Disable automatic migration for load balancing.
+ Normally the runtime will automatically try to schedule
+ threads across the available CPUs to make use of idle
+ CPUs; this option disables that behaviour. It is probably
+ only of use if you are explicitly scheduling threads onto
+ CPUs with <literal>GHC.Conc.forkOnIO</literal>.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term><option>-qw</option></term>
+ <indexterm><primary><option>-qw</option></primary><secondary>RTS
+ option</secondary></indexterm>
+ <listitem>
+ <para>Migrate a thread to the current CPU when it is woken
+ up. Normally when a thread is woken up after being
+ blocked it will be scheduled on the CPU it was running on
+ last; this option allows the thread to immediately migrate
+ to the CPU that unblocked it.</para>
+
+ <para>The rationale for allowing this eager migration is
+ that it tends to move threads that are communicating with
+ each other onto the same CPU; however there are
+ pathalogical situations where it turns out to be a poor
+ strategy. Depending on the communication pattern in your
+ program, it may or may not be a good idea.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
</sect2>
<sect2>
<title>Hints for using SMP parallelism</title>
- <para>Add the <literal>-sstderr</literal> RTS option when
+ <para>Add the <literal>-s</literal> RTS option when
running the program to see timing stats, which will help to tell you
whether your program got faster by using more CPUs or not. If the user
time is greater than
<indexterm><primary>intermediate code generation</primary></indexterm>
<para>GHC can dump its optimized intermediate code (said to be in “Core” format)
- to a file as a side-effect of compilation. Core files, which are given the suffix
- <filename>.hcr</filename>, can be read and processed by non-GHC back-end
- tools. The Core format is formally described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz">
+ to a file as a side-effect of compilation. Non-GHC back-end tools can read and process Core files; these files have the suffix
+ <filename>.hcr</filename>. The Core format is described in <ulink url="../ext-core/core.pdf">
<citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
- and sample tools (in Haskell)
- for manipulating Core files are available in the GHC source distribution
- directory <literal>/fptools/ghc/utils/ext-core</literal>.
+ and sample tools
+ for manipulating Core files (in Haskell) are in the GHC source distribution
+ directory under <literal>utils/ext-core</literal>.
Note that the format of <literal>.hcr</literal>
- files is <emphasis>different</emphasis> (though similar) to the Core output format generated
- for debugging purposes (<xref linkend="options-debugging"/>).</para>
+ files is <emphasis>different</emphasis> from the Core output format that GHC generates
+ for debugging purposes (<xref linkend="options-debugging"/>), though the two formats appear somewhat similar.</para>
<para>The Core format natively supports notes which you can add to
your source code using the <literal>CORE</literal> pragma (see <xref
</variablelist>
-<para>GHC can also read in External Core files as source; just give the <literal>.hcr</literal> file on
-the command line, instead of the <literal>.hs</literal> or <literal>.lhs</literal> Haskell source.
-A current infelicity is that you need to give the <literal>-fglasgow-exts</literal> flag too, because
-ordinary Haskell 98, when translated to External Core, uses things like rank-2 types.</para>
+<para>Currently (as of version 6.8.2), GHC does not have the ability to read in External Core files as source. If you would like GHC to have this ability, please <ulink url="http://hackage.haskell.org/trac/ghc/wiki/MailingListsAndIRC">make your wishes known to the GHC Team</ulink>.</para>
+
</sect1>
&debug;
projects / ghc-hetmet.git / blobdiff