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
<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>
interfaces, and include files (usually something like
<literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
is the value of
- <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary>
- </indexterm>in the package configuration file (see <xref
- linkend="packages"/>).</para>
+ <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary></indexterm>
+ in the package configuration file
+ (see <xref linkend="packages"/>).</para>
</listitem>
</varlistentry>
<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>
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>, 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>
- <listitem>
- <indexterm><primary><option>-w</option></primary></indexterm>
- <para>Turns off all warnings, including the standard ones.</para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
<term><option>-Wall</option>:</term>
<listitem>
<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:</para>
- <itemizedlist>
- <listitem><option>-fwarn-simple-patterns</option></listitem>
- <listitem><option>-fwarn-tabs</option></listitem>
- <listitem><option>-fwarn-incomplete-record-updates</option></listitem>
- <listitem><option>-fwarn-monomorphism-restriction</option></listitem>
- <listitem><option>-fwarn-implicit-prelude</option></listitem>
- </itemizedlist>
+ are
+ <option>-fwarn-simple-patterns</option>,
+ <option>-fwarn-tabs</option>,
+ <option>-fwarn-incomplete-record-updates</option>,
+ <option>-fwarn-monomorphism-restriction</option>, and
+ <option>-fwarn-implicit-prelude</option>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-w</option>:</term>
+ <listitem>
+ <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-warnings-deprecations</option>:</term>
<listitem>
- <indexterm><primary><option>-fwarn-deprecations</option></primary>
+ <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 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>
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>-fno-implicit-prelude</option> or a
+ <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>-fno-implicit-prelude</option>
+ 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
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>
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
- <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>
</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
</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>
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>
<term>
- <option>-fno-state-hack</option>
- <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
+ <option>-fspec-constr</option>
+ <indexterm><primary><option>-fspec-constr</option></primary></indexterm>
</term>
<listitem>
- <para>Turn off the "state hack" whereby any lambda with a
- <literal>State#</literal> token as argument is considered to be
- single-entry, hence it is considered OK to inline things inside
- it. This can improve performance of IO and ST monad code, but it
- runs the risk of reducing sharing.</para>
+ <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>-funfolding-update-in-place=<replaceable>n</replaceable></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=<replaceable>n</replaceable></option>:
<indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
<indexterm><primary>inlining, controlling</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>
+ <title>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
<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>Setting <option>-N</option> also has the effect of
+ setting <option>-g</option> (the number of OS threads to
+ use for garbage collection) to the same value.</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="http://www.haskell.org/ghc/docs/papers/core.ps.gz">
<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;
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