<indexterm><primary>extensions</primary><secondary>options controlling</secondary>
</indexterm>
- <para>These flags control what variation of the language are
+ <para>The language option flag control what variation of the language are
permitted. Leaving out all of them gives you standard Haskell
98.</para>
- <para>NB. turning on an option that enables special syntax
+ <para>Generally speaking, all the language options are introduced by "<option>-X</option>",
+ e.g. <option>-XTemplateHaskell</option>.
+ </para>
+
+ <para> All the language options can be turned off by using the prefix "<option>No</option>";
+ e.g. "<option>-XNoTemplateHaskell</option>".</para>
+
+ <para> Language options recognised by Cabal can also be enabled using the <literal>LANGUAGE</literal> pragma,
+ thus <literal>{-# LANGUAGE TemplateHaskell #-}</literal> (see <xref linkend="language-pragma"/>>). </para>
+
+ <para>Turning on an option that enables special syntax
<emphasis>might</emphasis> cause working Haskell 98 code to fail
to compile, perhaps because it uses a variable name which has
become a reserved word. So, together with each option below, we
<para>This simultaneously enables all of the extensions to
Haskell 98 described in <xref
linkend="ghc-language-features"/>, except where otherwise
- noted. </para>
+ noted. We are trying to move away from this portmanteau flag,
+ and towards enabling features individaully.</para>
<para>New reserved words: <literal>forall</literal> (only in
types), <literal>mdo</literal>.</para>
<replaceable>float</replaceable><literal>##</literal>,
<literal>(#</literal>, <literal>#)</literal>,
<literal>|)</literal>, <literal>{|</literal>.</para>
+
+ <para>Implies these specific language options:
+ <option>-XForeignFunctionInterface</option>,
+ <option>-XImplicitParams</option>,
+ <option>-XScopedTypeVariables</option>,
+ <option>-XGADTs</option>,
+ <option>-XTypeFamilies</option>. </para>
</listitem>
</varlistentry>
<varlistentry>
<term>
- <option>-ffi</option> and <option>-fffi</option>:
- <indexterm><primary><option>-ffi</option></primary></indexterm>
- <indexterm><primary><option>-fffi</option></primary></indexterm>
+ <option>-XForeignFunctionInterface</option>:
+ <indexterm><primary><option>-XForeignFunctionInterface</option></primary></indexterm>
</term>
<listitem>
<para>This option enables the language extension defined in the
<varlistentry>
<term>
- <option>-fno-monomorphism-restriction</option>,<option>-fno-mono-pat-binds</option>:
+ <option>-XMonomorphismRestriction</option>,<option>-XMonoPatBinds</option>:
</term>
<listitem>
<para> These two flags control how generalisation is done.
<varlistentry>
<term>
- <option>-fextended-default-rules</option>:
- <indexterm><primary><option>-fextended-default-rules</option></primary></indexterm>
+ <option>-XExtendedDefaultRules</option>:
+ <indexterm><primary><option>-XExtendedDefaultRules</option></primary></indexterm>
</term>
<listitem>
<para> Use GHCi's extended default rules in a regular module (<xref linkend="extended-default-rules"/>).
<varlistentry>
<term>
- <option>-fallow-overlapping-instances</option>
- <indexterm><primary><option>-fallow-overlapping-instances</option></primary></indexterm>
+ <option>-XOverlappingInstances</option>
+ <indexterm><primary><option>-XOverlappingInstances</option></primary></indexterm>
</term>
<term>
- <option>-fallow-undecidable-instances</option>
- <indexterm><primary><option>-fallow-undecidable-instances</option></primary></indexterm>
+ <option>-XUndecidableInstances</option>
+ <indexterm><primary><option>-XUndecidableInstances</option></primary></indexterm>
</term>
<term>
- <option>-fallow-incoherent-instances</option>
- <indexterm><primary><option>-fallow-incoherent-instances</option></primary></indexterm>
+ <option>-XIncoherentInstances</option>
+ <indexterm><primary><option>-XIncoherentInstances</option></primary></indexterm>
</term>
<term>
<option>-fcontext-stack=N</option>
<varlistentry>
<term>
- <option>-farrows</option>
- <indexterm><primary><option>-farrows</option></primary></indexterm>
+ <option>-XArrows</option>
+ <indexterm><primary><option>-XArrows</option></primary></indexterm>
</term>
<listitem>
<para>See <xref linkend="arrow-notation"/>. Independent of
<varlistentry>
<term>
- <option>-fgenerics</option>
- <indexterm><primary><option>-fgenerics</option></primary></indexterm>
+ <option>-XGenerics</option>
+ <indexterm><primary><option>-XGenerics</option></primary></indexterm>
</term>
<listitem>
<para>See <xref linkend="generic-classes"/>. Independent of
</varlistentry>
<varlistentry>
- <term><option>-fno-implicit-prelude</option></term>
+ <term><option>-XNoImplicitPrelude</option></term>
<listitem>
- <para><indexterm><primary>-fno-implicit-prelude
+ <para><indexterm><primary>-XNoImplicitPrelude
option</primary></indexterm> GHC normally imports
<filename>Prelude.hi</filename> files for you. If you'd
rather it didn't, then give it a
- <option>-fno-implicit-prelude</option> option. The idea is
+ <option>-XNoImplicitPrelude</option> option. The idea is
that you can then import a Prelude of your own. (But don't
call it <literal>Prelude</literal>; the Haskell module
namespace is flat, and you must not conflict with any
translation for list comprehensions continues to use
<literal>Prelude.map</literal> etc.</para>
- <para>However, <option>-fno-implicit-prelude</option> does
+ <para>However, <option>-XNoImplicitPrelude</option> does
change the handling of certain built-in syntax: see <xref
linkend="rebindable-syntax"/>.</para>
</listitem>
</varlistentry>
<varlistentry>
- <term><option>-fimplicit-params</option></term>
+ <term><option>-XImplicitParams</option></term>
<listitem>
<para>Enables implicit parameters (see <xref
linkend="implicit-parameters"/>). Currently also implied by
</varlistentry>
<varlistentry>
- <term><option>-foverloaded-strings</option></term>
+ <term><option>-XOverloadedStrings</option></term>
<listitem>
<para>Enables overloaded string literals (see <xref
linkend="overloaded-strings"/>).</para>
</varlistentry>
<varlistentry>
- <term><option>-fscoped-type-variables</option></term>
+ <term><option>-XScopedTypeVariables</option></term>
<listitem>
<para>Enables lexically-scoped type variables (see <xref
linkend="scoped-type-variables"/>). Implied by
</varlistentry>
<varlistentry>
- <term><option>-fth</option></term>
+ <term><option>-XTemplateHaskell</option></term>
<listitem>
<para>Enables Template Haskell (see <xref
linkend="template-haskell"/>). This flag must
</title>
<para> The recursive do-notation (also known as mdo-notation) is implemented as described in
-"A recursive do for Haskell",
-Levent Erkok, John Launchbury",
+<ulink url="http://citeseer.ist.psu.edu/erk02recursive.html">A recursive do for Haskell</ulink>,
+by Levent Erkok, John Launchbury,
Haskell Workshop 2002, pages: 29-37. Pittsburgh, Pennsylvania.
+This paper is essential reading for anyone making non-trivial use of mdo-notation,
+and we do not repeat it here.
</para>
<para>
The do-notation of Haskell does not allow <emphasis>recursive bindings</emphasis>,
</programlisting>
<para>
The function <literal>mfix</literal>
-dictates how the required recursion operation should be performed. If recursive bindings are required for a monad,
-then that monad must be declared an instance of the <literal>MonadFix</literal> class.
-For details, see the above mentioned reference.
+dictates how the required recursion operation should be performed. For example,
+<literal>justOnes</literal> desugars as follows:
+<programlisting>
+justOnes = mfix (\xs' -> do { xs <- Just (1:xs'); return xs }
+</programlisting>
+For full details of the way in which mdo is typechecked and desugared, see
+the paper <ulink url="http://citeseer.ist.psu.edu/erk02recursive.html">A recursive do for Haskell</ulink>.
+In particular, GHC implements the segmentation technique described in Section 3.2 of the paper.
</para>
<para>
+If recursive bindings are required for a monad,
+then that monad must be declared an instance of the <literal>MonadFix</literal> class.
The following instances of <literal>MonadFix</literal> are automatically provided: List, Maybe, IO.
Furthermore, the Control.Monad.ST and Control.Monad.ST.Lazy modules provide the instances of the MonadFix class
for Haskell's internal state monad (strict and lazy, respectively).
</para></listitem>
<listitem><para>
-You should <literal>import Control.Monad.Fix</literal>.
-(Note: Strictly speaking, this import is required only when you need to refer to the name
-<literal>MonadFix</literal> in your program, but the import is always safe, and the programmers
-are encouraged to always import this module when using the mdo-notation.)
-</para></listitem>
-
-<listitem><para>
As with other extensions, ghc should be given the flag <literal>-fglasgow-exts</literal>
</para></listitem>
</itemizedlist>
hierarchy. It completely defeats that purpose if the
literal "1" means "<literal>Prelude.fromInteger
1</literal>", which is what the Haskell Report specifies.
- So the <option>-fno-implicit-prelude</option> flag causes
+ So the <option>-XNoImplicitPrelude</option> flag causes
the following pieces of built-in syntax to refer to
<emphasis>whatever is in scope</emphasis>, not the Prelude
versions:
</sect2>
+<sect2 id="disambiguate-fields">
+<title>Record field disambiguation</title>
+<para>
+In record construction and record pattern matching
+it is entirely unambiguous which field is referred to, even if there are two different
+data types in scope with a common field name. For example:
+<programlisting>
+module M where
+ data S = MkS { x :: Int, y :: Bool }
+
+module Foo where
+ import M
+
+ data T = MkT { x :: Int }
+
+ ok1 (MkS { x = n }) = n+1 -- Unambiguous
+
+ ok2 n = MkT { x = n+1 } -- Unambiguous
+
+ bad1 k = k { x = 3 } -- Ambiguous
+ bad2 k = x k -- Ambiguous
+</programlisting>
+Even though there are two <literal>x</literal>'s in scope,
+it is clear that the <literal>x</literal> in the pattern in the
+definition of <literal>ok1</literal> can only mean the field
+<literal>x</literal> from type <literal>S</literal>. Similarly for
+the function <literal>ok2</literal>. However, in the record update
+in <literal>bad1</literal> and the record selection in <literal>bad2</literal>
+it is not clear which of the two types is intended.
+</para>
+<para>
+Haskell 98 regards all four as ambiguous, but with the
+<option>-fdisambiguate-record-fields</option> flag, GHC will accept
+the former two. The rules are precisely the same as those for instance
+declarations in Haskell 98, where the method names on the left-hand side
+of the method bindings in an instance declaration refer unambiguously
+to the method of that class (provided they are in scope at all), even
+if there are other variables in scope with the same name.
+This reduces the clutter of qualified names when you import two
+records from different modules that use the same field name.
+</para>
+</sect2>
</sect1>
may use different notation to that implemented in GHC.
</para>
<para>
-The rest of this section outlines the extensions to GHC that support GADTs.
+The rest of this section outlines the extensions to GHC that support GADTs. The extension is enabled with
+<option>-XGADTs</option>.
<itemizedlist>
<listitem><para>
A GADT can only be declared using GADT-style syntax (<xref linkend="gadt-style"/>);
</para>
<para>
Nevertheless, GHC allows you to experiment with more liberal rules. If you use
-the experimental flag <option>-fallow-undecidable-instances</option>
-<indexterm><primary>-fallow-undecidable-instances
-option</primary></indexterm>, both the Paterson Conditions and the Coverage Condition
+the experimental flag <option>-XUndecidableInstances</option>
+<indexterm><primary>-XUndecidableInstances</primary></indexterm>,
+both the Paterson Conditions and the Coverage Condition
(described in <xref linkend="instance-rules"/>) are lifted. Termination is ensured by having a
fixed-depth recursion stack. If you exceed the stack depth you get a
sort of backtrace, and the opportunity to increase the stack depth
In general, <emphasis>GHC requires that that it be unambiguous which instance
declaration
should be used to resolve a type-class constraint</emphasis>. This behaviour
-can be modified by two flags: <option>-fallow-overlapping-instances</option>
-<indexterm><primary>-fallow-overlapping-instances
+can be modified by two flags: <option>-XOverlappingInstances</option>
+<indexterm><primary>-XOverlappingInstances
</primary></indexterm>
-and <option>-fallow-incoherent-instances</option>
-<indexterm><primary>-fallow-incoherent-instances
+and <option>-XIncoherentInstances</option>
+<indexterm><primary>-XIncoherentInstances
</primary></indexterm>, as this section discusses. Both these
flags are dynamic flags, and can be set on a per-module basis, using
an <literal>OPTIONS_GHC</literal> pragma if desired (<xref linkend="source-file-options"/>).</para>
</para>
<para>
-The <option>-fallow-overlapping-instances</option> flag instructs GHC to allow
+The <option>-XOverlappingInstances</option> flag instructs GHC to allow
more than one instance to match, provided there is a most specific one. For
example, the constraint <literal>C Int [Int]</literal> matches instances (A),
(C) and (D), but the last is more specific, and hence is chosen. If there is no
GHC does not commit to instance (C), because in a particular
call of <literal>f</literal>, <literal>b</literal> might be instantiate
to <literal>Int</literal>, in which case instance (D) would be more specific still.
-So GHC rejects the program. If you add the flag <option>-fallow-incoherent-instances</option>,
+So GHC rejects the program.
+(If you add the flag <option>-XIncoherentInstances</option>,
GHC will instead pick (C), without complaining about
-the problem of subsequent instantiations.
+the problem of subsequent instantiations.)
+</para>
+<para>
+Notice that we gave a type signature to <literal>f</literal>, so GHC had to
+<emphasis>check</emphasis> that <literal>f</literal> has the specified type.
+Suppose instead we do not give a type signature, asking GHC to <emphasis>infer</emphasis>
+it instead. In this case, GHC will refrain from
+simplifying the constraint <literal>C Int [Int]</literal> (for the same reason
+as before) but, rather than rejecting the program, it will infer the type
+<programlisting>
+ f :: C Int b => [b] -> [b]
+</programlisting>
+That postpones the question of which instance to pick to the
+call site for <literal>f</literal>
+by which time more is known about the type <literal>b</literal>.
</para>
<para>
The willingness to be overlapped or incoherent is a property of
the <emphasis>instance declaration</emphasis> itself, controlled by the
-presence or otherwise of the <option>-fallow-overlapping-instances</option>
-and <option>-fallow-incoherent-instances</option> flags when that mdodule is
+presence or otherwise of the <option>-XOverlappingInstances</option>
+and <option>-XIncoherentInstances</option> flags when that mdodule is
being defined. Neither flag is required in a module that imports and uses the
instance declaration. Specifically, during the lookup process:
<itemizedlist>
<listitem><para>
An instance declaration is ignored during the lookup process if (a) a more specific
match is found, and (b) the instance declaration was compiled with
-<option>-fallow-overlapping-instances</option>. The flag setting for the
+<option>-XOverlappingInstances</option>. The flag setting for the
more-specific instance does not matter.
</para></listitem>
<listitem><para>
-Suppose an instance declaration does not matche the constraint being looked up, but
+Suppose an instance declaration does not match the constraint being looked up, but
does unify with it, so that it might match when the constraint is further
instantiated. Usually GHC will regard this as a reason for not committing to
some other constraint. But if the instance declaration was compiled with
-<option>-fallow-incoherent-instances</option>, GHC will skip the "does-it-unify?"
+<option>-XIncoherentInstances</option>, GHC will skip the "does-it-unify?"
check for that declaration.
</para></listitem>
</itemizedlist>
</para>
<para>
If an instance declaration is compiled without
-<option>-fallow-overlapping-instances</option>,
+<option>-XOverlappingInstances</option>,
then that instance can never be overlapped. This could perhaps be
inconvenient. Perhaps the rule should instead say that the
<emphasis>overlapping</emphasis> instance declaration should be compiled in
this way, rather than the <emphasis>overlapped</emphasis> one. Perhaps overlap
at a usage site should be permitted regardless of how the instance declarations
-are compiled, if the <option>-fallow-overlapping-instances</option> flag is
+are compiled, if the <option>-XOverlappingInstances</option> flag is
used at the usage site. (Mind you, the exact usage site can occasionally be
hard to pin down.) We are interested to receive feedback on these points.
</para>
-<para>The <option>-fallow-incoherent-instances</option> flag implies the
-<option>-fallow-overlapping-instances</option> flag, but not vice versa.
+<para>The <option>-XIncoherentInstances</option> flag implies the
+<option>-XOverlappingInstances</option> flag, but not vice versa.
</para>
</sect3>
due to Jeff Lewis.)</para>
<para>Implicit parameter support is enabled with the option
-<option>-fimplicit-params</option>.</para>
+<option>-XImplicitParams</option>.</para>
<para>
A variable is called <emphasis>dynamically bound</emphasis> when it is bound by the calling
<para>Following a suggestion of Mark Jones, in his paper
<ulink url="http://www.cse.ogi.edu/~mpj/thih/">Typing Haskell in
Haskell</ulink>,
-GHC implements a more general scheme. If <option>-fglasgow-exts</option> is
+GHC implements a more general scheme. If <option>-XRelaxedPolyRec</option> is
specified:
<emphasis>the dependency analysis ignores references to variables that have an explicit
type signature</emphasis>.
The same refined dependency analysis also allows the type signatures of
mutually-recursive functions to have different contexts, something that is illegal in
Haskell 98 (Section 4.5.2, last sentence). With
-<option>-fglasgow-exts</option>
+<option>-XRelaxedPolyRec</option>
GHC only insists that the type signatures of a <emphasis>refined</emphasis> group have identical
type signatures; in practice this means that only variables bound by the same
pattern binding must have the same context. For example, this is fine:
<para>
GHC supports <emphasis>overloaded string literals</emphasis>. Normally a
string literal has type <literal>String</literal>, but with overloaded string
-literals enabled (with <literal>-foverloaded-strings</literal>)
+literals enabled (with <literal>-XOverloadedStrings</literal>)
a string literal has type <literal>(IsString a) => a</literal>.
</para>
<para>
from module <literal>GHC.Exts</literal>.
</para>
<para>
-Haskell's defaulting mechanism is extended to cover string literals, when <option>-foverloaded-strings</option> is specified.
+Haskell's defaulting mechanism is extended to cover string literals, when <option>-XOverloadedStrings</option> is specified.
Specifically:
<itemizedlist>
<listitem><para>
guide when it has stabilised.
</para>
<para>
-Type families are enabled by the flag <option>-ftype-families</option>.
+Type families are enabled by the flag <option>-XTypeFamilies</option>.
</para>
<para> Template Haskell has the following new syntactic
constructions. You need to use the flag
- <option>-fth</option><indexterm><primary><option>-fth</option></primary>
+ <option>-XTemplateHaskell</option>
+ <indexterm><primary><option>-XTemplateHaskell</option></primary>
</indexterm>to switch these syntactic extensions on
- (<option>-fth</option> is no longer implied by
+ (<option>-XTemplateHaskell</option> is no longer implied by
<option>-fglasgow-exts</option>).</para>
<itemizedlist>
the quotation has type <literal>Expr</literal>.</para></listitem>
<listitem><para> <literal>[d| ... |]</literal>, where the "..." is a list of top-level declarations;
the quotation has type <literal>Q [Dec]</literal>.</para></listitem>
- <listitem><para> [Planned, but not implemented yet.] <literal>[t| ... |]</literal>, where the "..." is a type;
+ <listitem><para> <literal>[t| ... |]</literal>, where the "..." is a type;
the quotation has type <literal>Type</literal>.</para></listitem>
</itemizedlist></para></listitem>
<para>Now run the compiler (here we are a Cygwin prompt on Windows):
</para>
<programlisting>
-$ ghc --make -fth main.hs -o main.exe
+$ ghc --make -XTemplateHaskell main.hs -o main.exe
</programlisting>
<para>Run "main.exe" and here is your output:</para>
</itemizedlist>
and the arrows web page at
<ulink url="http://www.haskell.org/arrows/"><literal>http://www.haskell.org/arrows/</literal></ulink>.
-With the <option>-farrows</option> flag, GHC supports the arrow
+With the <option>-XArrows</option> flag, GHC supports the arrow
notation described in the second of these papers.
What follows is a brief introduction to the notation;
it won't make much sense unless you've read Hughes's paper.
than the material below.
</para>
<para>
-Bang patterns are enabled by the flag <option>-fbang-patterns</option>.
+Bang patterns are enabled by the flag <option>-XBangPatterns</option>.
</para>
<sect2 id="bang-patterns-informal">
<itemizedlist>
<listitem>
<para>Use the flags <option>-fglasgow-exts</option> (to enable the extra syntax),
- <option>-fgenerics</option> (to generate extra per-data-type code),
+ <option>-XGenerics</option> (to generate extra per-data-type code),
and <option>-package lang</option> (to make the <literal>Generics</literal> library
available. </para>
</listitem>
<sect2>
<title>Switching off the dreaded Monomorphism Restriction</title>
- <indexterm><primary><option>-fno-monomorphism-restriction</option></primary></indexterm>
+ <indexterm><primary><option>-XNoMonomorphismRestriction</option></primary></indexterm>
<para>Haskell's monomorphism restriction (see
<ulink url="http://haskell.org/onlinereport/decls.html#sect4.5.5">Section
4.5.5</ulink>
of the Haskell Report)
can be completely switched off by
-<option>-fno-monomorphism-restriction</option>.
+<option>-XNoMonomorphismRestriction</option>.
</para>
</sect2>
<sect2>
<title>Monomorphic pattern bindings</title>
- <indexterm><primary><option>-fno-mono-pat-binds</option></primary></indexterm>
- <indexterm><primary><option>-fmono-pat-binds</option></primary></indexterm>
+ <indexterm><primary><option>-XNoMonoPatBinds</option></primary></indexterm>
+ <indexterm><primary><option>-XMonoPatBinds</option></primary></indexterm>
<para> As an experimental change, we are exploring the possibility of
making pattern bindings monomorphic; that is, not generalised at all.
[x] = e -- A pattern binding
</programlisting>
Experimentally, GHC now makes pattern bindings monomorphic <emphasis>by
-default</emphasis>. Use <option>-fno-mono-pat-binds</option> to recover the
+default</emphasis>. Use <option>-XMonoPatBinds</option> to recover the
standard behaviour.
</para>
</sect2>
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