<tbody>
<row>
<entry><option>-fallow-overlapping-instances</option></entry>
- <entry>Enable overlapping instances</entry>
+ <entry>Enable <link linkend="instance-overlap">overlapping instances</link></entry>
<entry>dynamic</entry>
<entry><option>-fno-allow-overlapping-instances</option></entry>
</row>
<row>
- <entry><option>-fallow-undecidable-instances</option></entry>
- <entry>Enable undecidable instances</entry>
- <entry>dynamic</entry>
- <entry><option>-fno-allow-undecidable-instances</option></entry>
- </row>
- <row>
<entry><option>-fallow-incoherent-instances</option></entry>
- <entry>Enable incoherent instances.
+ <entry>Enable <link linkend="instance-overlap">incoherent instances</link>.
Implies <option>-fallow-overlapping-instances</option> </entry>
<entry>dynamic</entry>
<entry><option>-fno-allow-incoherent-instances</option></entry>
</row>
<row>
- <entry><option>-farrows</option></entry>
- <entry>Enable arrow notation extension</entry>
+ <entry><option>-fallow-undecidable-instances</option></entry>
+ <entry>Enable <link linkend="undecidable-instances">undecidable instances</link></entry>
<entry>dynamic</entry>
- <entry><option>-fno-arrows</option></entry>
+ <entry><option>-fno-allow-undecidable-instances</option></entry>
</row>
<row>
<entry><option>-fcontext-stack=N</option><replaceable>n</replaceable></entry>
- <entry>set the limit for context reduction</entry>
+ <entry>set the <link linkend="undecidable-instances">limit for context reduction</link></entry>
<entry>dynamic</entry>
<entry><option>20</option></entry>
</row>
<row>
+ <entry><option>-farrows</option></entry>
+ <entry>Enable <link linkend="arrow-notation">arrow
+ notation</link> extension</entry>
+ <entry>dynamic</entry>
+ <entry><option>-fno-arrows</option></entry>
+ </row>
+ <row>
<entry><option>-ffi</option> or <option>-fffi</option></entry>
- <entry>Enable foreign function interface (implied by
+ <entry>Enable <link linkend="ffi">foreign function interface</link> (implied by
<option>-fglasgow-exts</option>)</entry>
<entry>dynamic</entry>
<entry><option>-fno-ffi</option></entry>
</row>
<row>
<entry><option>-fgenerics</option></entry>
- <entry>Enable generics</entry>
+ <entry>Enable <link linkend="generic-classes">generic classes</link></entry>
<entry>dynamic</entry>
<entry><option>-fno-fgenerics</option></entry>
</row>
</row>
<row>
<entry><option>-fimplicit-params</option></entry>
- <entry>Enable Implicit Parameters.
+ <entry>Enable <link linkend="implicit-parameters">Implicit Parameters</link>.
Implied by <option>-fglasgow-exts</option>.</entry>
<entry>dynamic</entry>
<entry><option>-fno-implicit-params</option></entry>
</row>
<row>
<entry><option>-fno-monomorphism-restriction</option></entry>
- <entry>Disable the monomorphism restriction</entry>
+ <entry>Disable the <link linkend="monomorphism">monomorphism restriction</link></entry>
<entry>dynamic</entry>
<entry><option>-fmonomorphism-restriction</option></entry>
</row>
<row>
<entry><option>-fno-mono-pat-binds</option></entry>
- <entry>Make pattern bindings polymorphic</entry>
+ <entry>Make <link linkend="monomorphism">pattern bindings polymorphic</link></entry>
<entry>dynamic</entry>
<entry><option>-fmono-pat-binds</option></entry>
</row>
<row>
<entry><option>-fextended-default-rules</option></entry>
- <entry>Use GHCi's extended default rules in a normal module</entry>
+ <entry>Use GHCi's <link linkend="extended-default-rules">extended default rules</link> in a normal module</entry>
<entry>dynamic</entry>
<entry><option>-fno-extended-default-rules</option></entry>
</row>
<row>
<entry><option>-fscoped-type-variables</option></entry>
- <entry>Enable lexically-scoped type variables.
+ <entry>Enable <link linkend="scoped-type-variables">lexically-scoped type variables</link>.
Implied by <option>-fglasgow-exts</option>.</entry>
<entry>dynamic</entry>
<entry><option>-fno-scoped-type-variables</option></entry>
</row>
<row>
<entry><option>-fth</option></entry>
- <entry>Enable Template Haskell.
+ <entry>Enable <link linkend="template-haskell">Template Haskell</link>.
No longer implied by <option>-fglasgow-exts</option>.</entry>
<entry>dynamic</entry>
<entry><option>-fno-th</option></entry>
</row>
+ <row>
+ <entry><option>-fbang-patterns</option></entry>
+ <entry>Enable <link linkend="sec-bang-patterns">bang patterns</link>.</entry>
+ <entry>dynamic</entry>
+ <entry><option>-fno-bang-patterns</option></entry>
+ </row>
</tbody>
</tgroup>
</informaltable>
<varlistentry>
<term>
- <option>-fno-monomorphism-restriction</option>:
- <indexterm><primary><option>-fno-monomorphism-restriction</option></primary></indexterm>
- </term>
- <listitem>
- <para> Switch off the Haskell 98 monomorphism restriction.
- Independent of the <option>-fglasgow-exts</option>
- flag. </para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term>
- <option>-fno-mono-pat-binds</option>:
- <indexterm><primary><option>-fno-mono-pat-binds</option></primary></indexterm>
- <indexterm><primary><option>-fmono-pat-binds</option></primary></indexterm>
+ <option>-fno-monomorphism-restriction</option>,<option>-fno-monomorphism-restriction</option>:
</term>
<listitem>
- <para> As an experimental change, we are exploring the possibility of
- making pattern bindings monomorphic; that is, not generalised at all.
- A pattern binding is a binding whose LHS has no function arguments,
- and is not a simple variable. For example:
-<programlisting>
- f x = x -- Not a pattern binding
- f = \x -> x -- Not a pattern binding
- f :: Int -> Int = \x -> x -- Not a pattern binding
-
- (g,h) = e -- A pattern binding
- (f) = e -- A pattern binding
- [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
-standard behaviour.
+ <para> These two flags control how generalisation is done in
+ See <xlink linkend="monomorphism"/>.
</para>
</listitem>
</varlistentry>
</sect1>
+<!-- ==================== BANG PATTERNS ================= -->
+
+<sect1 id="sec-bang-patterns">
+<title>Bang patterns
+<indexterm><primary>Bang patterns</primary></indexterm>
+</title>
+<para>GHC supports an extension of pattern matching called <emphasis>bang
+patterns</emphasis>. Bang patterns are under consideration for Haskell Prime.
+The <ulink
+url="http://hackage.haskell.org/trac/haskell-prime/wiki/BangPatterns">the
+Haskell prime feature description</ulink> contains more discussion and examples
+than the material below.
+</para>
+<para>
+Bang patterns are enabled by the flag <option>-fbang-patterns</option>.
+</para>
+
+<sect2 id="sec-bang-patterns-informal">
+<title>Informal description of bang patterns
+</title>
+<para>
+The main idea is to add a single new production to the syntax of patterns:
+<programlisting>
+ pat ::= !pat
+</programlisting>
+Matching an expression <literal>e</literal> against a pattern <literal>!p</literal> is done by first
+evaluating <literal>e</literal> (to WHNF) and then matching the result against <literal>p</literal>.
+Example:
+<programlisting>
+f1 !x = True
+</programlisting>
+This definition makes <literal>f1</literal> is strict in <literal>x</literal>,
+whereas without the bang it would be lazy.
+Bang patterns can be nested of course:
+<programlisting>
+f2 (!x, y) = [x,y]
+</programlisting>
+Here, <literal>f2</literal> is strict in <literal>x</literal> but not in
+<literal>y</literal>.
+A bang only really has an effect if it precedes a variable or wild-card pattern:
+<programlisting>
+f3 !(x,y) = [x,y]
+f4 (x,y) = [x,y]
+</programlisting>
+Here, <literal>f3</literal> and <literal>f4</literal> are identical; putting a bang before a pattern that
+forces evaluation anyway does nothing.
+</para><para>
+Bang patterns work in <literal>case</literal> expressions too, of course:
+<programlisting>
+g5 x = let y = f x in body
+g6 x = case f x of { y -> body }
+g7 x = case f x of { !y -> body }
+</programlisting>
+The functions <literal>g5</literal> and <literal>g6</literal> mean exactly the same thing.
+But <literal>g7</literal> evalutes <literal>(f x)</literal>, binds <literal>y</literal> to the
+result, and then evaluates <literal>body</literal>.
+</para><para>
+Bang patterns work in <literal>let</literal> and <literal>where</literal>
+definitions too. For example:
+<programlisting>
+let ![x,y] = e in b
+</programlisting>
+is a strict pattern: operationally, it evaluates <literal>e</literal>, matches
+it against the pattern <literal>[x,y]</literal>, and then evaluates <literal>b</literal>
+The "<literal>!</literal>" should not be regarded as part of the pattern; after all,
+in a function argument <literal>![x,y]</literal> means the
+same as <literal>[x,y]</literal>. Rather, the "<literal>!</literal>"
+is part of the syntax of <literal>let</literal> bindings.
+</para>
+</sect2>
+
+
+<sect2 id="sec-bang-patterns-sem">
+<title>Syntax and semantics
+</title>
+<para>
+
+We add a single new production to the syntax of patterns:
+<programlisting>
+ pat ::= !pat
+</programlisting>
+There is one problem with syntactic ambiguity. Consider:
+<programlisting>
+f !x = 3
+</programlisting>
+Is this a definition of the infix function "<literal>(!)</literal>",
+or of the "<literal>f</literal>" with a bang pattern? GHC resolves this
+ambiguity inf favour of the latter. If you want to define
+<literal>(!)</literal> with bang-patterns enabled, you have to do so using
+prefix notation:
+<programlisting>
+(!) f x = 3
+</programlisting>
+The semantics of Haskell pattern matching is described in <ulink
+url="http://haskell.org/onlinereport/exps.html#sect3.17.2">
+Section 3.17.2</ulink> of the Haskell Report. To this description add
+one extra item 10, saying:
+<itemizedlist><listitem><para>Matching
+the pattern <literal>!pat</literal> against a value <literal>v</literal> behaves as follows:
+<itemizedlist><listitem><para>if <literal>v</literal> is bottom, the match diverges</para></listitem>
+ <listitem><para>otherwise, <literal>pat</literal> is matched against
+ <literal>v</literal></para></listitem>
+</itemizedlist>
+</para></listitem></itemizedlist>
+Similarly, in Figure 4 of <ulink url="http://haskell.org/onlinereport/exps.html#sect3.17.3">
+Section 3.17.3</ulink>, add a new case (t):
+<programlisting>
+case v of { !pat -> e; _ -> e' }
+ = v `seq` case v of { pat -> e; _ -> e' }
+</programlisting>
+</para><para>
+That leaves let expressions, whose translation is given in
+<ulink url="http://haskell.org/onlinereport/exps.html#sect3.12">Section
+3.12</ulink>
+of the Haskell Report.
+In the translation box, first apply
+the following transformation: for each pattern <literal>pi</literal> that is of
+form <literal>!qi = ei</literal>, transform it to <literal>(xi,!qi) = ((),ei)</literal>, and and replace <literal>e0</literal>
+by <literal>(xi `seq` e0)</literal>. Then, when none of the left-hand-side patterns
+have a bang at the top, apply the rules in the existing box.
+</para>
+<para>The effect of the let rule is to force complete matching of the pattern
+<literal>qi</literal> before evaluation of the body is begun. The bang is
+retained in the translated form in case <literal>qi</literal> is a variable,
+thus:
+<programlisting>
+ let !y = f x in b
+</programlisting>
+
+</para>
+<para>
+The let-binding can be recursive. However, it is much more common for
+the let-binding to be non-recursive, in which case the following law holds:
+<literal>(let !p = rhs in body)</literal>
+ is equivalent to
+<literal>(case rhs of !p -> body)</literal>
+</para>
+<para>
+A pattern with a bang at the outermost level is not allowed at the top level of
+a module.
+</para>
+</sect2>
+</sect1>
+
<!-- ==================== ASSERTIONS ================= -->
<sect1 id="sec-assertions">
</sect2>
</sect1>
+<sect1 id="monomorphism">
+<title>Control over monomorphism</title>
+
+<para>GHC supports two flags that control the way in which generalisation is
+carried out at let and where bindings.
+</para>
+
+<sect2>
+<title>Switching off the dreaded Monomorphism Restriction</title>
+ <indexterm><primary><option>-fno-monomorphism-restriction</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>.
+</para>
+</sect2>
+
+<sect2>
+<title>Monomorphic patteern bindings</title>
+ <indexterm><primary><option>-fno-mono-pat-binds</option></primary></indexterm>
+ <indexterm><primary><option>-fmono-pat-binds</option></primary></indexterm>
+
+ <para> As an experimental change, we are exploring the possibility of
+ making pattern bindings monomorphic; that is, not generalised at all.
+ A pattern binding is a binding whose LHS has no function arguments,
+ and is not a simple variable. For example:
+<programlisting>
+ f x = x -- Not a pattern binding
+ f = \x -> x -- Not a pattern binding
+ f :: Int -> Int = \x -> x -- Not a pattern binding
+
+ (g,h) = e -- A pattern binding
+ (f) = e -- A pattern binding
+ [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
+standard behaviour.
+</para>
+</sect2>
+</sect1>
+
<!-- Emacs stuff:
projects / ghc-hetmet.git / commitdiff