+<!-- ==================== BANG PATTERNS ================= -->
+
+<sect1 id="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">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="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="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 in 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>
+