</sect2>
+ <!-- ===================== n+k patterns =================== -->
+
+<sect2 id="n-k-patterns">
+<title>n+k patterns</title>
+<indexterm><primary><option>-XNoNPlusKPatterns</option></primary></indexterm>
+
+<para>
+<literal>n+k</literal> pattern support is enabled by default. To disable
+it, you can use the <option>-XNoNPlusKPatterns</option> flag.
+</para>
+
+</sect2>
+
<!-- ===================== Recursive do-notation =================== -->
<sect2 id="mdo-notation">
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
This reduces the clutter of qualified names when you import two
records from different modules that use the same field name.
</para>
+<para>
+Some details:
+<itemizedlist>
+<listitem><para>
+Field disambiguation can be combined with punning (see <xref linkend="record-puns"/>). For exampe:
+<programlisting>
+module Foo where
+ import M
+ x=True
+ ok3 (MkS { x }) = x+1 -- Uses both disambiguation and punning
+</programlisting>
+</para></listitem>
+
+<listitem><para>
+With <option>-XDisambiguateRecordFields</option> you can use <emphasis>unqualifed</emphasis>
+field names even if the correponding selector is only in scope <emphasis>qualified</emphasis>
+For example, assuming the same module <literal>M</literal> as in our earlier example, this is legal:
+<programlisting>
+module Foo where
+ import qualified M -- Note qualified
+
+ ok4 (M.MkS { x = n }) = n+1 -- Unambiguous
+</programlisting>
+Since the constructore <literal>MkS</literal> is only in scope qualified, you must
+name it <literal>M.MkS</literal>, but the field <literal>x</literal> does not need
+to be qualified even though <literal>M.x</literal> is in scope but <literal>x</literal>
+is not. (In effect, it is qualified by the constructor.)
+</para></listitem>
+</itemizedlist>
+</para>
+
</sect2>
<!-- ===================== Record puns =================== -->
</para>
<para>
-Note that puns and other patterns can be mixed in the same record:
-<programlisting>
-data C = C {a :: Int, b :: Int}
-f (C {a, b = 4}) = a
-</programlisting>
-and that puns can be used wherever record patterns occur (e.g. in
-<literal>let</literal> bindings or at the top-level).
-</para>
-
-<para>
+Note that:
+<itemizedlist>
+<listitem><para>
Record punning can also be used in an expression, writing, for example,
<programlisting>
let a = 1 in C {a}
<programlisting>
let a = 1 in C {a = a}
</programlisting>
-
-Note that this expansion is purely syntactic, so the record pun
+The expansion is purely syntactic, so the expanded right-hand side
expression refers to the nearest enclosing variable that is spelled the
same as the field name.
+</para></listitem>
+
+<listitem><para>
+Puns and other patterns can be mixed in the same record:
+<programlisting>
+data C = C {a :: Int, b :: Int}
+f (C {a, b = 4}) = a
+</programlisting>
+</para></listitem>
+
+<listitem><para>
+Puns can be used wherever record patterns occur (e.g. in
+<literal>let</literal> bindings or at the top-level).
+</para></listitem>
+
+<listitem><para>
+A pun on a qualified field name is expanded by stripping off the module qualifier.
+For example:
+<programlisting>
+f (C {M.a}) = a
+</programlisting>
+means
+<programlisting>
+f (M.C {M.a = a}) = a
+</programlisting>
+(This is useful if the field selector <literal>a</literal> for constructor <literal>M.C</literal>
+is only in scope in qualified form.)
+</para></listitem>
+</itemizedlist>
</para>
+
</sect2>
<!-- ===================== Record wildcards =================== -->
<para>
Record wildcards are enabled by the flag <literal>-XRecordWildCards</literal>.
+This flag implies <literal>-XDisambiguateRecordFields</literal>.
</para>
<para>
</para>
<para>
-Record wildcard syntax permits a (<literal>..</literal>) in a record
+Record wildcard syntax permits a "<literal>..</literal>" in a record
pattern, where each elided field <literal>f</literal> is replaced by the
pattern <literal>f = f</literal>. For example, the above pattern can be
written as
</para>
<para>
-Note that wildcards can be mixed with other patterns, including puns
+More details:
+<itemizedlist>
+<listitem><para>
+Wildcards can be mixed with other patterns, including puns
(<xref linkend="record-puns"/>); for example, in a pattern <literal>C {a
= 1, b, ..})</literal>. Additionally, record wildcards can be used
wherever record patterns occur, including in <literal>let</literal>
</programlisting>
defines <literal>b</literal>, <literal>c</literal>, and
<literal>d</literal>.
-</para>
+</para></listitem>
-<para>
+<listitem><para>
Record wildcards can also be used in expressions, writing, for example,
-
<programlisting>
let {a = 1; b = 2; c = 3; d = 4} in C {..}
</programlisting>
-
in place of
-
<programlisting>
let {a = 1; b = 2; c = 3; d = 4} in C {a=a, b=b, c=c, d=d}
</programlisting>
-
-Note that this expansion is purely syntactic, so the record wildcard
+The expansion is purely syntactic, so the record wildcard
expression refers to the nearest enclosing variables that are spelled
the same as the omitted field names.
+</para></listitem>
+
+<listitem><para>
+The "<literal>..</literal>" expands to the missing
+<emphasis>in-scope</emphasis> record fields, where "in scope"
+includes both unqualified and qualified-only.
+Any fields that are not in scope are not filled in. For example
+<programlisting>
+module M where
+ data R = R { a,b,c :: Int }
+module X where
+ import qualified M( R(a,b) )
+ f a b = R { .. }
+</programlisting>
+The <literal>{..}</literal> expands to <literal>{M.a=a,M.b=b}</literal>,
+omitting <literal>c</literal> since it is not in scope at all.
+</para></listitem>
+</itemizedlist>
</para>
</sect2>
<sect2 id="include-pragma">
<title>INCLUDE pragma</title>
- <para>The <literal>INCLUDE</literal> pragma is for specifying the names
- of C header files that should be <literal>#include</literal>'d into
- the C source code generated by the compiler for the current module (if
- compiling via C). For example:</para>
-
-<programlisting>
-{-# INCLUDE "foo.h" #-}
-{-# INCLUDE <stdio.h> #-}</programlisting>
-
- <para><literal>INCLUDE</literal> is a file-header pragma (see <xref linkend="pragmas"/>).</para>
-
- <para>An <literal>INCLUDE</literal> pragma is the preferred alternative
- to the <option>-#include</option> option (<xref
- linkend="options-C-compiler" />), because the
- <literal>INCLUDE</literal> pragma is understood by other
- compilers. Yet another alternative is to add the include file to each
- <literal>foreign import</literal> declaration in your code, but we
- don't recommend using this approach with GHC.</para>
+ <para>The <literal>INCLUDE</literal> used to be necessary for
+ specifying header files to be included when using the FFI and
+ compiling via C. It is no longer required for GHC, but is
+ accepted (and ignored) for compatibility with other
+ compilers.</para>
</sect2>
<sect2 id="warning-deprecated-pragma">
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