diff options
| -rw-r--r-- | docs/users_guide/glasgow_exts.xml | 16 |
1 files changed, 13 insertions, 3 deletions
diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index a6c43b8e78..befaf4d33c 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -2325,7 +2325,7 @@ f (C {a = 1, ..}) = b + c + d More details: <itemizedlist> <listitem><para> -Wildcards can be mixed with other patterns, including puns +Record wildcards in patterns 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> @@ -2338,7 +2338,7 @@ defines <literal>b</literal>, <literal>c</literal>, and </para></listitem> <listitem><para> -Record wildcards can also be used in expressions, writing, for example, +Record wildcards can also be used in an expression, when constructing a record. For example, <programlisting> let {a = 1; b = 2; c = 3; d = 4} in C {..} </programlisting> @@ -2352,7 +2352,15 @@ the same as the omitted field names. </para></listitem> <listitem><para> -The "<literal>..</literal>" expands to the missing +Record wildcards may <emphasis>not</emphasis> be used in record <emphasis>updates</emphasis>. For example this +is illegal: +<programlisting> +f r = r { x = 3, .. } +</programlisting> +</para></listitem> + +<listitem><para> +For both pattern and expression wildcards, the "<literal>..</literal>" expands to the missing <emphasis>in-scope</emphasis> record fields. Specifically the expansion of "<literal>C {..}</literal>" includes <literal>f</literal> if and only if: @@ -2369,6 +2377,8 @@ the variable <literal>f</literal> is in scope unqualified, apart from the binding of the record selector itself. </para></listitem> </itemizedlist> +These rules restrict record wildcards to the situations in which the user +could have written the expanded version. For example <programlisting> module M where |