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Merge pull request #2880 from Ma27/document-optional-attrs-with-at-pattern

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doc: clarify that optional attrs in a function argument will be ignored unless specified
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Eelco Dolstra 2019-05-22 14:14:56 +02:00 committed by GitHub
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20
doc/manual/expressions/language-constructs.xml
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@ -217,7 +217,25 @@ but can also be written as:
ellipsis(<literal>...</literal>) as you can access attribute names as
<literal>a</literal>, using <literal>args.a</literal>, which was given as an
additional attribute to the function.
</para></listitem>
</para>
<warning>
<para>
The <literal>args@</literal> expression is bound to the argument passed to the function which
means that attributes with defaults that aren't explicitly specified in the function call
won't cause an evaluation error, but won't exist in <literal>args</literal>.
</para>
<para>
For instance
<programlisting>
let
function = args@{ a ? 23, ... }: args;
in
function {}
</programlisting>
will evaluate to an empty attribute set.
</para>
</warning></listitem>
</itemizedlist>