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* Patterns.
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@ -937,17 +937,84 @@ set.</para>
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<para>Functions have the following form:
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<programlisting>
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{<replaceable>params</replaceable>}: <replaceable>body</replaceable></programlisting>
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<replaceable>pattern</replaceable>: <replaceable>body</replaceable></programlisting>
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This defines a function that must be called with an attribute set
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containing the attributes listed in <replaceable>params</replaceable>,
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which is a comma-separated list of attribute names. Optionally, for
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each parameter a <emphasis>default value</emphasis> may be specified
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by writing <literal><replaceable>param</replaceable> ?
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<replaceable>e</replaceable></literal>, where
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<replaceable>e</replaceable> is an arbitrary expression. If a
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parameter has a default, the corresponding attribute may be omitted in
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function calls.</para>
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The pattern specifies what the argument of the function must look
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like, and binds variables in the body to (parts of) the
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argument. There are three kinds of patterns:</para>
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<itemizedlist>
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<listitem><para>If a pattern is a single identifier, then the
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function matches any argument. Example:
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<programlisting>
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let negate = x: !x;
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concat = x: y: x + y;
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in if negate true then concat "foo" "bar" else ""</programlisting>
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Note that <function>concat</function> is a function that takes one
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argument and returns a function that takes another argument. This
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allows partial parameterisation (i.e., only filling some of the
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arguments of a function); e.g.,
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<programlisting>
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map (concat "foo") ["bar" "bla" "abc"]</programlisting>
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evaluates to <literal>["foobar" "foobla"
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"fooabc"]</literal>.</para></listitem>
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<listitem><para>An <emphasis>attribute set pattern</emphasis> of the
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form <literal>{name1, name2, …, nameN}</literal>
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matches an attribute set containing the listed attributes, and binds
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the values of those attributes to variables in the function body.
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For example, the function
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<programlisting>
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{x, y, z}: z + y + x</programlisting>
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can only be called with a set containing exactly the attributes
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<varname>x</varname>, <varname>y</varname> and
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<varname>z</varname>. No other attributes are allowed. If you want
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to allow additional arguments, you can use an ellipsis
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(<literal>...</literal>):
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<programlisting>
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{x, y, z, ....}: z + y + x</programlisting>
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This works on any set that contains at least the three named
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attributes.</para>
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<para>It is possible to provide <emphasis>default values</emphasis>
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for attributes, in which case they are allowed to be missing. A
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default value is specified by writing
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<literal><replaceable>name</replaceable> ?
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<replaceable>e</replaceable></literal>, where
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<replaceable>e</replaceable> is an arbitrary expression. For example,
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<programlisting>
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{x, y ? "foo", z ? "bar"}: z + y + x</programlisting>
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specifies a function that only requires an attribute named
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<varname>x</varname>, but optionally accepts <varname>y</varname>
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and <varname>z</varname>.</para></listitem>
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<listitem><para>An <literal>@</literal>-pattern requires that the
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argument matches with the patterns on the left- and right-hand side
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of the <literal>@</literal>-sign. For example:
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<programlisting>
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args@{x, y, z, ...}: z + y + x + args.a</programlisting>
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Here <varname>args</varname> is bound to the entire argument, which
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is further matches against the pattern <literal>{x, y, z,
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...}</literal>.</para></listitem>
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</itemizedlist>
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<para>Note that functions do not have names. If you want to give them
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a name, you can bind them to an attribute, e.g.,
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@ -958,31 +1025,6 @@ in concat {x = "foo"; y = "bar";}</programlisting>
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</para>
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<para>It is also possible to define a function that takes a single
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argument and that does not need to be called with an attribute set as
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argument. The syntax is
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<programlisting>
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<replaceable>var</replaceable>: <replaceable>body</replaceable></programlisting>
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where <replaceable>var</replaceable> is the name of the argument. It
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is not possible to define a default. Example:
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<programlisting>
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let negate = x: !x;
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concat = x: y: x + y;
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in if negate true then concat "foo" "bar" else ""</programlisting>
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Note that <function>concat</function> is a function that takes one
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arguments and returns a function that takes another argument. This
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allows partial parameterisation (i.e., only filling some of the
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arguments of a function); e.g.,
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<programlisting>
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map (concat "foo") ["bar" "bla" "abc"]</programlisting>
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evaluates to <literal>["foobar" "foobla" "fooabc"]</literal>.</para>
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</simplesect>
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