forked from lix-project/lix
* Document the new let.
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@ -118,6 +118,9 @@ irreversible.</para></warning>
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the availability of primop in a backwards-compatible
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way.</para></listitem>
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<listitem><para>Real let-expressions: <literal>let x = ...;
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... z = ...; in ...</literal>.</para></listitem>
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</itemizedlist>
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</para></listitem>
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@ -716,36 +716,27 @@ encountered</quote>).</para></footnote>.</para>
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</simplesect>
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<simplesect><title>Let expressions</title>
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<simplesect><title>Let-expressions</title>
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<para>A <literal>let</literal> expression is a simple short-hand for a
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<literal>rec</literal> expression followed by an attribute selection:
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<literal>let { <replaceable>attrs</replaceable> }</literal> translates
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to <literal>rec { <replaceable>attrs</replaceable>
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}.body</literal>.</para>
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<para>For instance,
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<para>A let-expression allows you define local
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variables for an expression. For instance,
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<programlisting>
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let {
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let
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x = "foo";
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y = "bar";
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body = x + y;
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}</programlisting>
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in x + y</programlisting>
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is equivalent to
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<programlisting>
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rec {
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x = "foo";
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y = "bar";
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body = x + y;
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}.body</programlisting>
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and evaluates to <literal>"foobar"</literal>.
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evaluates to <literal>"foobar"</literal>.
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</para>
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<note><para>There is also an obsolete form of let-expression,
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<literal>let { <replaceable>attrs</replaceable> }</literal>, which is
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translated to <literal>rec { <replaceable>attrs</replaceable>
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}.body</literal>. That is, the body of the let-expression is the
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<literal>body</literal> attribute of the attribute set.</para></note>
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</simplesect>
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@ -757,13 +748,13 @@ propagate attributes). This can be shortened using the
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<literal>inherit</literal> keyword. For instance,
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<programlisting>
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let {
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let
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x = 123;
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body = {
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in
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{
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inherit x;
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y = 456;
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};
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}</programlisting>
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}</programlisting>
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evaluates to <literal>{x = 123; y = 456;}</literal>. (Note that this
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works because <varname>x</varname> is added to the lexical scope by
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@ -819,10 +810,8 @@ function calls.</para>
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a name, you can bind them to an attribute, e.g.,
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<programlisting>
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let {
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concat = {x, y}: x + y;
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body = concat {x = "foo"; y = "bar";};
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}</programlisting>
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let concat = {x, y}: x + y;
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in concat {x = "foo"; y = "bar";}</programlisting>
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</para>
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@ -837,11 +826,9 @@ 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 {
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negate = x: !x;
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concat = x: y: x + y;
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body = if negate true then concat "foo" "bar" else "";
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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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@ -849,7 +836,7 @@ 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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map (concat "foo") ["bar", "bla", "abc"]</programlisting>
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evaluates to <literal>["foobar" "foobla" "fooabc"]</literal>.</para>
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@ -958,9 +945,9 @@ used in the Nix expression for Subversion.</para>
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<simplesect><title>With expressions</title>
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<simplesect><title>With-expressions</title>
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<para>A <emphasis>with</emphasis> expression,
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<para>A <emphasis>with-expression</emphasis>,
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<programlisting>
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with <replaceable>e1</replaceable>; <replaceable>e2</replaceable></programlisting>
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@ -970,11 +957,8 @@ lexical scope of the expression <replaceable>e2</replaceable>. For
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instance,
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<programlisting>
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let {
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as = {x = "foo"; y = "bar";};
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body = with as; x + y;
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}</programlisting>
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let as = {x = "foo"; y = "bar";};
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in with as; x + y</programlisting>
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evaluates to <literal>"foobar"</literal> since the
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<literal>with</literal> adds the <varname>x</varname> and
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