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* Document the new let.

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This commit is contained in:
Eelco Dolstra 2006-10-02 16:14:30 +00:00
parent ac19b333b3
commit 853252ac66
2 changed files with 29 additions and 42 deletions
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3
doc/manual/release-notes.xml
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@ -118,6 +118,9 @@ irreversible.</para></warning>
the availability of primop in a backwards-compatible
way.</para></listitem>
<listitem><para>Real let-expressions: <literal>let x = ...;
... z = ...; in ...</literal>.</para></listitem>
</itemizedlist>
</para></listitem>

68
doc/manual/writing-nix-expressions.xml
View File

@ -716,36 +716,27 @@ encountered</quote>).</para></footnote>.</para>
</simplesect>
<simplesect><title>Let expressions</title>
<simplesect><title>Let-expressions</title>
<para>A <literal>let</literal> expression is a simple short-hand for a
<literal>rec</literal> expression followed by an attribute selection:
<literal>let { <replaceable>attrs</replaceable> }</literal> translates
to <literal>rec { <replaceable>attrs</replaceable>
}.body</literal>.</para>
<para>For instance,
<para>A let-expression allows you define local
variables for an expression. For instance,
<programlisting>
let {
let
x = "foo";
y = "bar";
body = x + y;
}</programlisting>
in x + y</programlisting>
is equivalent to
<programlisting>
rec {
x = "foo";
y = "bar";
body = x + y;
}.body</programlisting>
and evaluates to <literal>"foobar"</literal>.
evaluates to <literal>"foobar"</literal>.
</para>
<note><para>There is also an obsolete form of let-expression,
<literal>let { <replaceable>attrs</replaceable> }</literal>, which is
translated to <literal>rec { <replaceable>attrs</replaceable>
}.body</literal>. That is, the body of the let-expression is the
<literal>body</literal> attribute of the attribute set.</para></note>
</simplesect>
@ -757,13 +748,13 @@ propagate attributes). This can be shortened using the
<literal>inherit</literal> keyword. For instance,
<programlisting>
let {
let
x = 123;
body = {
in
{
inherit x;
y = 456;
};
}</programlisting>
}</programlisting>
evaluates to <literal>{x = 123; y = 456;}</literal>. (Note that this
works because <varname>x</varname> is added to the lexical scope by
@ -819,10 +810,8 @@ function calls.</para>
a name, you can bind them to an attribute, e.g.,
<programlisting>
let {
concat = {x, y}: x + y;
body = concat {x = "foo"; y = "bar";};
}</programlisting>
let concat = {x, y}: x + y;
in concat {x = "foo"; y = "bar";}</programlisting>
</para>
@ -837,11 +826,9 @@ where <replaceable>var</replaceable> is the name of the argument. It
is not possible to define a default. Example:
<programlisting>
let {
negate = x: !x;
concat = x: y: x + y;
body = if negate true then concat "foo" "bar" else "";
}</programlisting>
let negate = x: !x;
concat = x: y: x + y;
in if negate true then concat "foo" "bar" else ""</programlisting>
Note that <function>concat</function> is a function that takes one
arguments and returns a function that takes another argument. This
@ -849,7 +836,7 @@ allows partial parameterisation (i.e., only filling some of the
arguments of a function); e.g.,
<programlisting>
map (concat "foo") ["bar", "bla", "abc"]</programlisting>
map (concat "foo") ["bar", "bla", "abc"]</programlisting>
evaluates to <literal>["foobar" "foobla" "fooabc"]</literal>.</para>
@ -958,9 +945,9 @@ used in the Nix expression for Subversion.</para>
<simplesect><title>With expressions</title>
<simplesect><title>With-expressions</title>
<para>A <emphasis>with</emphasis> expression,
<para>A <emphasis>with-expression</emphasis>,
<programlisting>
with <replaceable>e1</replaceable>; <replaceable>e2</replaceable></programlisting>
@ -970,11 +957,8 @@ lexical scope of the expression <replaceable>e2</replaceable>. For
instance,
<programlisting>
let {
as = {x = "foo"; y = "bar";};
body = with as; x + y;
}</programlisting>
let as = {x = "foo"; y = "bar";};
in with as; x + y</programlisting>
evaluates to <literal>"foobar"</literal> since the
<literal>with</literal> adds the <varname>x</varname> and