diff options
| -rw-r--r-- | doc/manual/release-notes.xml | 3 | ||||
| -rw-r--r-- | doc/manual/writing-nix-expressions.xml | 68 |
2 files changed, 29 insertions, 42 deletions
diff --git a/doc/manual/release-notes.xml b/doc/manual/release-notes.xml index cc04a5d808bb..a4a4f5034407 100644 --- a/doc/manual/release-notes.xml +++ b/doc/manual/release-notes.xml @@ -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> diff --git a/doc/manual/writing-nix-expressions.xml b/doc/manual/writing-nix-expressions.xml index ea719190f375..74ef3a868e77 100644 --- a/doc/manual/writing-nix-expressions.xml +++ b/doc/manual/writing-nix-expressions.xml @@ -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, - -<programlisting> -let { - x = "foo"; - y = "bar"; - body = x + y; -}</programlisting> - -is equivalent to +<para>A let-expression allows you define local +variables for an expression. For instance, <programlisting> -rec { +let x = "foo"; y = "bar"; - body = x + y; -}.body</programlisting> +in x + y</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 |