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Eelco Dolstra 2012-05-11 17:39:06 -04:00
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99
doc/manual/writing-nix-expressions.xml
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@ -11,6 +11,13 @@ the things that tell Nix how to build packages. It starts with a
simple example (a Nix expression for GNU Hello), and then moves
on to a more in-depth look at the Nix expression language.</para>
<note><para>This chapter is mostly about the Nix expression language.
For more extensive information on adding packages to the Nix Packages
collection (such as functions in the standard environment and coding
conventions), please consult <link
xlink:href="http://hydra.nixos.org/job/nixpkgs/trunk/tarball/latest/download-by-type/doc/manual">its
manual</link>.</para></note>
<section><title>A simple Nix expression</title>
@ -315,15 +322,15 @@ error check.</para>
rec { <co xml:id='ex-hello-composition-co-1' />
hello = (import ../applications/misc/hello/ex-1 <co xml:id='ex-hello-composition-co-2' />) { <co xml:id='ex-hello-composition-co-3' />
hello = import ../applications/misc/hello/ex-1 <co xml:id='ex-hello-composition-co-2' /> { <co xml:id='ex-hello-composition-co-3' />
inherit fetchurl stdenv perl;
};
perl = (import ../development/interpreters/perl) { <co xml:id='ex-hello-composition-co-4' />
perl = import ../development/interpreters/perl { <co xml:id='ex-hello-composition-co-4' />
inherit fetchurl stdenv;
};
fetchurl = (import ../build-support/fetchurl) {
fetchurl = import ../build-support/fetchurl {
inherit stdenv; ...
};
@ -390,6 +397,23 @@ some fragments of
<varname>stdenv.mkDerivation</varname> in <xref
linkend='ex-hello-nix' />).</para>
<note><para>Nixpkgs has a convenience function
<function>callPackage</function> that imports and calls a
function, filling in any missing arguments by passing the
corresponding attribute from the Nixpkgs set, like this:
<programlisting>
hello = callPackage ../applications/misc/hello/ex-1 { };
</programlisting>
If necessary, you can set or override arguments:
<programlisting>
hello = callPackage ../applications/misc/hello/ex-1 { stdenv = myStdenv; };
</programlisting>
</para></note>
</callout>
<callout arearefs='ex-hello-composition-co-4'>
@ -813,7 +837,23 @@ occur once.</para>
<programlisting>
{ a = "Foo"; b = "Bar"; }.a</programlisting>
evaluates to <literal>"Foo"</literal>.</para>
evaluates to <literal>"Foo"</literal>. It is possible to provide a
default value in an attribute selection using the
<literal>or</literal> keyword. For example,
<programlisting>
{ a = "Foo"; b = "Bar"; }.c or "Xyzzy"</programlisting>
will evaluate to <literal>"Xyzzy"</literal> because there is no
<varname>c</varname> attribute in the set.</para>
<para>You can use arbitrary string constants as attribute names by
enclosing them in quotes:
<programlisting>
{ "foo bar" = 123; "nix-1.0" = 456; }."foo bar" </programlisting>
This will evaluate to <literal>123</literal>.</para>
</simplesect>
@ -1189,12 +1229,17 @@ weakest binding).</para>
</thead>
<tbody>
<row>
<entry><replaceable>e</replaceable> .
<replaceable>id</replaceable></entry>
<entry><replaceable>e</replaceable> <literal>.</literal>
<replaceable>attrpath</replaceable>
[ <literal>or</literal> <replaceable>def</replaceable> ]
</entry>
<entry>none</entry>
<entry>Select attribute named <replaceable>id</replaceable>
from attribute set <replaceable>e</replaceable>. Abort
evaluation if the attribute doesn’t exist.</entry>
<entry>Select attribute denoted by the attribute path
<replaceable>attrpath</replaceable> from attribute set
<replaceable>e</replaceable>. (An attribute path is a
dot-separated list of attribute names.) If the attribute
doesn’t exist, return <replaceable>def</replaceable> if
provided, otherwise abort evaluation.</entry>
</row>
<row>
<entry><replaceable>e1</replaceable> <replaceable>e2</replaceable></entry>
@ -1203,31 +1248,31 @@ weakest binding).</para>
argument <replaceable>e2</replaceable>.</entry>
</row>
<row>
<entry><replaceable>e</replaceable> ?
<replaceable>id</replaceable></entry>
<entry><replaceable>e</replaceable> <literal>?</literal>
<replaceable>attrpath</replaceable></entry>
<entry>none</entry>
<entry>Test whether attribute set <replaceable>e</replaceable>
contains an attribute named <replaceable>id</replaceable>;
contains the attribute denoted by <replaceable>attrpath</replaceable>;
return <literal>true</literal> or
<literal>false</literal>.</entry>
</row>
<row>
<entry><replaceable>e1</replaceable> ++ <replaceable>e2</replaceable></entry>
<entry><replaceable>e1</replaceable> <literal>++</literal> <replaceable>e2</replaceable></entry>
<entry>right</entry>
<entry>List concatenation.</entry>
</row>
<row>
<entry><replaceable>e1</replaceable> + <replaceable>e2</replaceable></entry>
<entry><replaceable>e1</replaceable> <literal>+</literal> <replaceable>e2</replaceable></entry>
<entry>left</entry>
<entry>String or path concatenation.</entry>
</row>
<row>
<entry>! <replaceable>e</replaceable></entry>
<entry><literal>!</literal> <replaceable>e</replaceable></entry>
<entry>left</entry>
<entry>Boolean negation.</entry>
</row>
<row>
<entry><replaceable>e1</replaceable> //
<entry><replaceable>e1</replaceable> <literal>//</literal>
<replaceable>e2</replaceable></entry>
<entry>right</entry>
<entry>Return an attribute set consisting of the attributes in
@ -1236,31 +1281,31 @@ weakest binding).</para>
precedence over the former in case of equally named attributes).</entry>
</row>
<row>
<entry><replaceable>e1</replaceable> ==
<entry><replaceable>e1</replaceable> <literal>==</literal>
<replaceable>e2</replaceable></entry>
<entry>none</entry>
<entry>Equality.</entry>
</row>
<row>
<entry><replaceable>e1</replaceable> !=
<entry><replaceable>e1</replaceable> <literal>!=</literal>
<replaceable>e2</replaceable></entry>
<entry>none</entry>
<entry>Inequality.</entry>
</row>
<row>
<entry><replaceable>e1</replaceable> &amp;&amp;
<entry><replaceable>e1</replaceable> <literal>&amp;&amp;</literal>
<replaceable>e2</replaceable></entry>
<entry>left</entry>
<entry>Logical AND.</entry>
</row>
<row>
<entry><replaceable>e1</replaceable> ||
<entry><replaceable>e1</replaceable> <literal>||</literal>
<replaceable>e2</replaceable></entry>
<entry>left</entry>
<entry>Logical OR.</entry>
</row>
<row>
<entry><replaceable>e1</replaceable> ->
<entry><replaceable>e1</replaceable> <literal>-></literal>
<replaceable>e2</replaceable></entry>
<entry>none</entry>
<entry>Logical implication (equivalent to
@ -1662,7 +1707,17 @@ impureEnvVars = [ "http_proxy" "https_proxy" <replaceable>...</replaceable> ];
</varlistentry>
<varlistentry><term><varname>preferLocalBuild</varname></term>
<listitem><para>If this attribute is set to
<literal>true</literal> and <link
linkend="chap-distributed-builds">distributed building is
enabled</link>, then, if possible, perform this build locally
instead of forwarding it to a remote machine. This is appropriate
for trivial builders where the cost of doing a remote build would
exceed the cost of building locally.</para></listitem>
</varlistentry>
</variablelist>