* Move list of built-in functions to a separate file.
This commit is contained in:
Eelco Dolstra
parent
1db187ff69
commit
35ac16e75e
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@ -20,7 +20,7 @@ man1_MANS = nix-env.1 nix-build.1 nix-store.1 nix-instantiate.1 \
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FIGURES = figures/user-environments.png
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MANUAL_SRCS = manual.xml introduction.xml installation.xml \
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package-management.xml writing-nix-expressions.xml \
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package-management.xml writing-nix-expressions.xml builtins.xml \
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build-farm.xml \
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$(man1_MANS:.1=.xml) \
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troubleshooting.xml bugs.xml opt-common.xml opt-common-syn.xml \
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588
doc/manual/builtins.xml
Normal file
588
doc/manual/builtins.xml
Normal file
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@ -0,0 +1,588 @@
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<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink">
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<title>Built-in functions</title>
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<para>This section lists the functions and constants built into the
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Nix expression evaluator. (The built-in function
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<function>derivation</function> is discussed above.) Some built-ins,
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such as <function>derivation</function>, are always in scope of every
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Nix expression; you can just access them right away. But to prevent
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polluting the namespace too much, most built-ins are not in scope.
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Instead, you can access them through the <varname>builtins</varname>
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built-in value, which is an attribute set that contains all built-in
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functions and values. For instance, <function>derivation</function>
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is also available as <function>builtins.derivation</function>.</para>
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<variablelist >
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<varlistentry><term><function>abort</function> <replaceable>s</replaceable></term>
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<listitem><para>Abort Nix expression evaluation, print error
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message <replaceable>s</replaceable>.</para></listitem>
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</varlistentry>
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<varlistentry><term><function>builtins.add</function>
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<replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
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<listitem><para>Return the sum of the integers
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<replaceable>e1</replaceable> and
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<replaceable>e2</replaceable>.</para></listitem>
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</varlistentry>
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<varlistentry><term><function>builtins.attrNames</function>
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<replaceable>attrs</replaceable></term>
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<listitem><para>Return the names of the attributes in the
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attribute set <replaceable>attrs</replaceable> in a sorted list.
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For instance, <literal>builtins.attrNames {y = 1; x =
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"foo";}</literal> evaluates to <literal>["x" "y"]</literal>.
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There is no built-in function <function>attrValues</function>, but
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you can easily define it yourself:
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<programlisting>
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attrValues = attrs: map (name: builtins.getAttr name attrs) (builtins.attrNames attrs);</programlisting>
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</para></listitem>
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</varlistentry>
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<varlistentry><term><function>baseNameOf</function> <replaceable>s</replaceable></term>
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<listitem><para>Return the <emphasis>base name</emphasis> of the
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string <replaceable>s</replaceable>, that is, everything following
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the final slash in the string. This is similar to the GNU
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<command>basename</command> command.</para></listitem>
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</varlistentry>
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<varlistentry><term><varname>builtins</varname></term>
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<listitem><para>The attribute set <varname>builtins</varname>
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contains all the built-in functions and values. You can use
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<varname>builtins</varname> to test for the availability of
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features in the Nix installation, e.g.,
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<programlisting>
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if builtins ? getEnv then builtins.getEnv "PATH" else ""</programlisting>
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This allows a Nix expression to fall back gracefully on older Nix
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installations that don’t have the desired built-in function.
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However, in that case you should not write
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<programlisting>
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if builtins ? getEnv then __getEnv "PATH" else ""</programlisting>
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This Nix expression will trigger an “undefined variable” error on
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older Nix versions since <function>__getEnv</function> doesn’t
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exist. <literal>builtins.getEnv</literal>, on the other hand, is
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safe since <literal>builtins</literal> always exists and attribute
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selection is lazy, so it’s only performed if the test
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succeeds.</para></listitem>
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</varlistentry>
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<varlistentry
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xml:id='builtin-currentSystem'><term><varname>builtins.currentSystem</varname></term>
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<listitem><para>The built-in value <varname>currentSystem</varname>
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evaluates to the Nix platform identifier for the Nix installation
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on which the expression is being evaluated, such as
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<literal>"i686-linux"</literal> or
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<literal>"powerpc-darwin"</literal>.</para></listitem>
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</varlistentry>
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<!--
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<varlistentry><term><function>currentTime</function></term>
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<listitem><para>The built-in value <varname>currentTime</varname>
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returns the current system time in seconds since 00:00:00 1/1/1970
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UTC. Due to the evaluation model of Nix expressions
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(<emphasis>maximal laziness</emphasis>), it always yields the same
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value within an execution of Nix.</para></listitem>
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</varlistentry>
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-->
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<!--
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<varlistentry><term><function>dependencyClosure</function></term>
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<listitem><para>TODO</para></listitem>
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</varlistentry>
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-->
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<varlistentry><term><function>derivation</function>
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<replaceable>attrs</replaceable></term>
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<listitem><para><function>derivation</function> is described in
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<xref linkend='ssec-derivation' />.</para></listitem>
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</varlistentry>
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<varlistentry><term><function>dirOf</function> <replaceable>s</replaceable></term>
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<listitem><para>Return the directory part of the string
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<replaceable>s</replaceable>, that is, everything before the final
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slash in the string. This is similar to the GNU
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<command>dirname</command> command.</para></listitem>
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</varlistentry>
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<varlistentry><term><function>builtins.getAttr</function>
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<replaceable>s</replaceable> <replaceable>attrs</replaceable></term>
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<listitem><para><function>getAttr</function> returns the attribute
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named <replaceable>s</replaceable> from the attribute set
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<replaceable>attrs</replaceable>. Evaluation aborts if the
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attribute doesn’t exist. This is a dynamic version of the
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<literal>.</literal> operator, since <replaceable>s</replaceable>
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is an expression rather than an identifier.</para></listitem>
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</varlistentry>
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<varlistentry><term><function>builtins.getEnv</function>
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<replaceable>s</replaceable></term>
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<listitem><para><function>getEnv</function> returns the value of
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the environment variable <replaceable>s</replaceable>, or an empty
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string if the variable doesn’t exist. This function should be
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used with care, as it can introduce all sorts of nasty environment
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dependencies in your Nix expression.</para>
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<para><function>getEnv</function> is used in Nix Packages to
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locate the file <filename>~/.nixpkgs/config.nix</filename>, which
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contains user-local settings for Nix Packages. (That is, it does
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a <literal>getEnv "HOME"</literal> to locate the user’s home
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directory.)</para></listitem>
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</varlistentry>
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<varlistentry><term><function>builtins.hasAttr</function>
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<replaceable>s</replaceable> <replaceable>attrs</replaceable></term>
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<listitem><para><function>hasAttr</function> returns
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<literal>true</literal> if the attribute set
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<replaceable>attrs</replaceable> has an attribute named
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<replaceable>s</replaceable>, and <literal>false</literal>
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otherwise. This is a dynamic version of the <literal>?</literal>
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operator, since <replaceable>s</replaceable> is an expression
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rather than an identifier.</para></listitem>
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</varlistentry>
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<varlistentry><term><function>builtins.head</function>
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<replaceable>list</replaceable></term>
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<listitem><para>Return the first element of a list; abort
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evaluation if the argument isn’t a list or is an empty list. You
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can test whether a list is empty by comparing it with
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<literal>[]</literal>.</para></listitem>
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</varlistentry>
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<varlistentry><term><function>import</function>
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<replaceable>path</replaceable></term>
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<listitem><para>Load, parse and return the Nix expression in the
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file <replaceable>path</replaceable>. Evaluation aborts if the
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file doesn’t exist or contains an incorrect Nix
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expression. <function>import</function> implements Nix’s module
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system: you can put any Nix expression (such as an attribute set
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or a function) in a separate file, and use it from Nix expressions
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in other files.</para>
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<para>A Nix expression loaded by <function>import</function> must
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not contain any <emphasis>free variables</emphasis> (identifiers
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that are not defined in the Nix expression itself and are not
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built-in). Therefore, it cannot refer to variables that are in
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scope at the call site. For instance, if you have a calling
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expression
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<programlisting>
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rec {
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x = 123;
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y = import ./foo.nix;
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}</programlisting>
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then the following <filename>foo.nix</filename> will give an
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error:
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<programlisting>
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x + 456</programlisting>
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since <varname>x</varname> is not in scope in
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<filename>foo.nix</filename>. If you want <varname>x</varname>
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to be available in <filename>foo.nix</filename>, you should pass
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it as a function argument:
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<programlisting>
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rec {
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x = 123;
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y = import ./foo.nix x;
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}</programlisting>
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and
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<programlisting>
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x: x + 456</programlisting>
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(The function argument doesn’t have to be called
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<varname>x</varname> in <filename>foo.nix</filename>; any name
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would work.)</para></listitem>
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</varlistentry>
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<varlistentry><term><function>builtins.isList</function>
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<replaceable>e</replaceable></term>
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<listitem><para>Return <literal>true</literal> if
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<replaceable>e</replaceable> evaluates to a list, and
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<literal>false</literal> otherwise.</para></listitem>
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</varlistentry>
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<varlistentry><term><function>isNull</function>
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<replaceable>e</replaceable></term>
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<listitem><para>Return <literal>true</literal> if
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<replaceable>e</replaceable> evaluates to <literal>null</literal>,
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and <literal>false</literal> otherwise.</para>
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<warning><para>This function is <emphasis>deprecated</emphasis>;
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just write <literal>e == null</literal> instead.</para></warning>
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</listitem>
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</varlistentry>
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<varlistentry><term><function>builtins.lessThan</function>
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<replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
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<listitem><para>Return <literal>true</literal> if the integer
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<replaceable>e1</replaceable> is less than the integer
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<replaceable>e2</replaceable>, and <literal>false</literal>
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otherwise. Evaluation aborts if either
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<replaceable>e1</replaceable> or <replaceable>e2</replaceable>
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does not evaluate to an integer.</para></listitem>
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</varlistentry>
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<varlistentry><term><function>map</function>
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<replaceable>f</replaceable> <replaceable>list</replaceable></term>
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<listitem><para>Apply the function <replaceable>f</replaceable> to
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each element in the list <replaceable>list</replaceable>. For
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example,
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<programlisting>
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map (x: "foo" + x) ["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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</varlistentry>
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<varlistentry><term><function>builtins.pathExists</function>
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<replaceable>path</replaceable></term>
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<listitem><para>Return <literal>true</literal> if the path
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<replaceable>path</replaceable> exists, and
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<literal>false</literal> otherwise. One application of this
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function is to conditionally include a Nix expression containing
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user configuration:
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<programlisting>
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let
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fileName = builtins.getEnv "CONFIG_FILE";
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config =
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if fileName != "" && builtins.pathExists (builtins.toPath fileName)
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then import (builtins.toPath fileName)
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else { someSetting = false; }; <lineannotation># default configuration</lineannotation>
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in config.someSetting</programlisting>
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(Note that <envar>CONFIG_FILE</envar> must be an absolute path for
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this to work.)</para></listitem>
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</varlistentry>
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<!--
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<varlistentry><term><function>relativise</function></term>
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<listitem><para>TODO</para></listitem>
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</varlistentry>
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-->
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<varlistentry><term><function>removeAttrs</function>
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<replaceable>attrs</replaceable> <replaceable>list</replaceable></term>
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<listitem><para>Remove the attributes listed in
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<replaceable>list</replaceable> from the attribute set
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<replaceable>attrs</replaceable>. The attributes don’t have to
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exist in <replaceable>attrs</replaceable>. For instance,
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<screen>
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removeAttrs { x = 1; y = 2; z = 3; } ["a" "x" "z"]</screen>
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evaluates to <literal>{y = 2;}</literal>.</para></listitem>
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</varlistentry>
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<varlistentry><term><function>builtins.tail</function>
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<replaceable>list</replaceable></term>
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<listitem><para>Return the second to last elements of a list;
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abort evaluation if the argument isn’t a list or is an empty
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list.</para></listitem>
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</varlistentry>
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<varlistentry
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xml:id='builtin-toFile'><term><function>builtins.toFile</function>
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<replaceable>name</replaceable> <replaceable>s</replaceable></term>
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<listitem><para>Store the string <replaceable>s</replaceable> in a
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file in the Nix store and return its path. The file has suffix
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<replaceable>name</replaceable>. This file can be used as an
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input to derivations. One application is to write builders
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“inline”. For instance, the following Nix expression combines
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<xref linkend='ex-hello-nix' /> and <xref
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linkend='ex-hello-builder' /> into one file:
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<programlisting>
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{stdenv, fetchurl, perl}:
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stdenv.mkDerivation {
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name = "hello-2.1.1";
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builder = builtins.toFile "builder.sh" "
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source $stdenv/setup
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PATH=$perl/bin:$PATH
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tar xvfz $src
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cd hello-*
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./configure --prefix=$out
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make
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make install
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";
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src = fetchurl {
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url = http://nix.cs.uu.nl/dist/tarballs/hello-2.1.1.tar.gz;
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md5 = "70c9ccf9fac07f762c24f2df2290784d";
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};
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inherit perl;
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}</programlisting>
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</para>
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<para>It is even possible for one file to refer to another, e.g.,
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<programlisting>
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builder = let
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configFile = builtins.toFile "foo.conf" "
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# This is some dummy configuration file.
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<replaceable>...</replaceable>
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";
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in builtins.toFile "builder.sh" "
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source $stdenv/setup
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<replaceable>...</replaceable>
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cp ${configFile} $out/etc/foo.conf
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";</programlisting>
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Note that <literal>${configFile}</literal> is an antiquotation
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(see <xref linkend='ssec-values' />), so the result of the
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expression <literal>configFile</literal> (i.e., a path like
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<filename>/nix/store/m7p7jfny445k...-foo.conf</filename>) will be
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spliced into the resulting string.</para>
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<para>It is however <emphasis>not</emphasis> allowed to have files
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mutually referring to each other, like so:
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<programlisting>
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let
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foo = builtins.toFile "foo" "...${bar}...";
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bar = builtins.toFile "bar" "...${foo}...";
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in foo</programlisting>
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This is not allowed because it would cause a cyclic dependency in
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the computation of the cryptographic hashes for
|
||||
<varname>foo</varname> and <varname>bar</varname>.</para></listitem>
|
||||
|
||||
</varlistentry>
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||||
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||||
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<varlistentry><term><function>builtins.toPath</function> <replaceable>s</replaceable></term>
|
||||
|
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<listitem><para>Convert the string value
|
||||
<replaceable>s</replaceable> into a path value. The string
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||||
<replaceable>s</replaceable> must represent an absolute path
|
||||
(i.e., must start with <literal>/</literal>). The path need not
|
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exist. The resulting path is canonicalised, e.g.,
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||||
<literal>builtins.toPath "//foo/xyzzy/../bar/"</literal> returns
|
||||
<literal>/foo/bar</literal>.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
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||||
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||||
<varlistentry><term><function>toString</function> <replaceable>e</replaceable></term>
|
||||
|
||||
<listitem><para>Convert the expression
|
||||
<replaceable>e</replaceable> to a string.
|
||||
<replaceable>e</replaceable> can be a string (in which case
|
||||
<function>toString</function> is a no-op) or a path (e.g.,
|
||||
<literal>toString /foo/bar</literal> yields
|
||||
<literal>"/foo/bar"</literal>.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry xml:id='builtin-toXML'><term><function>builtins.toXML</function> <replaceable>e</replaceable></term>
|
||||
|
||||
<listitem><para>Return a string containing an XML representation
|
||||
of <replaceable>e</replaceable>. The main application for
|
||||
<function>toXML</function> is to communicate information with the
|
||||
builder in a more structured format than plain environment
|
||||
variables.</para>
|
||||
|
||||
<!-- TODO: more formally describe the schema of the XML
|
||||
representation -->
|
||||
|
||||
<para><xref linkend='ex-toxml' /> shows an example where this is
|
||||
the case. The builder is supposed to generate the configuration
|
||||
file for a <link xlink:href='http://jetty.mortbay.org/'>Jetty
|
||||
servlet container</link>. A servlet container contains a number
|
||||
of servlets (<filename>*.war</filename> files) each exported under
|
||||
a specific URI prefix. So the servlet configuration is a list of
|
||||
attribute sets containing the <varname>path</varname> and
|
||||
<varname>war</varname> of the servlet (<xref
|
||||
linkend='ex-toxml-co-servlets' />). This kind of information is
|
||||
difficult to communicate with the normal method of passing
|
||||
information through an environment variable, which just
|
||||
concatenates everything together into a string (which might just
|
||||
work in this case, but wouldn’t work if fields are optional or
|
||||
contain lists themselves). Instead the Nix expression is
|
||||
converted to an XML representation with
|
||||
<function>toXML</function>, which is unambiguous and can easily be
|
||||
processed with the appropriate tools. For instance, in the
|
||||
example an XSLT stylesheet (<xref linkend='ex-toxml-co-stylesheet'
|
||||
/>) is applied to it (<xref linkend='ex-toxml-co-apply' />) to
|
||||
generate the XML configuration file for the Jetty server. The XML
|
||||
representation produced from <xref linkend='ex-toxml-co-servlets'
|
||||
/> by <function>toXML</function> is shown in <xref
|
||||
linkend='ex-toxml-result' />.</para>
|
||||
|
||||
<para>Note that <xref linkend='ex-toxml' /> uses the <function
|
||||
linkend='builtin-toFile'>toFile</function> built-in to write the
|
||||
builder and the stylesheet “inline” in the Nix expression. The
|
||||
path of the stylesheet is spliced into the builder at
|
||||
<literal>xsltproc ${stylesheet}
|
||||
<replaceable>...</replaceable></literal>.</para>
|
||||
|
||||
<example xml:id='ex-toxml'><title>Passing information to a builder
|
||||
using <function>toXML</function></title>
|
||||
|
||||
<programlisting><![CDATA[
|
||||
{stdenv, fetchurl, libxslt, jira, uberwiki}:
|
||||
|
||||
stdenv.mkDerivation (rec {
|
||||
name = "web-server";
|
||||
|
||||
buildInputs = [libxslt];
|
||||
|
||||
builder = builtins.toFile "builder.sh" "
|
||||
source $stdenv/setup
|
||||
mkdir $out
|
||||
echo $servlets | xsltproc ${stylesheet} - > $out/server-conf.xml]]> <co xml:id='ex-toxml-co-apply' /> <![CDATA[
|
||||
";
|
||||
|
||||
stylesheet = builtins.toFile "stylesheet.xsl"]]> <co xml:id='ex-toxml-co-stylesheet' /> <![CDATA[
|
||||
"<?xml version='1.0' encoding='UTF-8'?>
|
||||
<xsl:stylesheet xmlns:xsl='http://www.w3.org/1999/XSL/Transform' version='1.0'>
|
||||
<xsl:template match='/'>
|
||||
<Configure>
|
||||
<xsl:for-each select='/expr/list/attrs'>
|
||||
<Call name='addWebApplication'>
|
||||
<Arg><xsl:value-of select=\"attr[@name = 'path']/string/@value\" /></Arg>
|
||||
<Arg><xsl:value-of select=\"attr[@name = 'war']/path/@value\" /></Arg>
|
||||
</Call>
|
||||
</xsl:for-each>
|
||||
</Configure>
|
||||
</xsl:template>
|
||||
</xsl:stylesheet>
|
||||
";
|
||||
|
||||
servlets = builtins.toXML []]> <co xml:id='ex-toxml-co-servlets' /> <![CDATA[
|
||||
{ path = "/bugtracker"; war = jira + "/lib/atlassian-jira.war"; }
|
||||
{ path = "/wiki"; war = uberwiki + "/uberwiki.war"; }
|
||||
];
|
||||
})]]></programlisting>
|
||||
|
||||
</example>
|
||||
|
||||
<example xml:id='ex-toxml-result'><title>XML representation produced by
|
||||
<function>toXML</function></title>
|
||||
|
||||
<programlisting><![CDATA[<?xml version='1.0' encoding='utf-8'?>
|
||||
<expr>
|
||||
<list>
|
||||
<attrs>
|
||||
<attr name="path">
|
||||
<string value="/bugtracker" />
|
||||
</attr>
|
||||
<attr name="war">
|
||||
<path value="/nix/store/d1jh9pasa7k2...-jira/lib/atlassian-jira.war" />
|
||||
</attr>
|
||||
</attrs>
|
||||
<attrs>
|
||||
<attr name="path">
|
||||
<string value="/wiki" />
|
||||
</attr>
|
||||
<attr name="war">
|
||||
<path value="/nix/store/y6423b1yi4sx...-uberwiki/uberwiki.war" />
|
||||
</attr>
|
||||
</attrs>
|
||||
</list>
|
||||
</expr>]]></programlisting>
|
||||
|
||||
</example>
|
||||
|
||||
</listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
</variablelist>
|
||||
|
||||
|
||||
</section>
|
||||
|
|
@ -1,6 +1,7 @@
|
|||
<chapter xmlns="http://docbook.org/ns/docbook"
|
||||
xmlns:xlink="http://www.w3.org/1999/xlink"
|
||||
xml:id='chap-writing-nix-expressions'>
|
||||
xml:id='chap-writing-nix-expressions'
|
||||
xmlns:xi="http://www.w3.org/2001/XInclude">
|
||||
|
||||
<title>Writing Nix Expressions</title>
|
||||
|
||||
|
|
@ -1339,589 +1340,8 @@ command-line argument. See <xref linkend='sec-standard-environment'
|
|||
</section>
|
||||
|
||||
|
||||
<section><title>Built-in functions</title>
|
||||
|
||||
<para>This section lists the functions and constants built into the
|
||||
Nix expression evaluator. (The built-in function
|
||||
<function>derivation</function> is discussed above.) Some built-ins,
|
||||
such as <function>derivation</function>, are always in scope of every
|
||||
Nix expression; you can just access them right away. But to prevent
|
||||
polluting the namespace too much, most built-ins are not in scope.
|
||||
Instead, you can access them through the <varname>builtins</varname>
|
||||
built-in value, which is an attribute set that contains all built-in
|
||||
functions and values. For instance, <function>derivation</function>
|
||||
is also available as <function>builtins.derivation</function>.</para>
|
||||
|
||||
|
||||
<variablelist>
|
||||
|
||||
|
||||
<varlistentry><term><function>abort</function> <replaceable>s</replaceable></term>
|
||||
|
||||
<listitem><para>Abort Nix expression evaluation, print error
|
||||
message <replaceable>s</replaceable>.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><function>builtins.add</function>
|
||||
<replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
|
||||
|
||||
<listitem><para>Return the sum of the integers
|
||||
<replaceable>e1</replaceable> and
|
||||
<replaceable>e2</replaceable>.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><function>builtins.attrNames</function>
|
||||
<replaceable>attrs</replaceable></term>
|
||||
|
||||
<listitem><para>Return the names of the attributes in the
|
||||
attribute set <replaceable>attrs</replaceable> in a sorted list.
|
||||
For instance, <literal>builtins.attrNames {y = 1; x =
|
||||
"foo";}</literal> evaluates to <literal>["x" "y"]</literal>.
|
||||
There is no built-in function <function>attrValues</function>, but
|
||||
you can easily define it yourself:
|
||||
|
||||
<programlisting>
|
||||
attrValues = attrs: map (name: builtins.getAttr name attrs) (builtins.attrNames attrs);</programlisting>
|
||||
|
||||
</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><function>baseNameOf</function> <replaceable>s</replaceable></term>
|
||||
|
||||
<listitem><para>Return the <emphasis>base name</emphasis> of the
|
||||
string <replaceable>s</replaceable>, that is, everything following
|
||||
the final slash in the string. This is similar to the GNU
|
||||
<command>basename</command> command.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><varname>builtins</varname></term>
|
||||
|
||||
<listitem><para>The attribute set <varname>builtins</varname>
|
||||
contains all the built-in functions and values. You can use
|
||||
<varname>builtins</varname> to test for the availability of
|
||||
features in the Nix installation, e.g.,
|
||||
|
||||
<programlisting>
|
||||
if builtins ? getEnv then builtins.getEnv "PATH" else ""</programlisting>
|
||||
|
||||
This allows a Nix expression to fall back gracefully on older Nix
|
||||
installations that don’t have the desired built-in function.
|
||||
However, in that case you should not write
|
||||
|
||||
<programlisting>
|
||||
if builtins ? getEnv then __getEnv "PATH" else ""</programlisting>
|
||||
|
||||
This Nix expression will trigger an “undefined variable” error on
|
||||
older Nix versions since <function>__getEnv</function> doesn’t
|
||||
exist. <literal>builtins.getEnv</literal>, on the other hand, is
|
||||
safe since <literal>builtins</literal> always exists and attribute
|
||||
selection is lazy, so it’s only performed if the test
|
||||
succeeds.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry
|
||||
xml:id='builtin-currentSystem'><term><varname>builtins.currentSystem</varname></term>
|
||||
|
||||
<listitem><para>The built-in value <varname>currentSystem</varname>
|
||||
evaluates to the Nix platform identifier for the Nix installation
|
||||
on which the expression is being evaluated, such as
|
||||
<literal>"i686-linux"</literal> or
|
||||
<literal>"powerpc-darwin"</literal>.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<!--
|
||||
<varlistentry><term><function>currentTime</function></term>
|
||||
|
||||
<listitem><para>The built-in value <varname>currentTime</varname>
|
||||
returns the current system time in seconds since 00:00:00 1/1/1970
|
||||
UTC. Due to the evaluation model of Nix expressions
|
||||
(<emphasis>maximal laziness</emphasis>), it always yields the same
|
||||
value within an execution of Nix.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
-->
|
||||
|
||||
|
||||
<!--
|
||||
<varlistentry><term><function>dependencyClosure</function></term>
|
||||
|
||||
<listitem><para>TODO</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
-->
|
||||
|
||||
|
||||
<varlistentry><term><function>derivation</function>
|
||||
<replaceable>attrs</replaceable></term>
|
||||
|
||||
<listitem><para><function>derivation</function> is described in
|
||||
<xref linkend='ssec-derivation' />.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><function>dirOf</function> <replaceable>s</replaceable></term>
|
||||
|
||||
<listitem><para>Return the directory part of the string
|
||||
<replaceable>s</replaceable>, that is, everything before the final
|
||||
slash in the string. This is similar to the GNU
|
||||
<command>dirname</command> command.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><function>builtins.getAttr</function>
|
||||
<replaceable>s</replaceable> <replaceable>attrs</replaceable></term>
|
||||
|
||||
<listitem><para><function>getAttr</function> returns the attribute
|
||||
named <replaceable>s</replaceable> from the attribute set
|
||||
<replaceable>attrs</replaceable>. Evaluation aborts if the
|
||||
attribute doesn’t exist. This is a dynamic version of the
|
||||
<literal>.</literal> operator, since <replaceable>s</replaceable>
|
||||
is an expression rather than an identifier.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><function>builtins.getEnv</function>
|
||||
<replaceable>s</replaceable></term>
|
||||
|
||||
<listitem><para><function>getEnv</function> returns the value of
|
||||
the environment variable <replaceable>s</replaceable>, or an empty
|
||||
string if the variable doesn’t exist. This function should be
|
||||
used with care, as it can introduce all sorts of nasty environment
|
||||
dependencies in your Nix expression.</para>
|
||||
|
||||
<para><function>getEnv</function> is used in Nix Packages to
|
||||
locate the file <filename>~/.nixpkgs/config.nix</filename>, which
|
||||
contains user-local settings for Nix Packages. (That is, it does
|
||||
a <literal>getEnv "HOME"</literal> to locate the user’s home
|
||||
directory.)</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><function>builtins.hasAttr</function>
|
||||
<replaceable>s</replaceable> <replaceable>attrs</replaceable></term>
|
||||
|
||||
<listitem><para><function>hasAttr</function> returns
|
||||
<literal>true</literal> if the attribute set
|
||||
<replaceable>attrs</replaceable> has an attribute named
|
||||
<replaceable>s</replaceable>, and <literal>false</literal>
|
||||
otherwise. This is a dynamic version of the <literal>?</literal>
|
||||
operator, since <replaceable>s</replaceable> is an expression
|
||||
rather than an identifier.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><function>builtins.head</function>
|
||||
<replaceable>list</replaceable></term>
|
||||
|
||||
<listitem><para>Return the first element of a list; abort
|
||||
evaluation if the argument isn’t a list or is an empty list. You
|
||||
can test whether a list is empty by comparing it with
|
||||
<literal>[]</literal>.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><function>import</function>
|
||||
<replaceable>path</replaceable></term>
|
||||
|
||||
<listitem><para>Load, parse and return the Nix expression in the
|
||||
file <replaceable>path</replaceable>. Evaluation aborts if the
|
||||
file doesn’t exist or contains an incorrect Nix
|
||||
expression. <function>import</function> implements Nix’s module
|
||||
system: you can put any Nix expression (such as an attribute set
|
||||
or a function) in a separate file, and use it from Nix expressions
|
||||
in other files.</para>
|
||||
|
||||
<para>A Nix expression loaded by <function>import</function> must
|
||||
not contain any <emphasis>free variables</emphasis> (identifiers
|
||||
that are not defined in the Nix expression itself and are not
|
||||
built-in). Therefore, it cannot refer to variables that are in
|
||||
scope at the call site. For instance, if you have a calling
|
||||
expression
|
||||
|
||||
<programlisting>
|
||||
rec {
|
||||
x = 123;
|
||||
y = import ./foo.nix;
|
||||
}</programlisting>
|
||||
|
||||
then the following <filename>foo.nix</filename> will give an
|
||||
error:
|
||||
|
||||
<programlisting>
|
||||
x + 456</programlisting>
|
||||
|
||||
since <varname>x</varname> is not in scope in
|
||||
<filename>foo.nix</filename>. If you want <varname>x</varname>
|
||||
to be available in <filename>foo.nix</filename>, you should pass
|
||||
it as a function argument:
|
||||
|
||||
<programlisting>
|
||||
rec {
|
||||
x = 123;
|
||||
y = import ./foo.nix x;
|
||||
}</programlisting>
|
||||
|
||||
and
|
||||
|
||||
<programlisting>
|
||||
x: x + 456</programlisting>
|
||||
|
||||
(The function argument doesn’t have to be called
|
||||
<varname>x</varname> in <filename>foo.nix</filename>; any name
|
||||
would work.)</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><function>builtins.isList</function>
|
||||
<replaceable>e</replaceable></term>
|
||||
|
||||
<listitem><para>Return <literal>true</literal> if
|
||||
<replaceable>e</replaceable> evaluates to a list, and
|
||||
<literal>false</literal> otherwise.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><function>isNull</function>
|
||||
<replaceable>e</replaceable></term>
|
||||
|
||||
<listitem><para>Return <literal>true</literal> if
|
||||
<replaceable>e</replaceable> evaluates to <literal>null</literal>,
|
||||
and <literal>false</literal> otherwise.</para>
|
||||
|
||||
<warning><para>This function is <emphasis>deprecated</emphasis>;
|
||||
just write <literal>e == null</literal> instead.</para></warning>
|
||||
|
||||
</listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><function>builtins.lessThan</function>
|
||||
<replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
|
||||
|
||||
<listitem><para>Return <literal>true</literal> if the integer
|
||||
<replaceable>e1</replaceable> is less than the integer
|
||||
<replaceable>e2</replaceable>, and <literal>false</literal>
|
||||
otherwise. Evaluation aborts if either
|
||||
<replaceable>e1</replaceable> or <replaceable>e2</replaceable>
|
||||
does not evaluate to an integer.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><function>map</function>
|
||||
<replaceable>f</replaceable> <replaceable>list</replaceable></term>
|
||||
|
||||
<listitem><para>Apply the function <replaceable>f</replaceable> to
|
||||
each element in the list <replaceable>list</replaceable>. For
|
||||
example,
|
||||
|
||||
<programlisting>
|
||||
map (x: "foo" + x) ["bar" "bla" "abc"]</programlisting>
|
||||
|
||||
evaluates to <literal>["foobar" "foobla"
|
||||
"fooabc"]</literal>.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><function>builtins.pathExists</function>
|
||||
<replaceable>path</replaceable></term>
|
||||
|
||||
<listitem><para>Return <literal>true</literal> if the path
|
||||
<replaceable>path</replaceable> exists, and
|
||||
<literal>false</literal> otherwise. One application of this
|
||||
function is to conditionally include a Nix expression containing
|
||||
user configuration:
|
||||
|
||||
<programlisting>
|
||||
let
|
||||
fileName = builtins.getEnv "CONFIG_FILE";
|
||||
config =
|
||||
if fileName != "" && builtins.pathExists (builtins.toPath fileName)
|
||||
then import (builtins.toPath fileName)
|
||||
else { someSetting = false; }; <lineannotation># default configuration</lineannotation>
|
||||
in config.someSetting</programlisting>
|
||||
|
||||
(Note that <envar>CONFIG_FILE</envar> must be an absolute path for
|
||||
this to work.)</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<!--
|
||||
<varlistentry><term><function>relativise</function></term>
|
||||
|
||||
<listitem><para>TODO</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
-->
|
||||
|
||||
|
||||
<varlistentry><term><function>removeAttrs</function>
|
||||
<replaceable>attrs</replaceable> <replaceable>list</replaceable></term>
|
||||
|
||||
<listitem><para>Remove the attributes listed in
|
||||
<replaceable>list</replaceable> from the attribute set
|
||||
<replaceable>attrs</replaceable>. The attributes don’t have to
|
||||
exist in <replaceable>attrs</replaceable>. For instance,
|
||||
|
||||
<screen>
|
||||
removeAttrs { x = 1; y = 2; z = 3; } ["a" "x" "z"]</screen>
|
||||
|
||||
evaluates to <literal>{y = 2;}</literal>.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><function>builtins.tail</function>
|
||||
<replaceable>list</replaceable></term>
|
||||
|
||||
<listitem><para>Return the second to last elements of a list;
|
||||
abort evaluation if the argument isn’t a list or is an empty
|
||||
list.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry
|
||||
xml:id='builtin-toFile'><term><function>builtins.toFile</function>
|
||||
<replaceable>name</replaceable> <replaceable>s</replaceable></term>
|
||||
|
||||
<listitem><para>Store the string <replaceable>s</replaceable> in a
|
||||
file in the Nix store and return its path. The file has suffix
|
||||
<replaceable>name</replaceable>. This file can be used as an
|
||||
input to derivations. One application is to write builders
|
||||
“inline”. For instance, the following Nix expression combines
|
||||
<xref linkend='ex-hello-nix' /> and <xref
|
||||
linkend='ex-hello-builder' /> into one file:
|
||||
|
||||
<programlisting>
|
||||
{stdenv, fetchurl, perl}:
|
||||
|
||||
stdenv.mkDerivation {
|
||||
name = "hello-2.1.1";
|
||||
|
||||
builder = builtins.toFile "builder.sh" "
|
||||
source $stdenv/setup
|
||||
|
||||
PATH=$perl/bin:$PATH
|
||||
|
||||
tar xvfz $src
|
||||
cd hello-*
|
||||
./configure --prefix=$out
|
||||
make
|
||||
make install
|
||||
";
|
||||
|
||||
src = fetchurl {
|
||||
url = http://nix.cs.uu.nl/dist/tarballs/hello-2.1.1.tar.gz;
|
||||
md5 = "70c9ccf9fac07f762c24f2df2290784d";
|
||||
};
|
||||
inherit perl;
|
||||
}</programlisting>
|
||||
|
||||
</para>
|
||||
|
||||
<para>It is even possible for one file to refer to another, e.g.,
|
||||
|
||||
<programlisting>
|
||||
builder = let
|
||||
configFile = builtins.toFile "foo.conf" "
|
||||
# This is some dummy configuration file.
|
||||
<replaceable>...</replaceable>
|
||||
";
|
||||
in builtins.toFile "builder.sh" "
|
||||
source $stdenv/setup
|
||||
<replaceable>...</replaceable>
|
||||
cp ${configFile} $out/etc/foo.conf
|
||||
";</programlisting>
|
||||
|
||||
Note that <literal>${configFile}</literal> is an antiquotation
|
||||
(see <xref linkend='ssec-values' />), so the result of the
|
||||
expression <literal>configFile</literal> (i.e., a path like
|
||||
<filename>/nix/store/m7p7jfny445k...-foo.conf</filename>) will be
|
||||
spliced into the resulting string.</para>
|
||||
|
||||
<para>It is however <emphasis>not</emphasis> allowed to have files
|
||||
mutually referring to each other, like so:
|
||||
|
||||
<programlisting>
|
||||
let
|
||||
foo = builtins.toFile "foo" "...${bar}...";
|
||||
bar = builtins.toFile "bar" "...${foo}...";
|
||||
in foo</programlisting>
|
||||
|
||||
This is not allowed because it would cause a cyclic dependency in
|
||||
the computation of the cryptographic hashes for
|
||||
<varname>foo</varname> and <varname>bar</varname>.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><function>builtins.toPath</function> <replaceable>s</replaceable></term>
|
||||
|
||||
<listitem><para>Convert the string value
|
||||
<replaceable>s</replaceable> into a path value. The string
|
||||
<replaceable>s</replaceable> must represent an absolute path
|
||||
(i.e., must start with <literal>/</literal>). The path need not
|
||||
exist. The resulting path is canonicalised, e.g.,
|
||||
<literal>builtins.toPath "//foo/xyzzy/../bar/"</literal> returns
|
||||
<literal>/foo/bar</literal>.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry><term><function>toString</function> <replaceable>e</replaceable></term>
|
||||
|
||||
<listitem><para>Convert the expression
|
||||
<replaceable>e</replaceable> to a string.
|
||||
<replaceable>e</replaceable> can be a string (in which case
|
||||
<function>toString</function> is a no-op) or a path (e.g.,
|
||||
<literal>toString /foo/bar</literal> yields
|
||||
<literal>"/foo/bar"</literal>.</para></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry xml:id='builtin-toXML'><term><function>builtins.toXML</function> <replaceable>e</replaceable></term>
|
||||
|
||||
<listitem><para>Return a string containing an XML representation
|
||||
of <replaceable>e</replaceable>. The main application for
|
||||
<function>toXML</function> is to communicate information with the
|
||||
builder in a more structured format than plain environment
|
||||
variables.</para>
|
||||
|
||||
<!-- TODO: more formally describe the schema of the XML
|
||||
representation -->
|
||||
|
||||
<para><xref linkend='ex-toxml' /> shows an example where this is
|
||||
the case. The builder is supposed to generate the configuration
|
||||
file for a <link xlink:href='http://jetty.mortbay.org/'>Jetty
|
||||
servlet container</link>. A servlet container contains a number
|
||||
of servlets (<filename>*.war</filename> files) each exported under
|
||||
a specific URI prefix. So the servlet configuration is a list of
|
||||
attribute sets containing the <varname>path</varname> and
|
||||
<varname>war</varname> of the servlet (<xref
|
||||
linkend='ex-toxml-co-servlets' />). This kind of information is
|
||||
difficult to communicate with the normal method of passing
|
||||
information through an environment variable, which just
|
||||
concatenates everything together into a string (which might just
|
||||
work in this case, but wouldn’t work if fields are optional or
|
||||
contain lists themselves). Instead the Nix expression is
|
||||
converted to an XML representation with
|
||||
<function>toXML</function>, which is unambiguous and can easily be
|
||||
processed with the appropriate tools. For instance, in the
|
||||
example an XSLT stylesheet (<xref linkend='ex-toxml-co-stylesheet'
|
||||
/>) is applied to it (<xref linkend='ex-toxml-co-apply' />) to
|
||||
generate the XML configuration file for the Jetty server. The XML
|
||||
representation produced from <xref linkend='ex-toxml-co-servlets'
|
||||
/> by <function>toXML</function> is shown in <xref
|
||||
linkend='ex-toxml-result' />.</para>
|
||||
|
||||
<para>Note that <xref linkend='ex-toxml' /> uses the <function
|
||||
linkend='builtin-toFile'>toFile</function> built-in to write the
|
||||
builder and the stylesheet “inline” in the Nix expression. The
|
||||
path of the stylesheet is spliced into the builder at
|
||||
<literal>xsltproc ${stylesheet}
|
||||
<replaceable>...</replaceable></literal>.</para>
|
||||
|
||||
<example xml:id='ex-toxml'><title>Passing information to a builder
|
||||
using <function>toXML</function></title>
|
||||
|
||||
<programlisting><![CDATA[
|
||||
{stdenv, fetchurl, libxslt, jira, uberwiki}:
|
||||
|
||||
stdenv.mkDerivation (rec {
|
||||
name = "web-server";
|
||||
|
||||
buildInputs = [libxslt];
|
||||
|
||||
builder = builtins.toFile "builder.sh" "
|
||||
source $stdenv/setup
|
||||
mkdir $out
|
||||
echo $servlets | xsltproc ${stylesheet} - > $out/server-conf.xml]]> <co xml:id='ex-toxml-co-apply' /> <![CDATA[
|
||||
";
|
||||
|
||||
stylesheet = builtins.toFile "stylesheet.xsl"]]> <co xml:id='ex-toxml-co-stylesheet' /> <![CDATA[
|
||||
"<?xml version='1.0' encoding='UTF-8'?>
|
||||
<xsl:stylesheet xmlns:xsl='http://www.w3.org/1999/XSL/Transform' version='1.0'>
|
||||
<xsl:template match='/'>
|
||||
<Configure>
|
||||
<xsl:for-each select='/expr/list/attrs'>
|
||||
<Call name='addWebApplication'>
|
||||
<Arg><xsl:value-of select=\"attr[@name = 'path']/string/@value\" /></Arg>
|
||||
<Arg><xsl:value-of select=\"attr[@name = 'war']/path/@value\" /></Arg>
|
||||
</Call>
|
||||
</xsl:for-each>
|
||||
</Configure>
|
||||
</xsl:template>
|
||||
</xsl:stylesheet>
|
||||
";
|
||||
|
||||
servlets = builtins.toXML []]> <co xml:id='ex-toxml-co-servlets' /> <![CDATA[
|
||||
{ path = "/bugtracker"; war = jira + "/lib/atlassian-jira.war"; }
|
||||
{ path = "/wiki"; war = uberwiki + "/uberwiki.war"; }
|
||||
];
|
||||
})]]></programlisting>
|
||||
|
||||
</example>
|
||||
|
||||
<example xml:id='ex-toxml-result'><title>XML representation produced by
|
||||
<function>toXML</function></title>
|
||||
|
||||
<programlisting><![CDATA[<?xml version='1.0' encoding='utf-8'?>
|
||||
<expr>
|
||||
<list>
|
||||
<attrs>
|
||||
<attr name="path">
|
||||
<string value="/bugtracker" />
|
||||
</attr>
|
||||
<attr name="war">
|
||||
<path value="/nix/store/d1jh9pasa7k2...-jira/lib/atlassian-jira.war" />
|
||||
</attr>
|
||||
</attrs>
|
||||
<attrs>
|
||||
<attr name="path">
|
||||
<string value="/wiki" />
|
||||
</attr>
|
||||
<attr name="war">
|
||||
<path value="/nix/store/y6423b1yi4sx...-uberwiki/uberwiki.war" />
|
||||
</attr>
|
||||
</attrs>
|
||||
</list>
|
||||
</expr>]]></programlisting>
|
||||
|
||||
</example>
|
||||
|
||||
</listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
</variablelist>
|
||||
|
||||
</section>
|
||||
<xi:include href="builtins.xml" />
|
||||
|
||||
|
||||
</section>
|
||||
|
|
|
|||
Loading…
Reference in a new issue