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* Document the built-in functions.
This commit is contained in:
Eelco Dolstra
parent
d20c3011a0
commit
3837fb233c
@ -190,9 +190,9 @@
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<varlistentry><term><literal>escapes</literal></term>
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<listitem><para>Indicate nesting using escape codes that can be
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interpreted by the <command>log2xml</command> tool in the Nix
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source distribution. The resulting XML file can be fed into the
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<command>log2html.xsl</command> stylesheet to create an HTML
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interpreted by the <command>nix-log2xml</command> tool in the
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Nix source distribution. The resulting XML file can be fed into
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the <command>log2html.xsl</command> stylesheet to create an HTML
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file that can be browsed interactively, using Javascript to
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expand and collapse parts of the output.</para></listitem>
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@ -41,17 +41,6 @@ div.section > div.titlepage h2 /* sections */
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margin-top: 1.5em;
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}
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div.refnamediv h2, div.refsynopsisdiv h2, div.refsection h2 /* refentry parts */
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{
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margin-top: 1.4em;
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font-size: 125%;
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}
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div.refsection h3
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{
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font-size: 110%;
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}
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h3 /* subsections */
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{
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font-size: 125%;
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@ -63,6 +52,16 @@ div.appendix h3
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margin-top: 1.5em;
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}
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div.refnamediv h2, div.refsynopsisdiv h2, div.refsection h2 /* refentry parts */
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{
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margin-top: 1.4em;
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font-size: 125%;
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}
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div.refsection h3
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{
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font-size: 110%;
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}
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/***************************************************************************
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@ -586,14 +586,18 @@ components.</para>
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language.</para>
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<section xml:id='ssec-values'><title>Values</title>
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<simplesect><title>Simple values</title>
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<para>Nix has the following basic datatypes:
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<itemizedlist>
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<listitem><para><emphasis>Strings</emphasis>, enclosed between
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double quotes, e.g., <literal>"foo bar"</literal>.</para></listitem>
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<listitem><para><emphasis>Strings</emphasis>,
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enclosed between double quotes, e.g., <literal>"foo bar"</literal>.
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TODO: antiquotations, escaping.</para></listitem>
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<listitem><para><emphasis>Integers</emphasis>, e.g.,
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<literal>123</literal>.</para></listitem>
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@ -681,6 +685,12 @@ evaluates to <literal>"Foo"</literal>.</para>
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</simplesect>
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</section>
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<section><title>Language constructs</title>
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<simplesect><title>Recursive attribute sets</title>
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<para>Recursive attribute sets are just normal attribute sets, but the
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@ -836,7 +846,7 @@ 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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@ -977,7 +987,19 @@ locally in a <literal>rec</literal>-expression.</para>
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</simplesect>
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<simplesect><title>Operators</title>
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<simplesect><title>Comments</title>
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<para>Comments can be single-line, started with a <literal>#</literal>
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character, or inline/multi-line, enclosed within <literal>/*
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... */</literal>.</para>
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</simplesect>
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</section>
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<section><title>Operators</title>
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<para><xref linkend='table-operators' /> lists the operators in the
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Nix expression language, in order of precedence (from strongest to
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@ -994,13 +1016,28 @@ weakest binding).</para>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><replaceable>e</replaceable> .
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<replaceable>id</replaceable></entry>
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<entry>none</entry>
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<entry>Select attribute named <replaceable>id</replaceable>
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from attribute set <replaceable>e</replaceable>. Abort
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evaluation if the attribute doesn’t exist.</entry>
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</row>
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<row>
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<entry><replaceable>e1</replaceable> <replaceable>e2</replaceable></entry>
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<entry>left</entry>
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<entry>Call function <replaceable>e1</replaceable> with
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argument <replaceable>e2</replaceable>.</entry>
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</row>
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<row>
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<entry><replaceable>e</replaceable> ?
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<replaceable>id</replaceable></entry>
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<entry>none</entry>
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<entry>Test whether attribute set <replaceable>e</replaceable>
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contains an attribute named
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<replaceable>id</replaceable>.</entry>
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contains an attribute named <replaceable>id</replaceable>;
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return <literal>true</literal> or
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<literal>false</literal>.</entry>
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</row>
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<row>
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<entry><replaceable>e1</replaceable> ++ <replaceable>e2</replaceable></entry>
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@ -1062,10 +1099,10 @@ weakest binding).</para>
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</tgroup>
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</table>
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</simplesect>
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</section>
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<simplesect xml:id="ssec-derivation"><title>Derivations</title>
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<section xml:id="ssec-derivation"><title>Derivations</title>
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<para>The most important built-in function is
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<function>derivation</function>, which is used to describe a
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@ -1230,23 +1267,457 @@ command-line argument. See <xref linkend='sec-standard-environment'
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</para>
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</simplesect>
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</section>
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<simplesect><title>Other built-in functions</title>
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<section><title>Built-in functions</title>
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<para>TODO</para>
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</simplesect>
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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>Add integers <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>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><term><varname>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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<simplesect><title>Comments</title>
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<!--
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<varlistentry><term><function>currentTime</function></term>
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<para>Comments can be single-line, started with a <literal>#</literal>
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character, or inline/multi-line, enclosed within <literal>/*
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... */</literal>.</para>
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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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</simplesect>
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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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|
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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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|
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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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||||
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</varlistentry>
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<!--
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||||
<varlistentry><term><function>relativise</function></term>
|
||||
|
||||
<listitem><para>TODO</para></listitem>
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||||
|
||||
</varlistentry>
|
||||
-->
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<varlistentry><term><function>removeAttrs</function>
|
||||
<replaceable>attrs</replaceable> <replaceable>list</replaceable></term>
|
||||
|
||||
<listitem><para>Remove the attributes listed in
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||||
<replaceable>list</replaceable> from the attribute set
|
||||
<replaceable>attrs</replaceable>. The attributes don’t have to
|
||||
exist in <replaceable>attrs</replaceable>. For instance,
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||||
|
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<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><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><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></listitem>
|
||||
|
||||
</varlistentry>
|
||||
|
||||
|
||||
</variablelist>
|
||||
|
||||
</section>
|
||||
|
||||
|
||||
</section>
|
||||
@ -1294,9 +1765,10 @@ of the following components:
|
||||
|
||||
<listitem><para>GNU Make. It has been patched to provide
|
||||
<quote>nested</quote> output that can be fed into the
|
||||
<command>log2xml</command> command and <command>log2html</command>
|
||||
stylesheet to create a structured, readable output of the build
|
||||
steps performed by Make.</para></listitem>
|
||||
<command>nix-log2xml</command> command and
|
||||
<command>log2html</command> stylesheet to create a structured,
|
||||
readable output of the build steps performed by
|
||||
Make.</para></listitem>
|
||||
|
||||
<listitem><para>Bash. This is the shell used for all builders in
|
||||
the Nix Packages collection. Not using <command>/bin/sh</command>
|
||||
|
||||
Loading…
Reference in New Issue
Block a user