forked from lix-project/lix
115 lines
3.6 KiB
XML
115 lines
3.6 KiB
XML
<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id='sec-arguments'>
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<title>Arguments and Variables</title>
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<para>The Nix expression in <xref linkend='ex-hello-nix' /> is a
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function; it is missing some arguments that have to be filled in
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somewhere. In the Nix Packages collection this is done in the file
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<filename>pkgs/top-level/all-packages.nix</filename>, where all Nix
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expressions for packages are imported and called with the appropriate
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arguments. Here are some fragments of
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<filename>all-packages.nix</filename>, with annotations of what they
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mean:</para>
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<programlisting>
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...
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rec { ①
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hello = import ../applications/misc/hello/ex-1 ② { ③
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inherit fetchurl stdenv perl;
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};
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perl = import ../development/interpreters/perl { ④
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inherit fetchurl stdenv;
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};
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fetchurl = import ../build-support/fetchurl {
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inherit stdenv; ...
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};
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stdenv = ...;
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}
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</programlisting>
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<orderedlist>
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<listitem>
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<para>This file defines a set of attributes, all of which are
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concrete derivations (i.e., not functions). In fact, we define a
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<emphasis>mutually recursive</emphasis> set of attributes. That
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is, the attributes can refer to each other. This is precisely
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what we want since we want to <quote>plug</quote> the
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various packages into each other.</para>
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</listitem>
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<listitem>
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<para>Here we <emphasis>import</emphasis> the Nix expression for
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GNU Hello. The import operation just loads and returns the
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specified Nix expression. In fact, we could just have put the
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contents of <xref linkend='ex-hello-nix' /> in
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<filename>all-packages.nix</filename> at this point. That
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would be completely equivalent, but it would make the file rather
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bulky.</para>
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<para>Note that we refer to
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<filename>../applications/misc/hello/ex-1</filename>, not
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<filename>../applications/misc/hello/ex-1/default.nix</filename>.
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When you try to import a directory, Nix automatically appends
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<filename>/default.nix</filename> to the file name.</para>
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</listitem>
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<listitem>
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<para>This is where the actual composition takes place. Here we
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<emphasis>call</emphasis> the function imported from
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<filename>../applications/misc/hello/ex-1</filename> with a set
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containing the things that the function expects, namely
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<varname>fetchurl</varname>, <varname>stdenv</varname>, and
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<varname>perl</varname>. We use inherit again to use the
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attributes defined in the surrounding scope (we could also have
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written <literal>fetchurl = fetchurl;</literal>, etc.).</para>
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<para>The result of this function call is an actual derivation
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that can be built by Nix (since when we fill in the arguments of
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the function, what we get is its body, which is the call to
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<varname>stdenv.mkDerivation</varname> in <xref
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linkend='ex-hello-nix' />).</para>
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<note><para>Nixpkgs has a convenience function
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<function>callPackage</function> that imports and calls a
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function, filling in any missing arguments by passing the
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corresponding attribute from the Nixpkgs set, like this:
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<programlisting>
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hello = callPackage ../applications/misc/hello/ex-1 { };
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</programlisting>
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If necessary, you can set or override arguments:
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<programlisting>
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hello = callPackage ../applications/misc/hello/ex-1 { stdenv = myStdenv; };
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</programlisting>
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</para></note>
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</listitem>
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<listitem>
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<para>Likewise, we have to instantiate Perl,
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<varname>fetchurl</varname>, and the standard environment.</para>
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</listitem>
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</orderedlist>
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</section>
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