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146 lines
5.7 KiB
XML
146 lines
5.7 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-expression-syntax'>
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<title>Expression Syntax</title>
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<para>Here is a Nix expression for GNU Hello:</para>
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<programlisting>
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{ stdenv, fetchurl, perl }: ①
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stdenv.mkDerivation { ②
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name = "hello-2.1.1"; ③
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builder = ./builder.sh; ④
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src = fetchurl { ⑤
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url = "ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz";
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sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
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};
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inherit perl; ⑥
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}</programlisting>
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<para>This file is actually already in the Nix Packages collection in
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<filename>pkgs/applications/misc/hello/ex-1/default.nix</filename>.
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It is customary to place each package in a separate directory and call
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the single Nix expression in that directory
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<filename>default.nix</filename>. The file has the following elements
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(referenced from the figure by number):
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<orderedlist>
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<listitem>
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<para>This states that the expression is a
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<emphasis>function</emphasis> that expects to be called with three
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arguments: <varname>stdenv</varname>, <varname>fetchurl</varname>,
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and <varname>perl</varname>. They are needed to build Hello, but
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we don't know how to build them here; that's why they are function
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arguments. <varname>stdenv</varname> is a package that is used
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by almost all Nix Packages packages; it provides a
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<quote>standard</quote> environment consisting of the things you
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would expect in a basic Unix environment: a C/C++ compiler (GCC,
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to be precise), the Bash shell, fundamental Unix tools such as
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<command>cp</command>, <command>grep</command>,
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<command>tar</command>, etc. <varname>fetchurl</varname> is a
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function that downloads files. <varname>perl</varname> is the
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Perl interpreter.</para>
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<para>Nix functions generally have the form <literal>{ x, y, ...,
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z }: e</literal> where <varname>x</varname>, <varname>y</varname>,
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etc. are the names of the expected arguments, and where
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<emphasis>e</emphasis> is the body of the function. So
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here, the entire remainder of the file is the body of the
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function; when given the required arguments, the body should
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describe how to build an instance of the Hello package.</para>
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</listitem>
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<listitem>
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<para>So we have to build a package. Building something from
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other stuff is called a <emphasis>derivation</emphasis> in Nix (as
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opposed to sources, which are built by humans instead of
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computers). We perform a derivation by calling
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<varname>stdenv.mkDerivation</varname>.
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<varname>mkDerivation</varname> is a function provided by
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<varname>stdenv</varname> that builds a package from a set of
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<emphasis>attributes</emphasis>. A set is just a list of
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key/value pairs where each key is a string and each value is an
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arbitrary Nix expression. They take the general form <literal>{
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<emphasis>name1</emphasis> =
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<emphasis>expr1</emphasis>; <emphasis>...</emphasis>
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<emphasis>nameN</emphasis> =
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<emphasis>exprN</emphasis>; }</literal>.</para>
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</listitem>
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<listitem>
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<para>The attribute <varname>name</varname> specifies the symbolic
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name and version of the package. Nix doesn't really care about
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these things, but they are used by for instance <command>nix-env
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-q</command> to show a <quote>human-readable</quote> name for
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packages. This attribute is required by
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<varname>mkDerivation</varname>.</para>
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</listitem>
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<listitem>
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<para>The attribute <varname>builder</varname> specifies the
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builder. This attribute can sometimes be omitted, in which case
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<varname>mkDerivation</varname> will fill in a default builder
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(which does a <literal>configure; make; make install</literal>, in
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essence). Hello is sufficiently simple that the default builder
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would suffice, but in this case, we will show an actual builder
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for educational purposes. The value
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<command>./builder.sh</command> refers to the shell script shown
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in <xref linkend='ex-hello-builder' />, discussed below.</para>
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</listitem>
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<listitem>
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<para>The builder has to know what the sources of the package
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are. Here, the attribute <varname>src</varname> is bound to the
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result of a call to the <command>fetchurl</command> function.
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Given a URL and a SHA-256 hash of the expected contents of the file
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at that URL, this function builds a derivation that downloads the
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file and checks its hash. So the sources are a dependency that
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like all other dependencies is built before Hello itself is
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built.</para>
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<para>Instead of <varname>src</varname> any other name could have
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been used, and in fact there can be any number of sources (bound
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to different attributes). However, <varname>src</varname> is
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customary, and it's also expected by the default builder (which we
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don't use in this example).</para>
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</listitem>
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<listitem>
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<para>Since the derivation requires Perl, we have to pass the
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value of the <varname>perl</varname> function argument to the
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builder. All attributes in the set are actually passed as
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environment variables to the builder, so declaring an attribute
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<programlisting>
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perl = perl;</programlisting>
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will do the trick: it binds an attribute <varname>perl</varname>
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to the function argument which also happens to be called
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<varname>perl</varname>. However, it looks a bit silly, so there
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is a shorter syntax. The <literal>inherit</literal> keyword
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causes the specified attributes to be bound to whatever variables
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with the same name happen to be in scope.</para>
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</listitem>
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</orderedlist>
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</para>
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</section>
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