224 lines
8.5 KiB
XML
224 lines
8.5 KiB
XML
<chapter id='chap-writing-nix-expressions'><title>Writing Nix Expressions</title>
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<para>This chapter shows you how to write Nix expressions, which are
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the things that tell Nix how to build components. It starts with a
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simple example (a Nix expression for GNU Hello), and then moves
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on to a more in-depth look at the Nix expression language.</para>
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<sect1><title>A simple Nix expression</title>
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<para>This section shows how to add and test the <ulink
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url='http://www.gnu.org/software/hello/hello.html'>GNU Hello
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package</ulink> to the Nix Packages collection. Hello is a program
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that prints out the text <quote>Hello, world!</quote>.</para>
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<para>To add a component to the Nix Packages collection, you generally
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need to do three things:
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<orderedlist>
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<listitem><para>Write a Nix expression for the component. This is a
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file that describes all the inputs involved in building the
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component, such as dependencies (other components required by the
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component), sources, and so on.</para></listitem>
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<listitem><para>Write a <emphasis>builder</emphasis>. This is a
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shell script<footnote><para>In fact, it can be written in any
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language, but typically it's a <command>bash</command> shell
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script.</para></footnote> that actually builds the component from
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the inputs.</para></listitem>
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<listitem><para>Add the component to the file
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<filename>pkgs/system/all-packages-generic.nix</filename>. The Nix
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expression written in the first step is a
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<emphasis>function</emphasis>; it requires other components in order
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to build it. In this step you put it all together, i.e., you call
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the function with the right arguments to build the actual
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component.</para></listitem>
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</orderedlist>
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</para>
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<sect2><title>The Nix expression</title>
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<example id='ex-hello-nix'><title>Nix expression for GNU Hello</title>
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<programlisting>
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{stdenv, fetchurl, perl}: <co id='ex-hello-nix-co-1' />
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stdenv.mkDerivation { <co id='ex-hello-nix-co-2' />
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name = "hello-2.1.1"; <co id='ex-hello-nix-co-3' />
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builder = ./builder.sh; <co id='ex-hello-nix-co-4' />
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src = fetchurl { <co id='ex-hello-nix-co-5' />
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url = ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz;
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md5 = "70c9ccf9fac07f762c24f2df2290784d";
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};
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inherit perl; <co id='ex-hello-nix-co-6' />
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}</programlisting>
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</example>
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<para><xref linkend='ex-hello-nix' /> shows a Nix expression for GNU
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Hello. It's 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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<calloutlist>
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<callout arearefs='ex-hello-nix-co-1'>
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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 component that is used
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by almost all Nix Packages components; 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. (See
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<filename>pkgs/stdenv/nix/path.nix</filename> to see what's in
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<command>stdenv</command>.) <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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<replaceable>e</replaceable> 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 component.</para>
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</callout>
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<callout arearefs='ex-hello-nix-co-2'>
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<para>So we have to build a component. 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 component from a set of
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<emphasis>attributes</emphasis>. An attribute set is just a list
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of key/value pairs where the value is an arbitrary Nix expression.
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They take the general form
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<literal>{<replaceable>name1</replaceable> =
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<replaceable>expr1</replaceable>; <replaceable>...</replaceable>
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<replaceable>name1</replaceable> =
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<replaceable>expr1</replaceable>;</literal>.</para>
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</callout>
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<callout arearefs='ex-hello-nix-co-3'>
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<para>The attribute <varname>name</varname> specifies the symbolic
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name and version of the component. 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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components. This attribute is required by
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<varname>mkDerivation</varname>.</para>
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</callout>
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<callout arearefs='ex-hello-nix-co-4'>
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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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</callout>
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<callout arearefs='ex-hello-nix-co-5'>
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<para>The builder has to know what the sources of the component
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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 MD5 hash of the expected contents of the file at
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that URL, this function actually builds a derivation that
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downloads the file and checks its hash. So the sources are a
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dependency that like all other dependencies is built before Hello
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itself is 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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</callout>
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<callout arearefs='ex-hello-nix-co-6'>
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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 trink: 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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</callout>
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</calloutlist>
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</para>
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</sect2>
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<sect2><title>The builder</title>
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<example id='ex-hello-builder'><title>Build script for GNU Hello</title>
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<programlisting>
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. $stdenv/setup
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PATH=$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</programlisting>
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</example>
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<para><xref linkend='ex-hello-builder' /> shows the builder referenced
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from Hello's Nix expression (stored in
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<filename>pkgs/applications/misc/hello/ex-1/builder.sh</filename>).</para>
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<para>TODO</para>
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<para>If you are wondering about the absence of error checking on the
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result of various commands called in the builder: this is because the
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shell script is evaluated with Bash's <option>-e</option> option,
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which causes the script to be aborted if any command fails without an
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error check.</para>
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</sect2>
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</sect1>
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</chapter>
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