1294 lines
45 KiB
XML
1294 lines
45 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
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(<filename>default.nix</filename>)</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
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(<filename>builder.sh</filename>)</title>
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<programlisting>
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. $stdenv/setup <co id='ex-hello-builder-co-1' />
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PATH=$perl/bin:$PATH <co id='ex-hello-builder-co-2' />
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tar xvfz $src <co id='ex-hello-builder-co-3' />
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cd hello-*
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./configure --prefix=$out <co id='ex-hello-builder-co-4' />
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make <co id='ex-hello-builder-co-5' />
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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>).
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The builder can actually be made a lot shorter by using the
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<emphasis>generic builder</emphasis> functions provided by
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<varname>stdenv</varname>, but here we write out the build steps to
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elucidate what a builder does. It performs the following
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steps:</para>
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<calloutlist>
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<callout arearefs='ex-hello-builder-co-1'>
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<para>When Nix runs a builder, it initially completely clears the
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environment. For instance, the <envar>PATH</envar> variable is
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empty<footnote><para>Actually, it's initialised to
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<filename>/path-not-set</filename> to prevent Bash from setting it
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to a default value.</para></footnote>. This is done to prevent
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undeclared inputs from being used in the build process. If for
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example the <envar>PATH</envar> contained
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<filename>/usr/bin</filename>, then you might accidentally use
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<filename>/usr/bin/gcc</filename>.</para>
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<para>So the first step is to set up the environment. This is
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done by calling the <filename>setup</filename> script of the
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standard environment. The environment variable
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<envar>stdenv</envar> points to the location of the standard
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environment being used. (It wasn't specified explicitly as an
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attribute in <xref linkend='ex-hello-nix' />, but
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<varname>mkDerivation</varname> adds it automatically.)</para>
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</callout>
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<callout arearefs='ex-hello-builder-co-2'>
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<para>Since Hello needs Perl, we have to make sure that Perl is in
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the <envar>PATH</envar>. The <envar>perl</envar> environment
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variable points to the location of the Perl component (since it
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was passed in as an attribute to the derivation), so
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<filename><replaceable>$perl</replaceable>/bin</filename> is the
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directory containing the Perl interpreter.</para>
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</callout>
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<callout arearefs='ex-hello-builder-co-3'>
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<para>Now we have to unpack the sources. The
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<varname>src</varname> attribute was bound to the result of
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fetching the Hello source tarball from the network, so the
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<envar>src</envar> environment variable points to the location in
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the Nix store to which the tarball was downloaded. After
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unpacking, we <command>cd</command> to the resulting source
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directory.</para>
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<para>The whole build is performed in a temporary directory
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created in <varname>/tmp</varname>, by the way. This directory is
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removed after the builder finishes, so there is no need to clean
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up the sources afterwards. Also, the temporary directory is
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always newly created, so you don't have to worry about files from
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previous builds interfering with the current build.</para>
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</callout>
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<callout arearefs='ex-hello-builder-co-4'>
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<para>GNU Hello is a typical Autoconf-based package, so we first
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have to run its <filename>configure</filename> script. In Nix
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every component is stored in a separate location in the Nix store,
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for instance
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<filename>/nix/store/9a54ba97fb71b65fda531012d0443ce2-hello-2.1.1</filename>.
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Nix computes this path by cryptographically hashing all attributes
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of the derivation. The path is passed to the builder through the
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<envar>out</envar> environment variable. So here we give
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<filename>configure</filename> the parameter
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<literal>--prefix=$out</literal> to cause Hello to be installed in
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the expected location.</para>
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</callout>
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<callout arearefs='ex-hello-builder-co-5'>
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<para>Finally we build Hello (<literal>make</literal>) and install
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it into the location specified by <envar>out</envar>
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(<literal>make install</literal>).</para>
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</callout>
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</calloutlist>
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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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<sect2><title>Composition</title>
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<example id='ex-hello-composition'><title>Composing GNU Hello
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(<filename>all-packages-generic.nix</filename>)</title>
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<programlisting>
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...
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rec { <co id='ex-hello-composition-co-1' />
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hello = (import ../applications/misc/hello/ex-1 <co id='ex-hello-composition-co-2' />) { <co id='ex-hello-composition-co-3' />
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inherit fetchurl stdenv perl;
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};
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perl = (import ../development/interpreters/perl) { <co id='ex-hello-composition-co-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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</example>
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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/system/all-packages-generic.nix</filename>, where all
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Nix expressions for components are imported and called with the
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appropriate arguments. <xref linkend='ex-hello-composition' /> shows
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some fragments of
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<filename>all-packages-generic.nix</filename>.</para>
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<calloutlist>
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<callout arearefs='ex-hello-composition-co-1'>
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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 components into each other.</para>
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</callout>
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<callout arearefs='ex-hello-composition-co-2'>
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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-generic.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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</callout>
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<callout arearefs='ex-hello-composition-co-3'>
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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 an
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attribute set containing the things that the function expects,
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namely <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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</callout>
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<callout arearefs='ex-hello-composition-co-4'>
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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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</callout>
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</calloutlist>
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</sect2>
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<sect2><title>Testing</title>
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<para>You can now try to build Hello. The simplest way to do that is
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by using <command>nix-env</command>:
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<screen>
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$ nix-env -f pkgs/system/i686-linux.nix -i hello
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installing `hello-2.1.1'
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building path `/nix/store/632d2b22514dcebe704887c3da15448d-hello-2.1.1'
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hello-2.1.1/
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hello-2.1.1/intl/
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hello-2.1.1/intl/ChangeLog
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<replaceable>...</replaceable>
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</screen>
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This will build Hello and install it into your profile. The file
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<filename>i686-linux</filename> is just a simple Nix expression that
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imports <filename>all-packages-generic.nix</filename> and instantiates
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it for Linux on the x86 platform.</para>
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<para>Note that the <literal>hello</literal> argument here refers to
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the symbolic name given to the Hello derivation (the
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<varname>name</varname> attribute in <xref linkend='ex-hello-nix' />),
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<emphasis>not</emphasis> the <varname>hello</varname> attribute in
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<filename>all-packages-generic.nix</filename>.
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<command>nix-env</command> simply walks through all derivations
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defined in the latter file, looking for one with a
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<varname>name</varname> attribute matching the command-line
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argument.</para>
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<para>You can test whether it works:
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<screen>
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$ hello
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Hello, world!</screen>
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</para>
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<para>Generally, however, using <command>nix-env</command> is not the
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best way to test components, since you may not want to install them
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into your profile right away (they might not work properly, after
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all). A better way is to write a short file containging the
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following:
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<programlisting>
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(import pkgs/system/i686-linux.nix).hello</programlisting>
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Call it <filename>test.nix</filename>. You can then build it without
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installing it using the command <command>nix-build</command>:
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<screen>
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$ nix-build ./test.nix
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...
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/nix/store/632d2b22514dcebe704887c3da15448d-hello-2.1.1</screen>
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<command>nix-build</command> will build the derivation and print the
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output path. It also creates a symlink to the output path called
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<filename>result</filename> in the current directory. This is
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convenient for testing the component:
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<screen>
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$ ./result/bin/hello
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Hello, world!</screen>
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</para>
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|
|
<para>Nix has a transactional semantics. Once a build finishes
|
|
succesfully, Nix makes a note of this in its database: it registers
|
|
that the path denoted by <envar>out</envar> is now
|
|
<quote>valid</quote>. If you try to build the derivation again, Nix
|
|
will see that the path is already valid and finish immediately. If a
|
|
build fails, either because it returns a non-zero exit code, because
|
|
Nix or the builder are killed, or because the machine crashes, then
|
|
the output path will not be registered as valid. If you try to build
|
|
the derivation again, Nix will remove the output path if it exists
|
|
(e.g., because the builder died half-way through <literal>make
|
|
install</literal>) and try again. Note that there is no
|
|
<quote>negative caching</quote>: Nix doesn't remember that a build
|
|
failed, and so a failed build can always be repeated. This is because
|
|
Nix cannot distinguish between permanent failures (e.g., a compiler
|
|
error due to a syntax error in the source) and transient failures
|
|
(e.g., a disk full condition).</para>
|
|
|
|
<para>Nix also performs locking. If you run multiple Nix builds
|
|
simultaneously, and they try to build the same derivation, the first
|
|
Nix instance that gets there will perform the build, while the others
|
|
block (or perform other derivations if available) until the build
|
|
finishes. So it is always safe to run multiple instances of Nix in
|
|
parallel (contrary to, say, <command>make</command>).</para>
|
|
|
|
<para>If you have a system with multiple CPUs, you may want to have
|
|
Nix build different derivations in parallel (insofar as possible).
|
|
Just pass the option <option>-j <replaceable>N</replaceable></option>,
|
|
where <replaceable>N</replaceable> is the maximum number of jobs to be
|
|
run in parallel. Typically this should be the number of CPUs.</para>
|
|
|
|
</sect2>
|
|
|
|
|
|
<sect2><title>The generic builder</title>
|
|
|
|
<para>Recall from <xref linkend='ex-hello-builder' /> that the builder
|
|
looked something like this:
|
|
|
|
<programlisting>
|
|
PATH=$perl/bin:$PATH
|
|
tar xvfz $src
|
|
cd hello-*
|
|
./configure --prefix=$out
|
|
make
|
|
make install</programlisting>
|
|
|
|
The builders for almost all Unix packages look like this — set up some
|
|
environment variables, unpack the sources, configure, build, and
|
|
install. For this reason the standard environment provides some Bash
|
|
functions that automate the build process. A builder using the
|
|
generic build facilities in shown in <xref linkend='ex-hello-builder2'
|
|
/>.</para>
|
|
|
|
<example id='ex-hello-builder2'><title>Build script using the generic
|
|
build functions</title>
|
|
<programlisting>
|
|
buildInputs="$perl" <co id='ex-hello-builder2-co-1' />
|
|
|
|
. $stdenv/setup <co id='ex-hello-builder2-co-2' />
|
|
|
|
genericBuild <co id='ex-hello-builder2-co-3' /></programlisting>
|
|
</example>
|
|
|
|
<calloutlist>
|
|
|
|
<callout arearefs='ex-hello-builder2-co-1'>
|
|
|
|
<para>The <envar>buildInputs</envar> variable tells
|
|
<filename>setup</filename> to use the indicated components as
|
|
<quote>inputs</quote>. This means that if a component provides a
|
|
<filename>bin</filename> subdirectory, it's added to
|
|
<envar>PATH</envar>; if it has a <filename>include</filename>
|
|
subdirectory, it's added to GCC's header search path; and so
|
|
on.</para>
|
|
|
|
</callout>
|
|
|
|
<callout arearefs='ex-hello-builder2-co-2'>
|
|
|
|
<para>The function <function>genericBuild</function> is defined in
|
|
the file <literal>$stdenv/setup</literal>.</para>
|
|
|
|
</callout>
|
|
|
|
<callout arearefs='ex-hello-builder2-co-3'>
|
|
|
|
<para>The final step calls the shell function
|
|
<function>genericBuild</function>, which performs the steps that
|
|
were done explicitly in <xref linkend='ex-hello-builder' />. The
|
|
generic builder is smart enough to figure out whether to unpack
|
|
the sources using <command>gzip</command>,
|
|
<command>bzip2</command>, etc. It can be customised in many ways;
|
|
see <xref linkend='sec-standard-environment' />.</para>
|
|
|
|
</callout>
|
|
|
|
</calloutlist>
|
|
|
|
<para>Discerning readers will note that the
|
|
<envar>buildInputs</envar> could just as well have been set in the Nix
|
|
expression, like this:
|
|
|
|
<programlisting>
|
|
buildInputs = [perl];</programlisting>
|
|
|
|
The <varname>perl</varname> attribute can then be removed, and the
|
|
builder becomes even shorter:
|
|
|
|
<programlisting>
|
|
. $stdenv/setup
|
|
genericBuilder</programlisting>
|
|
|
|
In fact, <varname>mkDerivation</varname> provides a default builder
|
|
that looks exactly like that, so it is actually possible to omit the
|
|
builder for Hello entirely.</para>
|
|
|
|
</sect2>
|
|
|
|
|
|
</sect1>
|
|
|
|
|
|
|
|
<sect1><title>The Nix expression language</title>
|
|
|
|
<para>The Nix expression language is a pure, lazy, functional
|
|
language. Purity means that operations in the language don't have
|
|
side-effects (for instance, there is no variable assignment).
|
|
Laziness means that arguments to functions are evaluated only when
|
|
they are needed. Functional means that functions are
|
|
<quote>normal</quote> values that can be passed around and
|
|
manipulated in interesting ways.</para>
|
|
|
|
<para>The language is not a full-featured, general purpose language.
|
|
It's main job is to describe components, compositions of components,
|
|
and the variability within components. For this a functional language
|
|
is perfectly suited.</para>
|
|
|
|
<para>This section presents the various features of the
|
|
language.</para>
|
|
|
|
|
|
<simplesect><title>Simple values</title>
|
|
|
|
<para>Nix has the following basic datatypes:
|
|
|
|
<itemizedlist>
|
|
|
|
<listitem><para><emphasis>Strings</emphasis>, enclosed between
|
|
double quotes, e.g., <literal>"foo bar"</literal>.</para></listitem>
|
|
|
|
<listitem><para><emphasis>Integers</emphasis>, e.g.,
|
|
<literal>123</literal>.</para></listitem>
|
|
|
|
<listitem><para><emphasis>URIs</emphasis> as defined in appendix B
|
|
of <ulink url='http://www.ietf.org/rfc/rfc2396.txt'>RFC
|
|
2396</ulink>, e.g.,
|
|
<literal>https://svn.cs.uu.nl:12443/dist/trace/trace-nix-trunk.tar.bz2</literal>.</para></listitem>
|
|
|
|
<listitem><para><emphasis>Paths</emphasis>, e.g.,
|
|
<filename>/bin/sh</filename> or <filename>./builder.sh</filename>.
|
|
A path must contain at least one slash to be recognised as such; for
|
|
instance, <filename>builder.sh</filename> is not a
|
|
path<footnote><para>It's parsed as an expression that selects the
|
|
attribute <varname>sh</varname> from the variable
|
|
<varname>builder</varname>.</para></footnote>. If the filename is
|
|
relative, i.e., if it does not begin with a slash, it is made
|
|
absolute at parse time relative to the directory of the Nix
|
|
expression that contained it. For instance, if a Nix expression in
|
|
<filename>/foo/bar/bla.nix</filename> refers to
|
|
<filename>../xyzzy/fnord.nix</filename>, the absolutised path is
|
|
<filename>/foo/xyzzy/fnord.nix</filename>.</para></listitem>
|
|
|
|
<listitem><para><emphasis>Booleans</emphasis> with values
|
|
<literal>true</literal> and
|
|
<literal>false</literal>.</para></listitem>
|
|
|
|
</itemizedlist>
|
|
|
|
</para>
|
|
|
|
</simplesect>
|
|
|
|
|
|
<simplesect><title>Lists</title>
|
|
|
|
<para>Lists are formed by enclosing a whitespace-separated list of
|
|
values between square bracktes. For example,
|
|
|
|
<programlisting>
|
|
[ 123 ./foo.nix "abc" (f {x=y;}) ]</programlisting>
|
|
|
|
defines a list of four elements, the last being the result of a call
|
|
to the function <varname>f</varname>. Note that function calls have
|
|
to be enclosed in parentheses. If they had been omitted, e.g.,
|
|
|
|
<programlisting>
|
|
[ 123 ./foo.nix "abc" f {x=y;} ]</programlisting>
|
|
|
|
the result would be a list of five elements, the fourth one being a
|
|
function and the fifth being an attribute set.</para>
|
|
|
|
</simplesect>
|
|
|
|
|
|
<simplesect><title>Attribute sets</title>
|
|
|
|
<para>Attribute sets are really the core of the language, since
|
|
ultimately it's all about creating derivations, which are really just
|
|
sets of attributes to be passed to build scripts.</para>
|
|
|
|
<para>Attribute sets are just a list of name/value pairs enclosed in
|
|
curly brackets, where each value is an arbitrary expression terminated
|
|
by a semicolon. For example:
|
|
|
|
<programlisting>
|
|
{ x = 123;
|
|
text = "Hello";
|
|
y = f { bla = 456; };
|
|
}</programlisting>
|
|
|
|
This defines an attribute set with attributes named
|
|
<varname>x</varname>, <varname>test</varname>, <varname>y</varname>.
|
|
The order of the attributes is irrelevant. An attribute name may only
|
|
occur once.</para>
|
|
|
|
<para>Attributes can be selected from an attribute set using the
|
|
<literal>.</literal> operator. For instance,
|
|
|
|
<programlisting>
|
|
{ a = "Foo"; b = "Bar"; }.a</programlisting>
|
|
|
|
evaluates to <literal>"Foo"</literal>.</para>
|
|
|
|
</simplesect>
|
|
|
|
|
|
<simplesect><title>Recursive attribute sets</title>
|
|
|
|
<para>Recursive attribute sets are just normal attribute sets, but the
|
|
attributes can refer to each other. For example,
|
|
|
|
<programlisting>
|
|
rec {
|
|
x = y;
|
|
y = 123;
|
|
}.x
|
|
</programlisting>
|
|
|
|
evaluates to <literal>123</literal>. Note that without
|
|
<literal>rec</literal> the binding <literal>x = y;</literal> would
|
|
refer to the variable <varname>y</varname> in the surrounding scope,
|
|
if one exists, and would be invalid if no such variable exists. That
|
|
is, in a normal (non-recursive) attribute set, attributes are not
|
|
added to the lexical scope; in a recursive set, they are.</para>
|
|
|
|
<para>Recursive attribute sets of course introduce the danger of
|
|
infinite recursion. For example,
|
|
|
|
<programlisting>
|
|
rec {
|
|
x = y;
|
|
y = x;
|
|
}.x</programlisting>
|
|
|
|
does not terminate<footnote><para>Actually, Nix detects infinite
|
|
recursion in this case and aborts (<quote>infinite recursion
|
|
encountered</quote>).</para></footnote>.</para>
|
|
|
|
</simplesect>
|
|
|
|
|
|
<simplesect><title>Let expressions</title>
|
|
|
|
<para>A <literal>let</literal> expression is a simple short-hand for a
|
|
<literal>rec</literal> expression followed by an attribute selection:
|
|
<literal>let { <replaceable>attrs</replaceable> }</literal> translates
|
|
to <literal>rec { <replaceable>attrs</replaceable>
|
|
}.body</literal>.</para>
|
|
|
|
<para>For instance,
|
|
|
|
<programlisting>
|
|
let {
|
|
x = "foo";
|
|
y = "bar";
|
|
body = x + y;
|
|
}</programlisting>
|
|
|
|
is equivalent to
|
|
|
|
<programlisting>
|
|
rec {
|
|
x = "foo";
|
|
y = "bar";
|
|
body = x + y;
|
|
}.body</programlisting>
|
|
|
|
and evaluates to <literal>"foobar"</literal>.
|
|
|
|
</para>
|
|
|
|
</simplesect>
|
|
|
|
|
|
<simplesect><title>Inheriting attributes</title>
|
|
|
|
<para>When defining an attribute set itt is often convenient to copy
|
|
variables from the surrounding lexical scope (e.g., when you want to
|
|
propagate attributes). This can be shortened using the
|
|
<literal>inherit</literal> keyword. For instance,
|
|
|
|
<programlisting>
|
|
let {
|
|
x = 123;
|
|
body = {
|
|
inherit x;
|
|
y = 456;
|
|
};
|
|
}</programlisting>
|
|
|
|
evaluates to <literal>{x = 123; y = 456;}</literal>. (Note that this
|
|
works because <varname>x</varname> is added to the lexical scope by
|
|
the <literal>let</literal> construct.) It is also possible to inherit
|
|
attributes from another attribute set. For instance, in this fragment
|
|
from <filename>all-packages-generic.nix</filename>,
|
|
|
|
<programlisting>
|
|
graphviz = (import ../tools/graphics/graphviz) {
|
|
inherit fetchurl stdenv libpng libjpeg expat x11 yacc;
|
|
inherit (xlibs) libXaw;
|
|
};
|
|
|
|
xlibs = {
|
|
libX11 = ...;
|
|
libXaw = ...;
|
|
...
|
|
}
|
|
|
|
libpng = ...;
|
|
libjpg = ...;
|
|
...</programlisting>
|
|
|
|
the attribute set used in the function call to the function defined in
|
|
<filename>../tools/graphics/graphviz</filename> inherits a number of
|
|
variables from the surrounding scope (<varname>fetchurl</varname>
|
|
... <varname>yacc</varname>), but also inherits
|
|
<varname>libXaw</varname> (the X Athena Widgets) from the
|
|
<varname>xlibs</varname> (X11 client-side libraries) attribute
|
|
set.</para>
|
|
|
|
</simplesect>
|
|
|
|
|
|
<simplesect><title>Functions</title>
|
|
|
|
<para>Functions have the following form:
|
|
|
|
<programlisting>
|
|
{<replaceable>params</replaceable>}: <replaceable>body</replaceable></programlisting>
|
|
|
|
This defines a function that must be called with an attribute set
|
|
containing the attributes listed in <replaceable>params</replaceable>,
|
|
which is a comma-separated list of attribute names. Optionally, for
|
|
each parameter a <emphasis>default value</emphasis> may be specified
|
|
by writing <literal><replaceable>param</replaceable> ?
|
|
<replaceable>e</replaceable></literal>, where
|
|
<replaceable>e</replaceable> is an arbitrary expression. If a
|
|
parameter has a default, the corresponding attribute may be omitted in
|
|
function calls.</para>
|
|
|
|
<para>Note that functions do not have names. If you want to give them
|
|
a name, you can bind them to an attribute, e.g.,
|
|
|
|
<programlisting>
|
|
let {
|
|
concat = {x, y}: x + y;
|
|
body = concat {x = "foo"; y = "bar";};
|
|
}</programlisting>
|
|
|
|
</para>
|
|
|
|
<para>It is also possible to define a function that takes a single
|
|
argument and that does need to be called with an attribute set as
|
|
argument. The syntax is
|
|
|
|
<programlisting>
|
|
<replaceable>var</replaceable>: <replaceable>body</replaceable></programlisting>
|
|
|
|
where <replaceable>var</replaceable> is the name of the argument. It
|
|
is not possible to define a default. Example:
|
|
|
|
<programlisting>
|
|
let {
|
|
negate = x: !x;
|
|
concat = x: y: x + y;
|
|
body = if negate true then concat "foo" "bar" else "";
|
|
}</programlisting>
|
|
|
|
Note that <function>concat</function> is a function that takes one
|
|
arguments and returns a function that takes another argument. This
|
|
allows partial parameterisation (i.e., only filling some of the
|
|
arguments of a function); e.g.,
|
|
|
|
<programlisting>
|
|
map (concat "foo") ["bar", "bla", "abc"]</programlisting>
|
|
|
|
evaluates to <literal>["foobar" "foobla" "fooabc"]</literal>.</para>
|
|
|
|
</simplesect>
|
|
|
|
|
|
<simplesect><title>Conditionals</title>
|
|
|
|
<para>Conditionals look like this:
|
|
|
|
<programlisting>
|
|
if <replaceable>e1</replaceable> then <replaceable>e2</replaceable> else <replaceable>e3</replaceable></programlisting>
|
|
|
|
where <replaceable>e1</replaceable> is an expression that should
|
|
evaluate to a boolean value (<literal>true</literal> or
|
|
<literal>false</literal>).</para>
|
|
|
|
</simplesect>
|
|
|
|
|
|
<simplesect><title>Assertions</title>
|
|
|
|
<para>Assertions are generally used to check that certain requirements
|
|
on or between features and dependencies hold. They look like this:
|
|
|
|
<programlisting>
|
|
assert <replaceable>e1</replaceable>; <replaceable>e2</replaceable></programlisting>
|
|
|
|
where <replaceable>e1</replaceable> is an expression that should
|
|
evaluate to a boolean value. If it evaluates to
|
|
<literal>true</literal>, <replaceable>e2</replaceable> is returned;
|
|
otherwise expression evaluation is aborted and a backtrace is printed.</para>
|
|
|
|
<example id='ex-subversion-nix'><title>Nix expression for Subversion</title>
|
|
<programlisting>
|
|
{ localServer ? false
|
|
, httpServer ? false
|
|
, sslSupport ? false
|
|
, pythonBindings ? false
|
|
, javaSwigBindings ? false
|
|
, javahlBindings ? false
|
|
, stdenv, fetchurl
|
|
, openssl ? null, httpd ? null, db4 ? null, expat, swig ? null, j2sdk ? null
|
|
}:
|
|
|
|
assert localServer -> db4 != null; <co id='ex-subversion-nix-co-1' />
|
|
assert httpServer -> httpd != null && httpd.expat == expat; <co id='ex-subversion-nix-co-2' />
|
|
assert sslSupport -> openssl != null && (httpServer -> httpd.openssl == openssl); <co id='ex-subversion-nix-co-3' />
|
|
assert pythonBindings -> swig != null && swig.pythonSupport;
|
|
assert javaSwigBindings -> swig != null && swig.javaSupport;
|
|
assert javahlBindings -> j2sdk != null;
|
|
|
|
stdenv.mkDerivation {
|
|
name = "subversion-1.1.1";
|
|
...
|
|
openssl = if sslSupport then openssl else null; <co id='ex-subversion-nix-co-4' />
|
|
...
|
|
}</programlisting>
|
|
</example>
|
|
|
|
<para><xref linkend='ex-subversion-nix' /> show how assertions are
|
|
used in the Nix expression for Subversion.</para>
|
|
|
|
<calloutlist>
|
|
|
|
<callout arearefs='ex-subversion-nix-co-1'>
|
|
<para>This assertion states that if Subversion is to have support
|
|
for local repositories, then Berkeley DB is needed. So if the
|
|
Subversion function is called with the
|
|
<varname>localServer</varname> argument set to
|
|
<literal>true</literal> but the <varname>db4</varname> argument
|
|
set to <literal>null</literal>, then the evaluation fails.</para>
|
|
</callout>
|
|
|
|
<callout arearefs='ex-subversion-nix-co-2'>
|
|
<para>This is a more subtle condition: if Subversion is built with
|
|
Apache (<literal>httpServer</literal>) support, then the Expat
|
|
library (an XML library) used by Subversion should be same as the
|
|
one used by Apache. This is because in this configuration
|
|
Subversion code ends up being linked with Apache code, and if the
|
|
Expat libraries do not match, a build- or runtime link error or
|
|
incompatibility might occur.</para>
|
|
</callout>
|
|
|
|
<callout arearefs='ex-subversion-nix-co-2'>
|
|
<para>This assertion says that in order for Subversion to have SSL
|
|
support (so that it can access <literal>https</literal> URLs), an
|
|
OpenSSL library must be passed. Additionally, it says
|
|
<emphasis>if</emphasis> Apache support is enabled, then Apache's
|
|
OpenSSL should much Subversion's. (Note that if Apache support is
|
|
not enabled, we don't care about Apache's OpenSSL.)</para>
|
|
</callout>
|
|
|
|
<callout arearefs='ex-subversion-nix-co-4'>
|
|
<para>The conditional here is not really related to assertions,
|
|
but is worth pointing out: it ensures that if SSL support is
|
|
disabled, then the Subversion derivation is not dependent on
|
|
OpenSSL, even if a non-<literal>null</literal> value was passed.
|
|
This prevents an unnecessary rebuild of Subversion if OpenSSL
|
|
changes.</para>
|
|
</callout>
|
|
|
|
</calloutlist>
|
|
|
|
</simplesect>
|
|
|
|
|
|
|
|
<simplesect><title>With expressions</title>
|
|
|
|
<para>A <emphasis>with</emphasis> expression,
|
|
|
|
<programlisting>
|
|
with <replaceable>e1</replaceable>; <replaceable>e2</replaceable></programlisting>
|
|
|
|
introduces the attribute set <replaceable>e1</replaceable> into the
|
|
lexical scope of the expression <replaceable>e2</replaceable>. For
|
|
instance,
|
|
|
|
<programlisting>
|
|
let {
|
|
as = {x = "foo"; y = "bar";};
|
|
|
|
body = with as; x + y;
|
|
}</programlisting>
|
|
|
|
evaluates to <literal>"foobar"</literal> since the
|
|
<literal>with</literal> adds the <varname>x</varname> and
|
|
<varname>y</varname> attributes of <varname>as</varname> to the
|
|
lexical scope in the expression <literal>x + y</literal>. The most
|
|
common use of <literal>with</literal> is in conjunction with the
|
|
<function>import</function> function. E.g.,
|
|
|
|
<programlisting>
|
|
with (import ./definitions.nix); ...</programlisting>
|
|
|
|
makes all attributes defined in the file
|
|
<filename>definitions.nix</filename> available as if they were defined
|
|
locally in a <literal>rec</literal>-expression.</para>
|
|
|
|
</simplesect>
|
|
|
|
|
|
<simplesect><title>Operators</title>
|
|
|
|
<para><xref linkend='table-operators' /> lists the operators in the
|
|
Nix expression language, in order of precedence (from strongest to
|
|
weakest binding).</para>
|
|
|
|
<table id='table-operators'>
|
|
<title>Operators</title>
|
|
<tgroup cols='3'>
|
|
<thead>
|
|
<row>
|
|
<entry>Syntax</entry>
|
|
<entry>Associativity</entry>
|
|
<entry>Description</entry>
|
|
</row>
|
|
</thead>
|
|
<tbody>
|
|
<row>
|
|
<entry><replaceable>e1</replaceable> ~ <replaceable>e2</replaceable></entry>
|
|
<entry>none</entry>
|
|
<entry>Construct a reference to a subpath of a derivation.
|
|
E.g., <literal>hello ~ "/bin/sh"</literal> refers to the
|
|
<filename>/bin/sh</filename> path within the Hello derivation.
|
|
Useful in specifying derivation attributes.</entry>
|
|
</row>
|
|
<row>
|
|
<entry><replaceable>e</replaceable> ?
|
|
<replaceable>id</replaceable></entry>
|
|
<entry>none</entry>
|
|
<entry>Test whether attribute set <replaceable>e</replaceable>
|
|
contains an attribute named
|
|
<replaceable>id</replaceable>.</entry>
|
|
</row>
|
|
<row>
|
|
<entry><replaceable>e1</replaceable> + <replaceable>e2</replaceable></entry>
|
|
<entry>left</entry>
|
|
<entry>String or path concatenation.</entry>
|
|
</row>
|
|
<row>
|
|
<entry>! <replaceable>e</replaceable></entry>
|
|
<entry>left</entry>
|
|
<entry>Boolean negation.</entry>
|
|
</row>
|
|
<row>
|
|
<entry><replaceable>e1</replaceable> //
|
|
<replaceable>e2</replaceable></entry>
|
|
<entry>right</entry>
|
|
<entry>Return an attribute set consisting of the attributes in
|
|
<replaceable>e1</replaceable> and
|
|
<replaceable>e2</replaceable> (with the latter taking
|
|
precedence over the former in case of equally named attributes).</entry>
|
|
</row>
|
|
<row>
|
|
<entry><replaceable>e1</replaceable> ==
|
|
<replaceable>e2</replaceable></entry>
|
|
<entry>none</entry>
|
|
<entry>Equality.</entry>
|
|
</row>
|
|
<row>
|
|
<entry><replaceable>e1</replaceable> !=
|
|
<replaceable>e2</replaceable></entry>
|
|
<entry>none</entry>
|
|
<entry>Inequality.</entry>
|
|
</row>
|
|
<row>
|
|
<entry><replaceable>e1</replaceable> &&
|
|
<replaceable>e2</replaceable></entry>
|
|
<entry>left</entry>
|
|
<entry>Logical AND.</entry>
|
|
</row>
|
|
<row>
|
|
<entry><replaceable>e1</replaceable> ||
|
|
<replaceable>e2</replaceable></entry>
|
|
<entry>left</entry>
|
|
<entry>Logical OR.</entry>
|
|
</row>
|
|
<row>
|
|
<entry><replaceable>e1</replaceable> ->
|
|
<replaceable>e2</replaceable></entry>
|
|
<entry>none</entry>
|
|
<entry>Logical implication (equivalent to
|
|
<literal>!<replaceable>e1</replaceable> ||
|
|
<replaceable>e2</replaceable></literal>).</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
|
|
</simplesect>
|
|
|
|
|
|
<simplesect><title>Derivations</title>
|
|
|
|
<para>The most important built-in function is
|
|
<function>derivation</function>, which is used to describe a
|
|
single derivation (a build action). It takes as input an attribute
|
|
set, the attributes of which specify the inputs of the build.</para>
|
|
|
|
<itemizedlist>
|
|
|
|
<listitem><para>There must be an attribute named
|
|
<varname>system</varname> whose value must be a string specifying a
|
|
Nix platform identifier, such as <literal>"i686-linux"</literal> or
|
|
<literal>"powerpc-darwin"</literal><footnote><para>To figure out
|
|
your platform identifier, look at the line <quote>Checking for the
|
|
canonical Nix system name</quote> in the output of Nix's
|
|
<filename>configure</filename> script.</para></footnote> The build
|
|
can only be performed on a machine and operating system matching the
|
|
platform identifier. (Nix can automatically forward builds for
|
|
other platforms by forwarding them to other machines; see <xref
|
|
linkend='sec-distributed-builds' />.)</para></listitem>
|
|
|
|
<listitem><para>There must be an attribute named
|
|
<varname>name</varname> whose value must be a string. This is used
|
|
as a symbolic name for the component by <command>nix-env</command>,
|
|
and it is appended to the hash in the output path of the
|
|
derivation.</para></listitem>
|
|
|
|
<listitem><para>There must be an attribute named
|
|
<varname>builder</varname> that identifies the program that is
|
|
executed to perform the build. It can be either a derivation or a
|
|
source (a local file reference, e.g.,
|
|
<filename>./builder.sh</filename>).</para></listitem>
|
|
|
|
<listitem><para>Every attribute is passed as an environment variable
|
|
to the builder. Attribute values are translated to environment
|
|
variables as follows:
|
|
|
|
<itemizedlist>
|
|
|
|
<listitem><para>Strings, URIs, and integers are just passed
|
|
verbatim.</para></listitem>
|
|
|
|
<listitem><para>A <emphasis>path</emphasis> (e.g.,
|
|
<filename>../foo/sources.tar</filename>) causes the referenced
|
|
file to be copied to the store; its location in the store is put
|
|
in the environment variable. The idea is that all sources
|
|
should reside in the Nix store, since all inputs to a derivation
|
|
should reside in the Nix store.</para></listitem>
|
|
|
|
<listitem><para>A <emphasis>derivation</emphasis> causes that
|
|
derivation to be built prior to the present derivation; the
|
|
output path is put in the environment
|
|
variable.</para></listitem>
|
|
|
|
<listitem><para>Lists of the previous types are also allowed.
|
|
They are simply concatenated, separated by
|
|
spaces.</para></listitem>
|
|
|
|
</itemizedlist>
|
|
|
|
</para></listitem>
|
|
|
|
<listitem><para>The optional argument <varname>args</varname>
|
|
specifies command-line arguments to be passed to the builder. It
|
|
should be a list.</para></listitem>
|
|
|
|
</itemizedlist>
|
|
|
|
<para>(Note that <function>mkDerivation</function> in the standard
|
|
environment is a wrapper around <function>derivation</function> that
|
|
adds a default value for <varname>system</varname> and always uses
|
|
Bash as the builder, to which the supplied builder is passed as a
|
|
command-line argument. See <xref linkend='sec-standard-environment'
|
|
/>.)</para>
|
|
|
|
<para>The builder is executed as follows:
|
|
|
|
<itemizedlist>
|
|
|
|
<listitem><para>A temporary directory is created under the directory
|
|
specified by <envar>TMPDIR</envar> (default
|
|
<filename>/tmp</filename>) where the build will take place. The
|
|
current directory is changed to this directory.</para></listitem>
|
|
|
|
<listitem><para>The environment is cleared and set to the derivation
|
|
attributes, as specified above.</para></listitem>
|
|
|
|
<listitem><para>In addition, the following variables are set:
|
|
|
|
<itemizedlist>
|
|
|
|
<listitem><para><envar>NIX_BUILD_TOP</envar> contains the path of
|
|
the temporary directory for this build.</para></listitem>
|
|
|
|
<listitem><para>Also, <envar>TMPDIR</envar>,
|
|
<envar>TEMPDIR</envar>, <envar>TMP</envar>, <envar>TEMP</envar>
|
|
are set to point to the temporary directory. This is to prevent
|
|
the builder from accidentally writing temporary files anywhere
|
|
else. Doing so might cause interference by other
|
|
processes.</para></listitem>
|
|
|
|
<listitem><para><envar>PATH</envar> is set to
|
|
<filename>/path-not-set</filename> to prevent shells from
|
|
initialising it to their built-in default value.</para></listitem>
|
|
|
|
<listitem><para><envar>HOME</envar> is set to
|
|
<filename>/homeless-shelter</filename> to prevent programs from
|
|
using <filename>/etc/passwd</filename> or the like to find the
|
|
user's home directory, which could cause impurity. Usually, when
|
|
<envar>HOME</envar> is set, it is used as the location of the home
|
|
directory, even if it points to a non-existent
|
|
path.</para></listitem>
|
|
|
|
<listitem><para><envar>NIX_STORE</envar> is set to the path of the
|
|
top-level Nix store directory (typically,
|
|
<filename>/nix/store</filename>).</para></listitem>
|
|
|
|
<listitem><para><envar>out</envar> is set to point to the output
|
|
path of the derivation, which is a subdirectory of the Nix store.
|
|
The output path is a concatenation of the cryptographic hash of
|
|
all build inputs, and the <varname>name</varname>
|
|
attribute.</para></listitem>
|
|
|
|
</itemizedlist>
|
|
|
|
</para></listitem>
|
|
|
|
<listitem><para>If the output path already exists, it is removed.
|
|
Also, locks are acquired to prevent multiple Nix instances from
|
|
performing the same build at the same time.</para></listitem>
|
|
|
|
<listitem><para>A log of the combined standard output and error is
|
|
written to <filename>/nix/var/log/nix</filename>.</para></listitem>
|
|
|
|
<listitem><para>The builder is executed with the arguments specified
|
|
by the attribute <varname>args</varname>. If it exit with exit code
|
|
0, it is considered to have succeeded.</para></listitem>
|
|
|
|
<listitem><para>The temporary directory is removed (unless the
|
|
<option>-K</option> option was specified).</para></listitem>
|
|
|
|
<listitem><para>If the build was succesful, Nix scans the output for
|
|
references to the paths of the inputs. These so-called
|
|
<emphasis>retained dependencies</emphasis> could be used when the
|
|
output of the derivation is used (e.g., when it's executed or used
|
|
as input to another derivation), so if we deploy the derivation, we
|
|
should copy the retained dependencies as well. The scan is
|
|
performed by looking for the hash parts of file names of the
|
|
inputs.</para></listitem>
|
|
|
|
</itemizedlist>
|
|
|
|
</para>
|
|
|
|
</simplesect>
|
|
|
|
|
|
<simplesect><title>Other built-in functions</title>
|
|
|
|
<para>TODO</para>
|
|
|
|
</simplesect>
|
|
|
|
|
|
<simplesect><title>Comments</title>
|
|
|
|
<para>Comments can be single-line, started with a <literal>#</literal>
|
|
character, or inline/multi-line, enclosed within <literal>/*
|
|
... */</literal>.</para>
|
|
|
|
</simplesect>
|
|
|
|
|
|
</sect1>
|
|
|
|
|
|
|
|
<sect1 id='sec-standard-environment'><title>The standard environment</title>
|
|
|
|
<para>TODO</para>
|
|
|
|
</sect1>
|
|
|
|
|
|
</chapter>
|