path: root/doc/manual/writing-nix-expressions.xml

<chapter id='chap-writing-nix-expressions'><title>Writing Nix Expressions</title>

<para>This chapter shows you how to write Nix expressions, which are
the things that tell Nix how to build components.  It starts with a
simple example (a Nix expression for GNU Hello), and then moves
on to a more in-depth look at the Nix expression language.</para>


<sect1><title>A simple Nix expression</title>

<para>This section shows how to add and test the <ulink
url='http://www.gnu.org/software/hello/hello.html'>GNU Hello
package</ulink> to the Nix Packages collection.  Hello is a program
that prints out the text <quote>Hello, world!</quote>.</para>

<para>To add a component to the Nix Packages collection, you generally
need to do three things:

<orderedlist>

  <listitem><para>Write a Nix expression for the component.  This is a
  file that describes all the inputs involved in building the
  component, such as dependencies (other components required by the
  component), sources, and so on.</para></listitem>

  <listitem><para>Write a <emphasis>builder</emphasis>.  This is a
  shell script<footnote><para>In fact, it can be written in any
  language, but typically it's a <command>bash</command> shell
  script.</para></footnote> that actually builds the component from
  the inputs.</para></listitem>

  <listitem><para>Add the component to the file
  <filename>pkgs/system/all-packages-generic.nix</filename>.  The Nix
  expression written in the first step is a
  <emphasis>function</emphasis>; it requires other components in order
  to build it.  In this step you put it all together, i.e., you call
  the function with the right arguments to build the actual
  component.</para></listitem>

</orderedlist>

</para>


<sect2><title>The Nix expression</title>

<example id='ex-hello-nix'><title>Nix expression for GNU Hello
(<filename>default.nix</filename>)</title>
<programlisting>
{stdenv, fetchurl, perl}: <co id='ex-hello-nix-co-1' />

stdenv.mkDerivation { <co id='ex-hello-nix-co-2' />
  name = "hello-2.1.1"; <co id='ex-hello-nix-co-3' />
  builder = ./builder.sh; <co id='ex-hello-nix-co-4' />
  src = fetchurl { <co id='ex-hello-nix-co-5' />
    url = ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz;
    md5 = "70c9ccf9fac07f762c24f2df2290784d";
  };
  inherit perl; <co id='ex-hello-nix-co-6' />
}</programlisting>
</example>

<para><xref linkend='ex-hello-nix' /> shows a Nix expression for GNU
Hello.  It's actually already in the Nix Packages collection in
<filename>pkgs/applications/misc/hello/ex-1/default.nix</filename>.
It is customary to place each package in a separate directory and call
the single Nix expression in that directory
<filename>default.nix</filename>.  The file has the following elements
(referenced from the figure by number):

<calloutlist>

  <callout arearefs='ex-hello-nix-co-1'>

    <para>This states that the expression is a
    <emphasis>function</emphasis> that expects to be called with three
    arguments: <varname>stdenv</varname>, <varname>fetchurl</varname>,
    and <varname>perl</varname>.  They are needed to build Hello, but
    we don't know how to build them here; that's why they are function
    arguments.  <varname>stdenv</varname> is a component that is used
    by almost all Nix Packages components; it provides a
    <quote>standard</quote> environment consisting of the things you
    would expect in a basic Unix environment: a C/C++ compiler (GCC,
    to be precise), the Bash shell, fundamental Unix tools such as
    <command>cp</command>, <command>grep</command>,
    <command>tar</command>, etc.  (See
    <filename>pkgs/stdenv/nix/path.nix</filename> to see what's in
    <command>stdenv</command>.)  <varname>fetchurl</varname> is a
    function that downloads files.  <varname>perl</varname> is the
    Perl interpreter.</para>

    <para>Nix functions generally have the form <literal>{x, y, ...,
    z}: e</literal> where <varname>x</varname>, <varname>y</varname>,
    etc. are the names of the expected arguments, and where
    <replaceable>e</replaceable> is the body of the function.  So
    here, the entire remainder of the file is the body of the
    function; when given the required arguments, the body should
    describe how to build an instance of the Hello component.</para>
    
  </callout>

  <callout arearefs='ex-hello-nix-co-2'>

    <para>So we have to build a component.  Building something from
    other stuff is called a <emphasis>derivation</emphasis> in Nix (as
    opposed to sources, which are built by humans instead of
    computers).  We perform a derivation by calling
    <varname>stdenv.mkDerivation</varname>.
    <varname>mkDerivation</varname> is a function provided by
    <varname>stdenv</varname> that builds a component from a set of
    <emphasis>attributes</emphasis>.  An attribute set is just a list
    of key/value pairs where the value is an arbitrary Nix expression.
    They take the general form
    <literal>{<replaceable>name1</replaceable> =
    <replaceable>expr1</replaceable>; <replaceable>...</replaceable>
    <replaceable>name1</replaceable> =
    <replaceable>expr1</replaceable>;</literal>.</para>

  </callout>

  <callout arearefs='ex-hello-nix-co-3'>

    <para>The attribute <varname>name</varname> specifies the symbolic
    name and version of the component.  Nix doesn't really care about
    these things, but they are used by for instance <command>nix-env
    -q</command> to show a <quote>human-readable</quote> name for
    components.  This attribute is required by
    <varname>mkDerivation</varname>.</para>

  </callout>

  <callout arearefs='ex-hello-nix-co-4'>

    <para>The attribute <varname>builder</varname> specifies the
    builder.  This attribute can sometimes be omitted, in which case
    <varname>mkDerivation</varname> will fill in a default builder
    (which does a <literal>configure; make; make install</literal>, in
    essence).  Hello is sufficiently simple that the default builder
    would suffice, but in this case, we will show an actual builder
    for educational purposes.  The value
    <command>./builder.sh</command> refers to the shell script shown
    in <xref linkend='ex-hello-builder' />, discussed below.</para>

  </callout>

  <callout arearefs='ex-hello-nix-co-5'>

    <para>The builder has to know what the sources of the component
    are.  Here, the attribute <varname>src</varname> is bound to the
    result of a call to the <command>fetchurl</command> function.
    Given a URL and a MD5 hash of the expected contents of the file at
    that URL, this function actually builds a derivation that
    downloads the file and checks its hash.  So the sources are a
    dependency that like all other dependencies is built before Hello
    itself is built.</para>

    <para>Instead of <varname>src</varname> any other name could have
    been used, and in fact there can be any number of sources (bound
    to different attributes).  However, <varname>src</varname> is
    customary, and it's also expected by the default builder (which we
    don't use in this example).</para>

  </callout>

  <callout arearefs='ex-hello-nix-co-6'>

    <para>Since the derivation requires Perl, we have to pass the
    value of the <varname>perl</varname> function argument to the
    builder.  All attributes in the set are actually passed as
    environment variables to the builder, so declaring an attribute

    <programlisting>
perl = perl;</programlisting>

    will do the trink: it binds an attribute <varname>perl</varname>
    to the function argument which also happens to be called
    <varname>perl</varname>.  However, it looks a bit silly, so there
    is a shorter syntax.  The <literal>inherit</literal> keyword
    causes the specified attributes to be bound to whatever variables
    with the same name happen to be in scope.</para>

  </callout>
  
</calloutlist>

</para>

</sect2>


<sect2><title>The builder</title>

<example id='ex-hello-builder'><title>Build script for GNU Hello
(<filename>builder.sh</filename>)</title>
<programlisting>
. $stdenv/setup <co id='ex-hello-builder-co-1' />

PATH=$perl/bin:$PATH <co id='ex-hello-builder-co-2' />

tar xvfz $src <co id='ex-hello-builder-co-3' />
cd hello-*
./configure --prefix=$out <co id='ex-hello-builder-co-4' />
make <co id='ex-hello-builder-co-5' />
make install</programlisting>
</example>

<para><xref linkend='ex-hello-builder' /> shows the builder referenced
from Hello's Nix expression (stored in
<filename>pkgs/applications/misc/hello/ex-1/builder.sh</filename>).
The builder can actually be made a lot shorter by using the
<emphasis>generic builder</emphasis> functions provided by
<varname>stdenv</varname>, but here we write out the build steps to
elucidate what a builder does.  It performs the following
steps:</para>

<calloutlist>

  <callout arearefs='ex-hello-builder-co-1'>

    <para>When Nix runs a builder, it initially completely clears the
    environment.  For instance, the <envar>PATH</envar> variable is
    empty<footnote><para>Actually, it's initialised to
    <filename>/path-not-set</filename> to prevent Bash from setting it
    to a default value.</para></footnote>.  This is done to prevent
    undeclared inputs from being used in the build process.  If for
    example the <envar>PATH</envar> contained
    <filename>/usr/bin</filename>, then you might accidentally use
    <filename>/usr/bin/gcc</filename>.</para>

    <para>So the first step is to set up the environment.  This is
    done by calling the <filename>setup</filename> script of the
    standard environment.  The environment variable
    <envar>stdenv</envar> points to the location of the standard
    environment being used.  (It wasn't specified explicitly as an
    attribute in <xref linkend='ex-hello-nix' />, but
    <varname>mkDerivation</varname> adds it automatically.)</para>

  </callout>

  <callout arearefs='ex-hello-builder-co-2'>

    <para>Since Hello needs Perl, we have to make sure that Perl is in
    the <envar>PATH</envar>.  The <envar>perl</envar> environment
    variable points to the location of the Perl component (since it
    was passed in as an attribute to the derivation), so
    <filename><replaceable>$perl</replaceable>/bin</filename> is the
    directory containing the Perl interpreter.</para>

  </callout>

  <callout arearefs='ex-hello-builder-co-3'>

    <para>Now we have to unpack the sources.  The
    <varname>src</varname> attribute was bound to the result of
    fetching the Hello source tarball from the network, so the
    <envar>src</envar> environment variable points to the location in
    the Nix store to which the tarball was downloaded.  After
    unpacking, we <command>cd</command> to the resulting source
    directory.</para>

    <para>The whole build is performed in a temporary directory
    created in <varname>/tmp</varname>, by the way.  This directory is
    removed after the builder finishes, so there is no need to clean
    up the sources afterwards.  Also, the temporary directory is
    always newly created, so you don't have to worry about files from
    previous builds interfering with the current build.</para>

  </callout>

  <callout arearefs='ex-hello-builder-co-4'>

    <para>GNU Hello is a typical Autoconf-based package, so we first
    have to run its <filename>configure</filename> script.  In Nix
    every component is stored in a separate location in the Nix store,
    for instance
    <filename>/nix/store/9a54ba97fb71b65fda531012d0443ce2-hello-2.1.1</filename>.
    Nix computes this path by cryptographically hashing all attributes
    of the derivation.  The path is passed to the builder through the
    <envar>out</envar> environment variable.  So here we give
    <filename>configure</filename> the parameter
    <literal>--prefix=$out</literal> to cause Hello to be installed in
    the expected location.</para>

  </callout>

  <callout arearefs='ex-hello-builder-co-5'>

    <para>Finally we build Hello (<literal>make</literal>) and install
    it into the location specified by <envar>out</envar>
    (<literal>make install</literal>).</para>

  </callout>
  
</calloutlist>

<para>If you are wondering about the absence of error checking on the
result of various commands called in the builder: this is because the
shell script is evaluated with Bash's <option>-e</option> option,
which causes the script to be aborted if any command fails without an
error check.</para>

</sect2>


<sect2><title>Composition</title>

<example id='ex-hello-composition'><title>Composing GNU Hello
(<filename>all-packages-generic.nix</filename>)</title>
<programlisting>
...

rec { <co id='ex-hello-composition-co-1' />
  
  hello = (import ../applications/misc/hello/ex-1 <co id='ex-hello-composition-co-2' />) { <co id='ex-hello-composition-co-3' />
    inherit fetchurl stdenv perl;
  };

  perl = (import ../development/interpreters/perl) { <co id='ex-hello-composition-co-4' />
    inherit fetchurl stdenv;
  };

  fetchurl = (import ../build-support/fetchurl) { 
    inherit stdenv; ...
  };
  
  stdenv = ...;

}
</programlisting>
</example>

<para>The Nix expression in <xref linkend='ex-hello-nix' /> is a
function; it is missing some arguments that have to be filled in
somewhere.  In the Nix Packages collection this is done in the file
<filename>pkgs/system/all-packages-generic.nix</filename>, where all
Nix expressions for components are imported and called with the
appropriate arguments.  <xref linkend='ex-hello-composition' /> shows
some fragments of
<filename>all-packages-generic.nix</filename>.</para>

<calloutlist>

  <callout arearefs='ex-hello-composition-co-1'>

    <para>This file defines a set of attributes, all of which are
    concrete derivations (i.e., not functions).  In fact, we define a
    <emphasis>mutually recursive</emphasis> set of attributes.  That
    is, the attributes can refer to each other.  This is precisely
    what we want since we want to <quote>plug</quote> the
    various components into each other.</para>

  </callout>

  <callout arearefs='ex-hello-composition-co-2'>

    <para>Here we <emphasis>import</emphasis> the Nix expression for
    GNU Hello.  The import operation just loads and returns the
    specified Nix expression. In fact, we could just have put the
    contents of <xref linkend='ex-hello-nix' /> in
    <filename>all-packages-generic.nix</filename> at this point.  That
    would be completely equivalent, but it would make the file rather
    bulky.</para>

    <para>Note that we refer to
    <filename>../applications/misc/hello/ex-1</filename>, not
    <filename>../applications/misc/hello/ex-1/default.nix</filename>.
    When you try to import a directory, Nix automatically appends
    <filename>/default.nix</filename> to the file name.</para>

  </callout>

  <callout arearefs='ex-hello-composition-co-3'>

    <para>This is where the actual composition takes place.  Here we
    <emphasis>call</emphasis> the function imported from
    <filename>../applications/misc/hello/ex-1</filename> with an
    attribute set containing the things that the function expects,
    namely <varname>fetchurl</varname>, <varname>stdenv</varname>, and
    <varname>perl</varname>.  We use inherit again to use the
    attributes defined in the surrounding scope (we could also have
    written <literal>fetchurl = fetchurl;</literal>, etc.).</para>

    <para>The result of this function call is an actual derivation
    that can be built by Nix (since when we fill in the arguments of
    the function, what we get is its body, which is the call to
    <varname>stdenv.mkDerivation</varname> in <xref
    linkend='ex-hello-nix ' />).</para>

  </callout>

  <callout arearefs='ex-hello-composition-co-4'>

    <para>Likewise, we have to instantiate Perl,
    <varname>fetchurl</varname>, and the standard environment.</para>

  </callout>

</calloutlist>

</sect2>


<sect2><title>Testing</title>

<para>You can now try to build Hello.  The simplest way to do that is
by using <command>nix-env</command>:

<screen>
$ nix-env -f pkgs/system/i686-linux.nix -i hello
installing `hello-2.1.1'
building path `/nix/store/632d2b22514dcebe704887c3da15448d-hello-2.1.1'
hello-2.1.1/
hello-2.1.1/intl/
hello-2.1.1/intl/ChangeLog
<replaceable>...</replaceable>
</screen>

This will build Hello and install it into your profile.  The file
<filename>i686-linux</filename> is just a simple Nix expression that
imports <filename>all-packages-generic.nix</filename> and instantiates
it for Linux on the x86 platform.</para>

<para>Note that the <literal>hello</literal> argument here refers to
the symbolic name given to the Hello derivation (the
<varname>name</varname> attribute in <xref linkend='ex-hello-nix' />),
<emphasis>not</emphasis> the <varname>hello</varname> attribute in
<filename>all-packages-generic.nix</filename>.
<command>nix-env</command> simply walks through all derivations
defined in the latter file, looking for one with a
<varname>name</varname> attribute matching the command-line
argument.</para>

<para>You can test whether it works:

<screen>
$ hello
Hello, world!</screen>

</para>

<para>Generally, however, using <command>nix-env</command> is not the
best way to test components, since you may not want to install them
into your profile right away (they might not work properly, after
all).  A better way is to write a short file containging the
following:

<programlisting>
(import pkgs/system/i686-linux.nix).hello</programlisting>

Call it <filename>test.nix</filename>.  Then you can build it without
installing it using the command <command>nix-build</command>:

<screen>
$ nix-build ./test.nix
...
/nix/store/632d2b22514dcebe704887c3da15448d-hello-2.1.1</screen>

<command>nix-build</command> will build the derivation and print the
output path.  It also creates a symlink to the output path called
<filename>result</filename> in the current directory.  This is
convenient for testing the component:

<screen>
$ ./result/bin/hello
Hello, world!</screen>

</para>

<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>


</sect1>


</chapter>