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<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</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</title>
<programlisting>
. $stdenv/setup
PATH=$perl/bin:$PATH
tar xvfz $src
cd hello-*
./configure --prefix=$out
make
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>).</para>
<para>TODO</para>
<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>
</sect1>
</chapter>
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