forked from lix-project/lix
* Manual: writing Nix expressions.
This commit is contained in:
Eelco Dolstra
parent
feb3ceaee0
commit
8b934694f2
|
|
@ -9,7 +9,7 @@ build farm, since:
|
|||
<listitem><para>Nix supports distributed builds: a local Nix
|
||||
installation can forward Nix builds to other machines over the
|
||||
network. This allows multiple builds to be performed in parallel
|
||||
(thus improving performce), but more in importantly, it allows Nix
|
||||
(thus improving performance), but more in importantly, it allows Nix
|
||||
to perform multi-platform builds in a semi-transparent way. For
|
||||
instance, if you perform a build for a
|
||||
<literal>powerpc-darwin</literal> on an
|
||||
|
|
@ -38,8 +38,8 @@ build farm, since:
|
|||
once.</para></listitem>
|
||||
|
||||
<listitem><para>The results of a Nix build farm can be made
|
||||
available through a channel, so that users can use succesfull builds
|
||||
immediately.</para></listitem>
|
||||
available through a channel, so successful builds can be deployed to
|
||||
users immediately.</para></listitem>
|
||||
|
||||
</itemizedlist>
|
||||
|
||||
|
|
|
|||
|
|
@ -1,146 +1,221 @@
|
|||
<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 write simple Nix expressions — the
|
||||
things that describe how to build a package.</para>
|
||||
<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' />
|
||||
|
||||
derivation { <co id='ex-hello-nix-co-2' />
|
||||
stdenv.mkDerivation { <co id='ex-hello-nix-co-2' />
|
||||
name = "hello-2.1.1"; <co id='ex-hello-nix-co-3' />
|
||||
system = stdenv.system; <co id='ex-hello-nix-co-4' />
|
||||
builder = ./builder.sh; <co id='ex-hello-nix-co-5' />
|
||||
src = fetchurl { <co id='ex-hello-nix-co-6' />
|
||||
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";
|
||||
};
|
||||
stdenv = stdenv; <co id='ex-hello-nix-co-7' />
|
||||
perl = perl;
|
||||
inherit perl; <co id='ex-hello-nix-co-6' />
|
||||
}</programlisting>
|
||||
</example>
|
||||
|
||||
<para>A simple Nix expression is shown in <xref linkend='ex-hello-nix'
|
||||
/>. It describes how to the build the <ulink
|
||||
url='http://www.gnu.org/directory/GNU/hello.html'>GNU Hello
|
||||
package</ulink>. This package has several dependencies. First, it
|
||||
requires a number of other packages, such as a C compiler, standard
|
||||
Unix shell tools, and Perl. Rather than have this Nix expression
|
||||
refer to and use specific versions of these packages, it should be
|
||||
generic; that is, it should be a <emphasis>function</emphasis> that
|
||||
takes the required packages as inputs and yield a build of the GNU
|
||||
Hello package as a result. This Nix expression defines a function
|
||||
with three arguments <xref linkend='ex-hello-nix-co-1' />, namely:
|
||||
<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):
|
||||
|
||||
<orderedlist>
|
||||
<listitem><para><varname>stdenv</varname>, which should be a
|
||||
<emphasis>standard environment package</emphasis>. The standard
|
||||
environment is a set of tools and other components that would be
|
||||
expected in a fairly minimal Unix-like environment: a C compiler
|
||||
and linker, Unix shell tools, and so on.</para></listitem>
|
||||
<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>
|
||||
|
||||
<listitem><para><varname>fetchurl</varname>, which should be a
|
||||
function that given parameters <varname>url</varname> and
|
||||
<varname>md5</varname>, will fetch a file from the specified
|
||||
location and check that this file has the given MD5 hash code.
|
||||
The hash is required because build operations must be
|
||||
<emphasis>pure</emphasis>: given the same inputs they should
|
||||
always yield the same output. Since network resources can change
|
||||
at any time, we must in some way guarantee what the result will
|
||||
be.</para></listitem>
|
||||
|
||||
<listitem><para><varname>perl</varname>, which should be a Perl
|
||||
interpreter.</para></listitem>
|
||||
|
||||
</orderedlist>
|
||||
</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>
|
||||
|
||||
<para>The remainder of the file is the body of the function, which
|
||||
happens to be a <emphasis>derivation</emphasis> <xref
|
||||
linkend='ex-hello-nix-co-2' />, which is the built-in function
|
||||
<varname>derivation</varname> applied to a set of attributes that
|
||||
encode all the necessary information for building the GNU Hello
|
||||
package.</para>
|
||||
</sect2>
|
||||
|
||||
<example><title>Build script (<filename>builder.sh</filename>) for GNU
|
||||
Hello</title>
|
||||
|
||||
<sect2><title>The builder</title>
|
||||
|
||||
<example id='ex-hello-builder'><title>Build script for GNU Hello</title>
|
||||
<programlisting>
|
||||
#! /bin/sh
|
||||
. $stdenv/setup
|
||||
|
||||
buildinputs="$perl"
|
||||
. $stdenv/setup || exit 1
|
||||
PATH=$perl/bin:$PATH
|
||||
|
||||
tar xvfz $src || exit 1
|
||||
cd hello-* || exit 1
|
||||
./configure --prefix=$out || exit 1
|
||||
make || exit 1
|
||||
make install || exit 1</programlisting>
|
||||
tar xvfz $src
|
||||
cd hello-*
|
||||
./configure --prefix=$out
|
||||
make
|
||||
make install</programlisting>
|
||||
</example>
|
||||
|
||||
</sect1>
|
||||
<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>
|
||||
|
||||
<sect1><title>A more complex Nix expression</title>
|
||||
<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>
|
||||
|
||||
<example id='ex-svn-nix'><title>Nix expression for Subversion</title>
|
||||
<programlisting>
|
||||
{ localServer ? false <co id='ex-svn-nix-co-1' />
|
||||
, httpServer ? false
|
||||
, sslSupport ? false
|
||||
, swigBindings ? false
|
||||
, stdenv, fetchurl
|
||||
, openssl ? null, httpd ? null, db4 ? null, expat, swig ? null
|
||||
}:
|
||||
</sect2>
|
||||
|
||||
assert !isNull expat; <co id='ex-svn-nix-co-2' />
|
||||
assert localServer -> !isNull db4;
|
||||
assert httpServer -> !isNull httpd && httpd.expat == expat; <co id='ex-svn-nix-co-3' />
|
||||
assert sslSupport -> !isNull openssl && (httpServer -> httpd.openssl == openssl);
|
||||
assert swigBindings -> !isNull swig;
|
||||
|
||||
derivation {
|
||||
name = "subversion-0.32.1";
|
||||
system = stdenv.system;
|
||||
|
||||
builder = ./builder.sh;
|
||||
src = fetchurl {
|
||||
url = http://svn.collab.net/tarballs/subversion-0.32.1.tar.gz;
|
||||
md5 = "b06717a8ef50db4b5c4d380af00bd901";
|
||||
};
|
||||
|
||||
localServer = localServer;
|
||||
httpServer = httpServer;
|
||||
sslSupport = sslSupport;
|
||||
swigBindings = swigBindings;
|
||||
|
||||
stdenv = stdenv;
|
||||
openssl = if sslSupport then openssl else null; <co id='ex-svn-nix-co-4' />
|
||||
httpd = if httpServer then httpd else null;
|
||||
expat = expat;
|
||||
db4 = if localServer then db4 else null;
|
||||
swig = if swigBindings then swig else null;
|
||||
}</programlisting>
|
||||
</example>
|
||||
|
||||
<para>This example shows several features. Default parameters <xref
|
||||
linkend='ex-svn-nix-co-1'/> can be used to simplify call sites: if an
|
||||
argument that has a default is omitted, its default value is
|
||||
used.</para>
|
||||
|
||||
<para>You can use <emphasis>assertions</emphasis> to test whether
|
||||
arguments satisfy certain constraints. The simple assertion <xref
|
||||
linkend='ex-svn-nix-co-2'/> tests whether the <varname>expat</varname>
|
||||
argument is not a null value. The more complex assertion <xref
|
||||
linkend='ex-svn-nix-co-3'/> says that if Subversion is built with
|
||||
Apache support, then <varname>httpd</varname> (the Apache package)
|
||||
must not be null and it must have been built using the same instance
|
||||
of the <varname>expat</varname> library as was passed to the
|
||||
Subversion expression. This is since the Subversion code is
|
||||
dynamically linked against the Apache code and they both use Expat,
|
||||
they must be linked against the same instance — otherwise a conflict
|
||||
might occur.</para>
|
||||
|
||||
</sect1>
|
||||
|
||||
|
|
|
|||
Loading…
Reference in a new issue