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Diffstat (limited to 'doc')
| -rw-r--r-- | doc/manual/build-farm.xml | 6 | ||||
| -rw-r--r-- | doc/manual/writing-nix-expressions.xml | 303 |
2 files changed, 192 insertions, 117 deletions
diff --git a/doc/manual/build-farm.xml b/doc/manual/build-farm.xml index 2dd0932fc..b0d3fb8b6 100644 --- a/doc/manual/build-farm.xml +++ b/doc/manual/build-farm.xml @@ -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> diff --git a/doc/manual/writing-nix-expressions.xml b/doc/manual/writing-nix-expressions.xml index eb366c249..b16d00b92 100644 --- a/doc/manual/writing-nix-expressions.xml +++ b/doc/manual/writing-nix-expressions.xml @@ -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: - - <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> +<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> - <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 -}: - -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> +</sect2> -<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> |