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-<chapter xmlns="http://docbook.org/ns/docbook"
-         xmlns:xlink="http://www.w3.org/1999/xlink"
-         xml:id='chap-writing-nix-expressions'
-         xmlns:xi="http://www.w3.org/2001/XInclude">
-
-<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 packages.  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>
-
-<note><para>This chapter is mostly about the Nix expression language.
-For more extensive information on adding packages to the Nix Packages
-collection (such as functions in the standard environment and coding
-conventions), please consult <link
-xlink:href="http://nixos.org/nixpkgs/manual/">its
-manual</link>.</para></note>
-
-
-<section><title>A simple Nix expression</title>
-
-<para>This section shows how to add and test the <link
-xlink:href='http://www.gnu.org/software/hello/hello.html'>GNU Hello
-package</link> to the Nix Packages collection.  Hello is a program
-that prints out the text <quote>Hello, world!</quote>.</para>
-
-<para>To add a package to the Nix Packages collection, you generally
-need to do three things:
-
-<orderedlist>
-
-  <listitem><para>Write a Nix expression for the package.  This is a
-  file that describes all the inputs involved in building the package,
-  such as dependencies, 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 package from
-  the inputs.</para></listitem>
-
-  <listitem><para>Add the package to the file
-  <filename>pkgs/top-level/all-packages.nix</filename>.  The Nix
-  expression written in the first step is a
-  <emphasis>function</emphasis>; it requires other packages 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
-  package.</para></listitem>
-
-</orderedlist>
-
-</para>
-
-
-<section><title>The Nix expression</title>
-
-<example xml:id='ex-hello-nix'><title>Nix expression for GNU Hello
-(<filename>default.nix</filename>)</title>
-<programlisting>
-{ stdenv, fetchurl, perl }: <co xml:id='ex-hello-nix-co-1' />
-
-stdenv.mkDerivation { <co xml:id='ex-hello-nix-co-2' />
-  name = "hello-2.1.1"; <co xml:id='ex-hello-nix-co-3' />
-  builder = ./builder.sh; <co xml:id='ex-hello-nix-co-4' />
-  src = fetchurl { <co xml: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 xml: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 package that is used
-    by almost all Nix Packages packages; 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.  <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 package.</para>
-
-  </callout>
-
-  <callout arearefs='ex-hello-nix-co-2'>
-
-    <para>So we have to build a package.  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 package from a set of
-    <emphasis>attributes</emphasis>.  A set is just a list of
-    key/value pairs where each key is a string and each value is an
-    arbitrary Nix expression.  They take the general form <literal>{
-    <replaceable>name1</replaceable> =
-    <replaceable>expr1</replaceable>; <replaceable>...</replaceable>
-    <replaceable>nameN</replaceable> =
-    <replaceable>exprN</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 package.  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
-    packages.  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 package
-    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 an MD5 hash of the expected contents of the file
-    at that URL, this function 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 trick: 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>
-
-</section>
-
-
-<section><title>The builder</title>
-
-<example xml:id='ex-hello-builder'><title>Build script for GNU Hello
-(<filename>builder.sh</filename>)</title>
-<programlisting>
-source $stdenv/setup <co xml:id='ex-hello-builder-co-1' />
-
-PATH=$perl/bin:$PATH <co xml:id='ex-hello-builder-co-2' />
-
-tar xvfz $src <co xml:id='ex-hello-builder-co-3' />
-cd hello-*
-./configure --prefix=$out <co xml:id='ex-hello-builder-co-4' />
-make <co xml: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 (except for the attributes declared in the
-    derivation).  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 package (since it
-    was passed in as an attribute to the derivation), so
-    <filename><replaceable>$perl</replaceable>/bin</filename> is the
-    directory containing the Perl interpreter.</para>
-
-  </callout>
-
-  <callout arearefs='ex-hello-builder-co-3'>
-
-    <para>Now we have to unpack the sources.  The
-    <varname>src</varname> attribute was bound to the result of
-    fetching the Hello source tarball from the network, so the
-    <envar>src</envar> environment variable points to the location in
-    the Nix store to which the tarball was downloaded.  After
-    unpacking, we <command>cd</command> to the resulting source
-    directory.</para>
-
-    <para>The whole build is performed in a temporary directory
-    created in <varname>/tmp</varname>, by the way.  This directory is
-    removed after the builder finishes, so there is no need to clean
-    up the sources afterwards.  Also, the temporary directory is
-    always newly created, so you don't have to worry about files from
-    previous builds interfering with the current build.</para>
-
-  </callout>
-
-  <callout arearefs='ex-hello-builder-co-4'>
-
-    <para>GNU Hello is a typical Autoconf-based package, so we first
-    have to run its <filename>configure</filename> script.  In Nix
-    every package is stored in a separate location in the Nix store,
-    for instance
-    <filename>/nix/store/9a54ba97fb71b65fda531012d0443ce2-hello-2.1.1</filename>.
-    Nix computes this path by cryptographically hashing all attributes
-    of the derivation.  The path is passed to the builder through the
-    <envar>out</envar> environment variable.  So here we give
-    <filename>configure</filename> the parameter
-    <literal>--prefix=$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>
-
-</section>
-
-
-<section><title>Composition</title>
-
-<example xml:id='ex-hello-composition'><title>Composing GNU Hello
-(<filename>all-packages.nix</filename>)</title>
-<programlisting>
-...
-
-rec { <co xml:id='ex-hello-composition-co-1' />
-
-  hello = import ../applications/misc/hello/ex-1 <co xml:id='ex-hello-composition-co-2' /> { <co xml:id='ex-hello-composition-co-3' />
-    inherit fetchurl stdenv perl;
-  };
-
-  perl = import ../development/interpreters/perl { <co xml: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/top-level/all-packages.nix</filename>, where all
-Nix expressions for packages are imported and called with the
-appropriate arguments.  <xref linkend='ex-hello-composition' /> shows
-some fragments of
-<filename>all-packages.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 packages 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.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 a 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>
-
-    <note><para>Nixpkgs has a convenience function
-    <function>callPackage</function> that imports and calls a
-    function, filling in any missing arguments by passing the
-    corresponding attribute from the Nixpkgs set, like this:
-
-<programlisting>
-hello = callPackage ../applications/misc/hello/ex-1 { };
-</programlisting>
-
-    If necessary, you can set or override arguments:
-
-<programlisting>
-hello = callPackage ../applications/misc/hello/ex-1 { stdenv = myStdenv; };
-</programlisting>
-
-    </para></note>
-
-  </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>
-
-</section>
-
-
-<section><title>Testing</title>
-
-<para>You can now try to build Hello.  Of course, you could do
-<literal>nix-env -f pkgs/top-level/all-packages.nix -i hello</literal>,
-but you may not want to install a possibly broken package just yet.
-The best way to test the package is by using the command <command
-linkend="sec-nix-build">nix-build</command>, which builds a Nix
-expression and creates a symlink named <filename>result</filename> in
-the current directory:
-
-<screen>
-$ nix-build pkgs/top-level/all-packages.nix -A hello
-building path `/nix/store/632d2b22514d...-hello-2.1.1'
-hello-2.1.1/
-hello-2.1.1/intl/
-hello-2.1.1/intl/ChangeLog
-<replaceable>...</replaceable>
-
-$ ls -l result
-lrwxrwxrwx ... 2006-09-29 10:43 result -> /nix/store/632d2b22514d...-hello-2.1.1
-
-$ ./result/bin/hello
-Hello, world!</screen>
-
-The <link linkend='opt-attr'><option>-A</option></link> option selects
-the <literal>hello</literal> attribute from
-<filename>all-packages.nix</filename>.  This is faster than using the
-symbolic package name specified by the <literal>name</literal>
-attribute (which also happens to be <literal>hello</literal>) and is
-unambiguous (there can be multiple packages with the symbolic name
-<literal>hello</literal>, but there can be only one attribute in a set
-named <literal>hello</literal>).</para>
-
-<para><command>nix-build</command> registers the
-<filename>./result</filename> symlink as a garbage collection root, so
-unless and until you delete the <filename>./result</filename> symlink,
-the output of the build will be safely kept on your system.  You can
-use <command>nix-build</command>’s <option
-linkend='opt-out-link'>-o</option> switch to give the symlink another
-name.</para>
-
-<para>Nix has a transactional semantics.  Once a build finishes
-successfully, 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 paths will not be registered as valid.  If you try to build
-the derivation again, Nix will remove the output paths if they exist
-(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:
-
-<screen>
-$ nix-build pkgs/top-level/all-packages.nix -A hello
-waiting for lock on `/nix/store/0h5b7hp8d4hqfrw8igvx97x1xawrjnac-hello-2.1.1x'</screen>
-
-So it is always safe to run multiple instances of Nix in parallel
-(which isn’t the case with, 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 <link linkend='opt-max-jobs'><option>-j
-<replaceable>N</replaceable></option></link>, where
-<replaceable>N</replaceable> is the maximum number of jobs to be run
-in parallel, or set.  Typically this should be the number of
-CPUs.</para>
-
-</section>
-
-
-<section><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 xml:id='ex-hello-builder2'><title>Build script using the generic
-build functions</title>
-<programlisting>
-buildInputs="$perl" <co xml:id='ex-hello-builder2-co-1' />
-
-source $stdenv/setup <co xml:id='ex-hello-builder2-co-2' />
-
-genericBuild <co xml:id='ex-hello-builder2-co-3' /></programlisting>
-</example>
-
-<calloutlist>
-
-  <callout arearefs='ex-hello-builder2-co-1'>
-
-    <para>The <envar>buildInputs</envar> variable tells
-    <filename>setup</filename> to use the indicated packages as
-    <quote>inputs</quote>.  This means that if a package provides a
-    <filename>bin</filename> subdirectory, it's added to
-    <envar>PATH</envar>; if it has a <filename>include</filename>
-    subdirectory, it's added to GCC's header search path; and so
-    on.<footnote><para>How does it work? <filename>setup</filename>
-    tries to source the file
-    <filename><replaceable>pkg</replaceable>/nix-support/setup-hook</filename>
-    of all dependencies.  These “setup hooks” can then set up whatever
-    environment variables they want; for instance, the setup hook for
-    Perl sets the <envar>PERL5LIB</envar> environment variable to
-    contain the <filename>lib/site_perl</filename> directories of all
-    inputs.</para></footnote>
-    </para>
-
-  </callout>
-
-  <callout arearefs='ex-hello-builder2-co-2'>
-
-    <para>The function <function>genericBuild</function> is defined in
-    the file <literal>$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>
-source $stdenv/setup
-genericBuild</programlisting>
-
-In fact, <varname>mkDerivation</varname> provides a default builder
-that looks exactly like that, so it is actually possible to omit the
-builder for Hello entirely.</para>
-
-</section>
-
-
-</section>
-
-
-
-<section><title>The Nix expression language</title>
-
-<para>The Nix expression language is a pure, lazy, functional
-language.  Purity means that operations in the language don't have
-side-effects (for instance, there is no variable assignment).
-Laziness means that arguments to functions are evaluated only when
-they are needed.  Functional means that functions are
-<quote>normal</quote> values that can be passed around and manipulated
-in interesting ways.  The language is not a full-featured, general
-purpose language.  Its main job is to describe packages,
-compositions of packages, and the variability within
-packages.</para>
-
-<para>This section presents the various features of the
-language.</para>
-
-
-<section xml:id='ssec-values'><title>Values</title>
-
-
-<simplesect><title>Simple values</title>
-
-<para>Nix has the following basic data types:
-
-<itemizedlist>
-
-  <listitem>
-
-    <para><emphasis>Strings</emphasis> can be written in three
-    ways.</para>
-
-    <para>The most common way is to enclose the string between double
-    quotes, e.g., <literal>"foo bar"</literal>.  Strings can span
-    multiple lines.  The special characters <literal>"</literal> and
-    <literal>\</literal> and the character sequence
-    <literal>${</literal> must be escaped by prefixing them with a
-    backslash (<literal>\</literal>).  Newlines, carriage returns and
-    tabs can be written as <literal>\n</literal>,
-    <literal>\r</literal> and <literal>\t</literal>,
-    respectively.</para>
-
-    <para>You can include the result of an expression into a string by
-    enclosing it in
-    <literal>${<replaceable>...</replaceable>}</literal>, a feature
-    known as <emphasis>antiquotation</emphasis>.  The enclosed
-    expression must evaluate to something that can be coerced into a
-    string (meaning that it must be a string, a path, or a
-    derivation).  For instance, rather than writing
-
-<programlisting>
-"--with-freetype2-library=" + freetype + "/lib"</programlisting>
-
-    (where <varname>freetype</varname> is a derivation), you can
-    instead write the more natural
-
-<programlisting>
-"--with-freetype2-library=${freetype}/lib"</programlisting>
-
-    The latter is automatically translated to the former.  A more
-    complicated example (from the Nix expression for <link
-    xlink:href='http://www.trolltech.com/products/qt'>Qt</link>):
-
-<programlisting>
-configureFlags = "
-  -system-zlib -system-libpng -system-libjpeg
-  ${if openglSupport then "-dlopen-opengl
-    -L${mesa}/lib -I${mesa}/include
-    -L${libXmu}/lib -I${libXmu}/include" else ""}
-  ${if threadSupport then "-thread" else "-no-thread"}
-";</programlisting>
-
-    Note that Nix expressions and strings can be arbitrarily nested;
-    in this case the outer string contains various antiquotations that
-    themselves contain strings (e.g., <literal>"-thread"</literal>),
-    some of which in turn contain expressions (e.g.,
-    <literal>${mesa}</literal>).</para>
-
-    <para>The second way to write string literals is as an
-    <emphasis>indented string</emphasis>, which is enclosed between
-    pairs of <emphasis>double single-quotes</emphasis>, like so:
-
-<programlisting>
-''
-  This is the first line.
-  This is the second line.
-    This is the third line.
-''</programlisting>
-
-    This kind of string literal intelligently strips indentation from
-    the start of each line.  To be precise, it strips from each line a
-    number of spaces equal to the minimal indentation of the string as
-    a whole (disregarding the indentation of empty lines).  For
-    instance, the first and second line are indented two space, while
-    the third line is indented four spaces.  Thus, two spaces are
-    stripped from each line, so the resulting string is
-
-<programlisting>
-"This is the first line.\nThis is the second line.\n  This is the third line.\n"</programlisting>
-
-    </para>
-
-    <para>Note that the whitespace and newline following the opening
-    <literal>''</literal> is ignored if there is no non-whitespace
-    text on the initial line.</para>
-
-    <para>Antiquotation
-    (<literal>${<replaceable>expr</replaceable>}</literal>) is
-    supported in indented strings.</para>
-
-    <para>Since <literal>${</literal> and <literal>''</literal> have
-    special meaning in indented strings, you need a way to quote them.
-    <literal>${</literal> can be escaped by prefixing it with
-    <literal>''</literal> (that is, two single quotes), i.e.,
-    <literal>''${</literal>.  <literal>''</literal> can be escaped by
-    prefixing it with <literal>'</literal>, i.e.,
-    <literal>'''</literal>.  Finally, linefeed, carriage-return and
-    tab characters can be written as <literal>''\n</literal>,
-    <literal>''\r</literal>, <literal>''\t</literal>.</para>
-
-    <para>Indented strings are primarily useful in that they allow
-    multi-line string literals to follow the indentation of the
-    enclosing Nix expression, and that less escaping is typically
-    necessary for strings representing languages such as shell scripts
-    and configuration files because <literal>''</literal> is much less
-    common than <literal>"</literal>.  Example:
-
-<programlisting>
-stdenv.mkDerivation {
-  <replaceable>...</replaceable>
-  postInstall =
-    ''
-      mkdir $out/bin $out/etc
-      cp foo $out/bin
-      echo "Hello World" > $out/etc/foo.conf
-      ${if enableBar then "cp bar $out/bin" else ""}
-    '';
-  <replaceable>...</replaceable>
-}
-</programlisting>
-
-    </para>
-
-    <para>Finally, as a convenience, <emphasis>URIs</emphasis> as
-    defined in appendix B of <link
-    xlink:href='http://www.ietf.org/rfc/rfc2396.txt'>RFC 2396</link>
-    can be written <emphasis>as is</emphasis>, without quotes.  For
-    instance, the string
-    <literal>"http://example.org/foo.tar.bz2"</literal>
-    can also be written as
-    <literal>http://example.org/foo.tar.bz2</literal>.</para>
-
-  </listitem>
-
-  <listitem><para><emphasis>Integers</emphasis>, e.g.,
-  <literal>123</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 file name 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 absolute 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>
-
-  <listitem><para>The null value, denoted as
-  <literal>null</literal>.</para></listitem>
-
-</itemizedlist>
-
-</para>
-
-</simplesect>
-
-
-<simplesect><title>Lists</title>
-
-<para>Lists are formed by enclosing a whitespace-separated list of
-values between square brackets.  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 a set.</para>
-
-</simplesect>
-
-
-<simplesect><title>Sets</title>
-
-<para>Sets are really the core of the language, since ultimately the
-Nix language is all about creating derivations, which are really just
-sets of attributes to be passed to build scripts.</para>
-
-<para>Sets are just a list of name/value pairs (called
-<emphasis>attributes</emphasis>) 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 a set with attributes named <varname>x</varname>,
-<varname>text</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 a set using the
-<literal>.</literal> operator.  For instance,
-
-<programlisting>
-{ a = "Foo"; b = "Bar"; }.a</programlisting>
-
-evaluates to <literal>"Foo"</literal>.  It is possible to provide a
-default value in an attribute selection using the
-<literal>or</literal> keyword.  For example,
-
-<programlisting>
-{ a = "Foo"; b = "Bar"; }.c or "Xyzzy"</programlisting>
-
-will evaluate to <literal>"Xyzzy"</literal> because there is no
-<varname>c</varname> attribute in the set.</para>
-
-<para>You can use arbitrary double-quoted strings as attribute
-names:
-
-<programlisting>
-{ "foo ${bar}" = 123; "nix-1.0" = 456; }."foo ${bar}"
-</programlisting>
-
-This will evaluate to <literal>123</literal> (Assuming
-<literal>bar</literal> is antiquotable). In the case where an
-attribute name is just a single antiquotation, the quotes can be
-dropped:
-
-<programlisting>
-{ foo = 123; }.${bar} or 456 </programlisting>
-
-This will evaluate to <literal>123</literal> if
-<literal>bar</literal> evaluates to <literal>"foo"</literal> when
-coerced to a string and <literal>456</literal> otherwise (again
-assuming <literal>bar</literal> is antiquotable).</para>
-
-<para>In the special case where an attribute name inside of a set declaration
-evaluates to <literal>null</literal> (which is normally an error, as
-<literal>null</literal> is not antiquotable), that attribute is simply not
-added to the set:
-
-<programlisting>
-{ ${if foo then "bar" else null} = true; }</programlisting>
-
-This will evaluate to <literal>{}</literal> if <literal>foo</literal>
-evaluates to <literal>false</literal>.</para>
-
-
-</simplesect>
-
-
-</section>
-
-
-<section><title>Language constructs</title>
-
-
-<simplesect><title>Recursive sets</title>
-
-<para>Recursive sets are just normal 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) set, attributes are not added to the
-lexical scope; in a recursive set, they are.</para>
-
-<para>Recursive 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 let-expression allows you define local variables for an
-expression.  For instance,
-
-<programlisting>
-let
-  x = "foo";
-  y = "bar";
-in x + y</programlisting>
-
-evaluates to <literal>"foobar"</literal>.
-
-</para>
-
-</simplesect>
-
-
-<simplesect><title>Inheriting attributes</title>
-
-<para>When defining a set it 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; in
-{ 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 set.  For instance, in this fragment
-from <filename>all-packages.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 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) set.</para>
-
-</simplesect>
-
-
-<simplesect xml:id="ss-functions"><title>Functions</title>
-
-<para>Functions have the following form:
-
-<programlisting>
-<replaceable>pattern</replaceable>: <replaceable>body</replaceable></programlisting>
-
-The pattern specifies what the argument of the function must look
-like, and binds variables in the body to (parts of) the
-argument.  There are three kinds of patterns:</para>
-
-<itemizedlist>
-
-
-  <listitem><para>If a pattern is a single identifier, then the
-  function matches any argument.  Example:
-
-  <programlisting>
-let negate = x: !x;
-    concat = x: y: x + y;
-in if negate true then concat "foo" "bar" else ""</programlisting>
-
-  Note that <function>concat</function> is a function that takes one
-  argument 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></listitem>
-
-
-  <listitem><para>A <emphasis>set pattern</emphasis> of the form
-  <literal>{ name1, name2, …, nameN }</literal> matches a set
-  containing the listed attributes, and binds the values of those
-  attributes to variables in the function body.  For example, the
-  function
-
-<programlisting>
-{ x, y, z }: z + y + x</programlisting>
-
-  can only be called with a set containing exactly the attributes
-  <varname>x</varname>, <varname>y</varname> and
-  <varname>z</varname>.  No other attributes are allowed.  If you want
-  to allow additional arguments, you can use an ellipsis
-  (<literal>...</literal>):
-
-<programlisting>
-{ x, y, z, ... }: z + y + x</programlisting>
-
-  This works on any set that contains at least the three named
-  attributes.</para>
-
-  <para>It is possible to provide <emphasis>default values</emphasis>
-  for attributes, in which case they are allowed to be missing.  A
-  default value is specified by writing
-  <literal><replaceable>name</replaceable> ?
-  <replaceable>e</replaceable></literal>, where
-  <replaceable>e</replaceable> is an arbitrary expression.  For example,
-
-<programlisting>
-{ x, y ? "foo", z ? "bar" }: z + y + x</programlisting>
-
-  specifies a function that only requires an attribute named
-  <varname>x</varname>, but optionally accepts <varname>y</varname>
-  and <varname>z</varname>.</para></listitem>
-
-
-  <listitem><para>An <literal>@</literal>-pattern provides a means of referring
-  to the whole value being matched:
-
-<programlisting>
-args@{ x, y, z, ... }: z + y + x + args.a</programlisting>
-
-  Here <varname>args</varname> is bound to the entire argument, which
-  is further matched against the pattern <literal>{ x, y, z,
-  ... }</literal>.</para></listitem>
-
-
-</itemizedlist>
-
-<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;
-in concat { x = "foo"; y = "bar"; }</programlisting>
-
-</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 xml: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 xml:id='ex-subversion-nix-co-1' />
-assert httpServer -> httpd != null &amp;&amp; httpd.expat == expat; <co xml:id='ex-subversion-nix-co-2' />
-assert sslSupport -> openssl != null &amp;&amp; (httpServer -> httpd.openssl == openssl); <co xml:id='ex-subversion-nix-co-3' />
-assert pythonBindings -> swig != null &amp;&amp; swig.pythonSupport;
-assert javaSwigBindings -> swig != null &amp;&amp; swig.javaSupport;
-assert javahlBindings -> j2sdk != null;
-
-stdenv.mkDerivation {
-  name = "subversion-1.1.1";
-  ...
-  openssl = if sslSupport then openssl else null; <co xml: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-3'>
-    <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 that
-    <emphasis>if</emphasis> Apache support is enabled, then Apache's
-    OpenSSL should match 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-expression</emphasis>,
-
-<programlisting>
-with <replaceable>e1</replaceable>; <replaceable>e2</replaceable></programlisting>
-
-introduces the set <replaceable>e1</replaceable> into the lexical
-scope of the expression <replaceable>e2</replaceable>.  For instance,
-
-<programlisting>
-let as = { x = "foo"; y = "bar"; };
-in 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>Comments</title>
-
-<para>Comments can be single-line, started with a <literal>#</literal>
-character, or inline/multi-line, enclosed within <literal>/*
-... */</literal>.</para>
-
-</simplesect>
-
-
-</section>
-
-
-<section><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 xml: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>e</replaceable> <literal>.</literal>
-        <replaceable>attrpath</replaceable>
-        [ <literal>or</literal> <replaceable>def</replaceable> ]
-        </entry>
-        <entry>none</entry>
-        <entry>Select attribute denoted by the attribute path
-        <replaceable>attrpath</replaceable> from set
-        <replaceable>e</replaceable>.  (An attribute path is a
-        dot-separated list of attribute names.)  If the attribute
-        doesn’t exist, return <replaceable>def</replaceable> if
-        provided, otherwise abort evaluation.</entry>
-      </row>
-      <row>
-        <entry><replaceable>e1</replaceable> <replaceable>e2</replaceable></entry>
-        <entry>left</entry>
-        <entry>Call function <replaceable>e1</replaceable> with
-        argument <replaceable>e2</replaceable>.</entry>
-      </row>
-      <row>
-        <entry><replaceable>e</replaceable> <literal>?</literal>
-        <replaceable>attrpath</replaceable></entry>
-        <entry>none</entry>
-        <entry>Test whether set <replaceable>e</replaceable> contains
-        the attribute denoted by <replaceable>attrpath</replaceable>;
-        return <literal>true</literal> or
-        <literal>false</literal>.</entry>
-      </row>
-      <row>
-        <entry><replaceable>e1</replaceable> <literal>++</literal> <replaceable>e2</replaceable></entry>
-        <entry>right</entry>
-        <entry>List concatenation.</entry>
-      </row>
-      <row>
-        <entry><replaceable>e1</replaceable> <literal>+</literal> <replaceable>e2</replaceable></entry>
-        <entry>left</entry>
-        <entry>String or path concatenation.</entry>
-      </row>
-      <row>
-        <entry><literal>!</literal> <replaceable>e</replaceable></entry>
-        <entry>left</entry>
-        <entry>Boolean negation.</entry>
-      </row>
-      <row>
-        <entry><replaceable>e1</replaceable> <literal>//</literal>
-        <replaceable>e2</replaceable></entry>
-        <entry>right</entry>
-        <entry>Return a 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> <literal>==</literal>
-        <replaceable>e2</replaceable></entry>
-        <entry>none</entry>
-        <entry>Equality.</entry>
-      </row>
-      <row>
-        <entry><replaceable>e1</replaceable> <literal>!=</literal>
-        <replaceable>e2</replaceable></entry>
-        <entry>none</entry>
-        <entry>Inequality.</entry>
-      </row>
-      <row>
-        <entry><replaceable>e1</replaceable> <literal>&amp;&amp;</literal>
-        <replaceable>e2</replaceable></entry>
-        <entry>left</entry>
-        <entry>Logical AND.</entry>
-      </row>
-      <row>
-        <entry><replaceable>e1</replaceable> <literal>||</literal>
-        <replaceable>e2</replaceable></entry>
-        <entry>left</entry>
-        <entry>Logical OR.</entry>
-      </row>
-      <row>
-        <entry><replaceable>e1</replaceable> <literal>-></literal>
-        <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>
-
-</section>
-
-
-<section xml:id="ssec-derivation"><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 a set, the attributes
-of which specify the inputs of the build.</para>
-
-<itemizedlist>
-
-  <listitem xml:id="attr-system"><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='chap-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 package by <command>nix-env</command>,
-  and it is appended to the output paths 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 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; its
-      default 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>
-
-      <listitem><para><literal>true</literal> is passed as the string
-      <literal>1</literal>, <literal>false</literal> and
-      <literal>null</literal> are passed as an empty string.
-      </para></listitem>
-    </itemizedlist>
-
-  </para></listitem>
-
-  <listitem><para>The optional attribute <varname>args</varname>
-  specifies command-line arguments to be passed to the builder.  It
-  should be a list.</para></listitem>
-
-  <listitem><para>The optional attribute <varname>outputs</varname>
-  specifies a list of symbolic outputs of the derivation.  By default,
-  a derivation produces a single output path, denoted as
-  <literal>out</literal>.  However, derivations can produce multiple
-  output paths.  This is useful because it allows outputs to be
-  downloaded or garbage-collected separately.  For instance, imagine a
-  library package that provides a dynamic library, header files, and
-  documentation.  A program that links against the library doesn’t
-  need the header files and documentation at runtime, and it doesn’t
-  need the documentation at build time.  Thus, the library package
-  could specify:
-<programlisting>
-outputs = [ "lib" "headers" "doc" ];
-</programlisting>
-  This will cause Nix to pass environment variables
-  <literal>lib</literal>, <literal>headers</literal> and
-  <literal>doc</literal> to the builder containing the intended store
-  paths of each output.  The builder would typically do something like
-<programlisting>
-./configure --libdir=$lib/lib --includedir=$headers/include --docdir=$doc/share/doc
-</programlisting>
-  for an Autoconf-style package.  You can refer to each output of a
-  derivation by selecting it as an attribute, e.g.
-<programlisting>
-buildInputs = [ pkg.lib pkg.headers ];
-</programlisting>
-  The first element of <varname>output</varname> determines the
-  <emphasis>default output</emphasis>.  Thus, you could also write
-<programlisting>
-buildInputs = [ pkg pkg.headers ];
-</programlisting>
-  since <literal>pkg</literal> is equivalent to
-  <literal>pkg.lib</literal>.</para></listitem>
-
-</itemizedlist>
-
-<para>The function <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>For each output declared in
-    <varname>outputs</varname>, the corresponding environment variable
-    is set to point to the intended path in the Nix store for that
-    output.  Each output path is a concatenation of the cryptographic
-    hash of all build inputs, the <varname>name</varname> attribute
-    and the output name.  (The output name is omitted if it’s
-    <literal>out</literal>.)</para></listitem>
-
-  </itemizedlist>
-
-  </para></listitem>
-
-  <listitem><para>If an 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 exits 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 successful, Nix scans each output
-  path for references to input paths by looking for the hash parts of
-  the input paths.  Since these are potential runtime dependencies,
-  Nix registers them as dependencies of the output
-  paths.</para></listitem>
-
-  <listitem><para>After the build, Nix sets the last-modified
-  timestamp on all files in the build result to 1 (00:00:01 1/1/1970
-  UTC), sets the group to the default group, and sets the mode of the
-  file to 0444 or 0555 (i.e., read-only, with execute permission
-  enabled if the file was originally executable).  Note that possible
-  <literal>setuid</literal> and <literal>setgid</literal> bits are
-  cleared.  Setuid and setgid programs are not currently supported by
-  Nix.  This is because the Nix archives used in deployment have no
-  concept of ownership information, and because it makes the build
-  result dependent on the user performing the build.</para></listitem>
-
-</itemizedlist>
-
-</para>
-
-
-<section><title>Advanced attributes</title>
-
-<para>Derivations can declare some infrequently used optional
-attributes.</para>
-
-<variablelist>
-
-  <varlistentry><term><varname>allowedReferences</varname></term>
-
-    <listitem><para>The optional attribute
-    <varname>allowedReferences</varname> specifies a list of legal
-    references (dependencies) of the output of the builder.  For
-    example,
-
-<programlisting>
-allowedReferences = [];
-</programlisting>
-
-    enforces that the output of a derivation cannot have any runtime
-    dependencies on its inputs.  To allow an output to have a runtime
-    dependency on itself, use <literal>"out"</literal> as a list item.
-    This is used in NixOS to check that generated files such as
-    initial ramdisks for booting Linux don’t have accidental
-    dependencies on other paths in the Nix store.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><varname>exportReferencesGraph</varname></term>
-
-    <listitem><para>This attribute allows builders access to the
-    references graph of their inputs.  The attribute is a list of
-    inputs in the Nix store whose references graph the builder needs
-    to know.  The value of this attribute should be a list of pairs
-    <literal>[ <replaceable>name1</replaceable>
-    <replaceable>path1</replaceable> <replaceable>name2</replaceable>
-    <replaceable>path2</replaceable> <replaceable>...</replaceable>
-    ]</literal>.  The references graph of each
-    <replaceable>pathN</replaceable> will be stored in a text file
-    <replaceable>nameN</replaceable> in the temporary build directory.
-    The text files have the format used by <command>nix-store
-    --register-validity</command> (with the deriver fields left
-    empty).  For example, when the following derivation is built:
-
-<programlisting>
-derivation {
-  ...
-  exportReferencesGraph = [ "libfoo-graph" libfoo ];
-};
-</programlisting>
-
-    the references graph of <literal>libfoo</literal> is placed in the
-    file <filename>libfoo-graph</filename> in the temporary build
-    directory.</para>
-
-    <para><varname>exportReferencesGraph</varname> is useful for
-    builders that want to do something with the closure of a store
-    path.  Examples include the builders in NixOS that generate the
-    initial ramdisk for booting Linux (a <command>cpio</command>
-    archive containing the closure of the boot script) and the
-    ISO-9660 image for the installation CD (which is populated with a
-    Nix store containing the closure of a bootable NixOS
-    configuration).</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry xml:id="fixed-output-drvs">
-    <term><varname>outputHash</varname></term>
-    <term><varname>outputHashAlgo</varname></term>
-    <term><varname>outputHashMode</varname></term>
-
-    <listitem><para>These attributes declare that the derivation is a
-    so-called <emphasis>fixed-output derivation</emphasis>, which
-    means that a cryptographic hash of the output is already known in
-    advance.  When the build of a fixed-output derivation finishes,
-    Nix computes the cryptographic hash of the output and compares it
-    to the hash declared with these attributes.  If there is a
-    mismatch, the build fails.</para>
-
-    <para>The rationale for fixed-output derivations is derivations
-    such as those produced by the <function>fetchurl</function>
-    function.  This function downloads a file from a given URL.  To
-    ensure that the downloaded file has not been modified, the caller
-    must also specify a cryptographic hash of the file.  For example,
-
-<programlisting>
-fetchurl {
-  url = http://ftp.gnu.org/pub/gnu/hello/hello-2.1.1.tar.gz;
-  md5 = "70c9ccf9fac07f762c24f2df2290784d";
-}
-</programlisting>
-
-    It sometimes happens that the URL of the file changes, e.g.,
-    because servers are reorganised or no longer available.  We then
-    must update the call to <function>fetchurl</function>, e.g.,
-
-<programlisting>
-fetchurl {
-  url = ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz;
-  md5 = "70c9ccf9fac07f762c24f2df2290784d";
-}
-</programlisting>
-
-    If a <function>fetchurl</function> derivation was treated like a
-    normal derivation, the output paths of the derivation and
-    <emphasis>all derivations depending on it</emphasis> would change.
-    For instance, if we were to change the URL of the Glibc source
-    distribution in Nixpkgs (a package on which almost all other
-    packages depend) massive rebuilds would be needed.  This is
-    unfortunate for a change which we know cannot have a real effect
-    as it propagates upwards through the dependency graph.</para>
-
-    <para>For fixed-output derivations, on the other hand, the name of
-    the output path only depends on the <varname>outputHash*</varname>
-    and <varname>name</varname> attributes, while all other attributes
-    are ignored for the purpose of computing the output path.  (The
-    <varname>name</varname> attribute is included because it is part
-    of the path.)</para>
-
-    <para>As an example, here is the (simplified) Nix expression for
-    <varname>fetchurl</varname>:
-
-<programlisting>
-{ stdenv, curl }: # The <command>curl</command> program is used for downloading.
-
-{ url, md5 }:
-
-stdenv.mkDerivation {
-  name = baseNameOf (toString url);
-  builder = ./builder.sh;
-  buildInputs = [ curl ];
-
-  # This is a fixed-output derivation; the output must be a regular
-  # file with MD5 hash <varname>md5</varname>.
-  outputHashMode = "flat";
-  outputHashAlgo = "md5";
-  outputHash = md5;
-
-  inherit url;
-}
-</programlisting>
-
-    </para>
-
-    <para>The <varname>outputHashAlgo</varname> attribute specifies
-    the hash algorithm used to compute the hash.  It can currently be
-    <literal>"md5"</literal>, <literal>"sha1"</literal> or
-    <literal>"sha256"</literal>.</para>
-
-    <para>The <varname>outputHashMode</varname> attribute determines
-    how the hash is computed.  It must be one of the following two
-    values:
-
-    <variablelist>
-
-      <varlistentry><term><literal>"flat"</literal></term>
-
-        <listitem><para>The output must be a non-executable regular
-        file.  If it isn’t, the build fails.  The hash is simply
-        computed over the contents of that file (so it’s equal to what
-        Unix commands like <command>md5sum</command> or
-        <command>sha1sum</command> produce).</para>
-
-        <para>This is the default.</para></listitem>
-
-      </varlistentry>
-
-      <varlistentry><term><literal>"recursive"</literal></term>
-
-        <listitem><para>The hash is computed over the NAR archive dump
-        of the output (i.e., the result of <link
-        linkend="refsec-nix-store-dump"><command>nix-store
-        --dump</command></link>).  In this case, the output can be
-        anything, including a directory tree.</para></listitem>
-
-      </varlistentry>
-
-    </variablelist>
-
-    </para>
-
-    <para>The <varname>outputHash</varname> attribute, finally, must
-    be a string containing the hash in either hexadecimal or base-32
-    notation.  (See the <link
-    linkend="sec-nix-hash"><command>nix-hash</command> command</link>
-    for information about converting to and from base-32
-    notation.)</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><varname>impureEnvVars</varname></term>
-
-    <listitem><para>This attribute allows you to specify a list of
-    environment variables that should be passed from the environment
-    of the calling user to the builder.  Usually, the environment is
-    cleared completely when the builder is executed, but with this
-    attribute you can allow specific environment variables to be
-    passed unmodified.  For example, <function>fetchurl</function> in
-    Nixpkgs has the line
-
-<programlisting>
-impureEnvVars = [ "http_proxy" "https_proxy" <replaceable>...</replaceable> ];
-</programlisting>
-
-    to make it use the proxy server configuration specified by the
-    user in the environment variables <envar>http_proxy</envar> and
-    friends.</para>
-
-    <para>This attribute is only allowed in <link
-    linkend="fixed-output-drvs">fixed-output derivations</link>, where
-    impurities such as these are okay since (the hash of) the output
-    is known in advance.  It is ignored for all other
-    derivations.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><varname>preferLocalBuild</varname></term>
-
-    <listitem><para>If this attribute is set to
-    <literal>true</literal>, it has two effects.  First, the
-    derivation will always be built, not substituted, even if a
-    substitute is available.  Second, if <link
-    linkend="chap-distributed-builds">distributed building is
-    enabled</link>, then, if possible, the derivaton will be built
-    locally instead of forwarded to a remote machine.  This is
-    appropriate for trivial builders where the cost of doing a
-    download or remote build would exceed the cost of building
-    locally.</para></listitem>
-
-  </varlistentry>
-
-</variablelist>
-
-</section>
-
-
-</section>
-
-
-
-<xi:include href="builtins.xml" />
-
-
-</section>
-
-
-
-<section xml:id='sec-standard-environment'><title>The standard environment</title>
-
-
-<para>The standard environment is used by passing it as an input
-called <envar>stdenv</envar> to the derivation, and then doing
-
-<programlisting>
-source $stdenv/setup</programlisting>
-
-at the top of the builder.</para>
-
-<para>Apart from adding the aforementioned commands to the
-<envar>PATH</envar>, <filename>setup</filename> also does the
-following:
-
-<itemizedlist>
-
-  <listitem><para>All input packages specified in the
-  <envar>buildInputs</envar> environment variable have their
-  <filename>/bin</filename> subdirectory added to <envar>PATH</envar>,
-  their <filename>/include</filename> subdirectory added to the C/C++
-  header file search path, and their <filename>/lib</filename>
-  subdirectory added to the linker search path.  This can be extended.
-  For instance, when the <command>pkgconfig</command> package is
-  used, the subdirectory <filename>/lib/pkgconfig</filename> of each
-  input is added to the <envar>PKG_CONFIG_PATH</envar> environment
-  variable.</para></listitem>
-
-  <listitem><para>The environment variable
-  <envar>NIX_CFLAGS_STRIP</envar> is set so that the compiler strips
-  debug information from object files.  This can be disabled by
-  setting <envar>NIX_STRIP_DEBUG</envar> to
-  <literal>0</literal>.</para></listitem>
-
-</itemizedlist>
-
-</para>
-
-<para>The <filename>setup</filename> script also exports a function
-called <function>genericBuild</function> that knows how to build
-typical Autoconf-style packages.  It can be customised to perform
-builds for any type of package.  It is advisable to use
-<function>genericBuild</function> since it provides facilities that
-are almost always useful such as unpacking of sources, patching of
-sources, nested logging, etc.</para>
-
-<para>The definitive, up-to-date documentation of the generic builder
-is the source itself, which resides in
-<filename>pkgs/stdenv/generic/setup.sh</filename>.</para>
-
-
-<section><title>Customising the generic builder</title>
-
-<para>The operation of the generic builder can be modified in many
-places by setting certain variables.  These <emphasis>hook
-variables</emphasis> are typically set to the name of some shell
-function defined by you.  For instance, to perform some additional
-steps after <command>make install</command> you would set the
-<varname>postInstall</varname> variable:
-
-<programlisting>
-postInstall=myPostInstall
-
-myPostInstall() {
-    mkdir $out/share/extra
-    cp extrafiles/* $out/share/extra
-}</programlisting>
-
-</para>
-
-
-</section>
-
-
-<section><title>Debugging failed builds</title>
-
-<para>At the beginning of each phase, the set of all shell variables
-is written to the file <filename>env-vars</filename> at the top-level
-build directory.  This is useful for debugging: it allows you to
-recreate the environment in which a build was performed.  For
-instance, if a build fails, then assuming you used the
-<option>-K</option> flag, you can go to the output directory and
-<quote>switch</quote> to the environment of the builder:
-
-<screen>
-$ nix-build -K ./foo.nix
-... fails, keeping build directory `/tmp/nix-1234-0'
-
-$ cd /tmp/nix-1234-0
-
-$ source env-vars
-
-<lineannotation>(edit some files...)</lineannotation>
-
-$ make
-
-<lineannotation>(execution continues with the same GCC, make, etc.)</lineannotation></screen>
-
-</para>
-
-</section>
-
-
-</section>
-
-
-</chapter>