Restructuring the Nix manual

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-<section xmlns="http://docbook.org/ns/docbook"
-         xmlns:xlink="http://www.w3.org/1999/xlink"
-         xml:id='ssec-builtins'>
-
-<title>Built-in functions</title>
-
-
-<para>This section lists the functions and constants built into the
-Nix expression evaluator.  (The built-in function
-<function>derivation</function> is discussed above.)  Some built-ins,
-such as <function>derivation</function>, are always in scope of every
-Nix expression; you can just access them right away.  But to prevent
-polluting the namespace too much, most built-ins are not in scope.
-Instead, you can access them through the <varname>builtins</varname>
-built-in value, which is a set that contains all built-in functions
-and values.  For instance, <function>derivation</function> is also
-available as <function>builtins.derivation</function>.</para>
-
-
-<variablelist>
-
-
-  <varlistentry><term><function>abort</function> <replaceable>s</replaceable></term>
-
-    <listitem><para>Abort Nix expression evaluation, print error
-    message <replaceable>s</replaceable>.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.add</function>
-  <replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
-
-    <listitem><para>Return the sum of the integers
-    <replaceable>e1</replaceable> and
-    <replaceable>e2</replaceable>.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.attrNames</function>
-  <replaceable>set</replaceable></term>
-
-    <listitem><para>Return the names of the attributes in the set
-    <replaceable>set</replaceable> in a sorted list.  For instance,
-    <literal>builtins.attrNames { y = 1; x = "foo"; }</literal>
-    evaluates to <literal>[ "x" "y" ]</literal>.  There is no built-in
-    function <function>attrValues</function>, but you can easily
-    define it yourself:
-
-<programlisting>
-attrValues = set: map (name: builtins.getAttr name set) (builtins.attrNames set);</programlisting>
-
-    </para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>baseNameOf</function> <replaceable>s</replaceable></term>
-
-    <listitem><para>Return the <emphasis>base name</emphasis> of the
-    string <replaceable>s</replaceable>, that is, everything following
-    the final slash in the string.  This is similar to the GNU
-    <command>basename</command> command.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><varname>builtins</varname></term>
-
-    <listitem><para>The set <varname>builtins</varname> contains all
-    the built-in functions and values.  You can use
-    <varname>builtins</varname> to test for the availability of
-    features in the Nix installation, e.g.,
-
-<programlisting>
-if builtins ? getEnv then builtins.getEnv "PATH" else ""</programlisting>
-
-    This allows a Nix expression to fall back gracefully on older Nix
-    installations that don’t have the desired built-in
-    function.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.compareVersions</function>
-  <replaceable>s1</replaceable> <replaceable>s2</replaceable></term>
-
-    <listitem><para>Compare two strings representing versions and
-    return <literal>-1</literal> if version
-    <replaceable>s1</replaceable> is older than version
-    <replaceable>s2</replaceable>, <literal>0</literal> if they are
-    the same, and <literal>1</literal> if
-    <replaceable>s1</replaceable> is newer than
-    <replaceable>s2</replaceable>.  The version comparison algorithm
-    is the same as the one used by <link
-    linkend="ssec-version-comparisons"><command>nix-env
-    -u</command></link>.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.concatLists</function>
-  <replaceable>lists</replaceable></term>
-
-    <listitem><para>Concatenate a list of lists into a single
-    list.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry
-  xml:id='builtin-currentSystem'><term><varname>builtins.currentSystem</varname></term>
-
-    <listitem><para>The built-in value <varname>currentSystem</varname>
-    evaluates to the Nix platform identifier for the Nix installation
-    on which the expression is being evaluated, such as
-    <literal>"i686-linux"</literal> or
-    <literal>"powerpc-darwin"</literal>.</para></listitem>
-
-  </varlistentry>
-
-
-  <!--
-  <varlistentry><term><function>currentTime</function></term>
-
-    <listitem><para>The built-in value <varname>currentTime</varname>
-    returns the current system time in seconds since 00:00:00 1/1/1970
-    UTC.  Due to the evaluation model of Nix expressions
-    (<emphasis>maximal laziness</emphasis>), it always yields the same
-    value within an execution of Nix.</para></listitem>
-
-  </varlistentry>
-  -->
-
-
-  <!--
-  <varlistentry><term><function>dependencyClosure</function></term>
-
-    <listitem><para>TODO</para></listitem>
-
-  </varlistentry>
-  -->
-
-
-  <varlistentry><term><function>derivation</function>
-  <replaceable>attrs</replaceable></term>
-
-    <listitem><para><function>derivation</function> is described in
-    <xref linkend='ssec-derivation' />.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>dirOf</function> <replaceable>s</replaceable></term>
-
-    <listitem><para>Return the directory part of the string
-    <replaceable>s</replaceable>, that is, everything before the final
-    slash in the string.  This is similar to the GNU
-    <command>dirname</command> command.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.div</function>
-  <replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
-
-    <listitem><para>Return the quotient of the integers
-    <replaceable>e1</replaceable> and
-    <replaceable>e2</replaceable>.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.elem</function>
-  <replaceable>x</replaceable> <replaceable>xs</replaceable></term>
-
-    <listitem><para>Return <literal>true</literal> if a value equal to
-    <replaceable>x</replaceable> occurs in the list
-    <replaceable>xs</replaceable>, and <literal>false</literal>
-    otherwise.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.elemAt</function>
-  <replaceable>xs</replaceable> <replaceable>n</replaceable></term>
-
-    <listitem><para>Return element <replaceable>n</replaceable> from
-    the list <replaceable>xs</replaceable>.  Elements are counted
-    starting from 0.  A fatal error occurs in the index is out of
-    bounds.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.filter</function>
-  <replaceable>f</replaceable> <replaceable>xs</replaceable></term>
-
-    <listitem><para>Return a list consisting of the elements of
-    <replaceable>xs</replaceable> for which the function
-    <replaceable>f</replaceable> returns
-    <literal>true</literal>.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.filterSource</function>
-  <replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
-
-    <listitem>
-
-      <para>This function allows you to copy sources into the Nix
-      store while filtering certain files.  For instance, suppose that
-      you want to use the directory <filename>source-dir</filename> as
-      an input to a Nix expression, e.g.
-
-<programlisting>
-stdenv.mkDerivation {
-  ...
-  src = ./source-dir;
-}
-</programlisting>
-
-      However, if <filename>source-dir</filename> is a Subversion
-      working copy, then all those annoying <filename>.svn</filename>
-      subdirectories will also be copied to the store.  Worse, the
-      contents of those directories may change a lot, causing lots of
-      spurious rebuilds.  With <function>filterSource</function> you
-      can filter out the <filename>.svn</filename> directories:
-
-<programlisting>
-  src = builtins.filterSource
-    (path: type: type != "directory" || baseNameOf path != ".svn")
-    ./source-dir;
-</programlisting>
-
-      </para>
-
-      <para>Thus, the first argument <replaceable>e1</replaceable>
-      must be a predicate function that is called for each regular
-      file, directory or symlink in the source tree
-      <replaceable>e2</replaceable>.  If the function returns
-      <literal>true</literal>, the file is copied to the Nix store,
-      otherwise it is omitted.  The function is called with two
-      arguments.  The first is the full path of the file.  The second
-      is a string that identifies the type of the file, which is
-      either <literal>"regular"</literal>,
-      <literal>"directory"</literal>, <literal>"symlink"</literal> or
-      <literal>"unknown"</literal> (for other kinds of files such as
-      device nodes or fifos — but note that those cannot be copied to
-      the Nix store, so if the predicate returns
-      <literal>true</literal> for them, the copy will fail).</para>
-
-    </listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.fromJSON</function> <replaceable>e</replaceable></term>
-
-    <listitem><para>Convert a JSON string to a Nix
-    value. For example,
-
-<programlisting>
-builtins.fromJSON ''{"x": [1, 2, 3], "y": null}''
-</programlisting>
-
-    returns the value <literal>{ x = [ 1 2 3 ]; y = null;
-    }</literal>. Floating point numbers are not
-    supported.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.getAttr</function>
-  <replaceable>s</replaceable> <replaceable>set</replaceable></term>
-
-    <listitem><para><function>getAttr</function> returns the attribute
-    named <replaceable>s</replaceable> from
-    <replaceable>set</replaceable>.  Evaluation aborts if the
-    attribute doesn’t exist.  This is a dynamic version of the
-    <literal>.</literal> operator, since <replaceable>s</replaceable>
-    is an expression rather than an identifier.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.getEnv</function>
-  <replaceable>s</replaceable></term>
-
-    <listitem><para><function>getEnv</function> returns the value of
-    the environment variable <replaceable>s</replaceable>, or an empty
-    string if the variable doesn’t exist.  This function should be
-    used with care, as it can introduce all sorts of nasty environment
-    dependencies in your Nix expression.</para>
-
-    <para><function>getEnv</function> is used in Nix Packages to
-    locate the file <filename>~/.nixpkgs/config.nix</filename>, which
-    contains user-local settings for Nix Packages.  (That is, it does
-    a <literal>getEnv "HOME"</literal> to locate the user’s home
-    directory.)</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.hasAttr</function>
-  <replaceable>s</replaceable> <replaceable>set</replaceable></term>
-
-    <listitem><para><function>hasAttr</function> returns
-    <literal>true</literal> if <replaceable>set</replaceable> has an
-    attribute named <replaceable>s</replaceable>, and
-    <literal>false</literal> otherwise.  This is a dynamic version of
-    the <literal>?</literal>  operator, since
-    <replaceable>s</replaceable> is an expression rather than an
-    identifier.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.hashString</function>
-  <replaceable>type</replaceable> <replaceable>s</replaceable></term>
-
-    <listitem><para>Return a base-16 representation of the
-    cryptographic hash of string <replaceable>s</replaceable>.  The
-    hash algorithm specified by <replaceable>type</replaceable> must
-    be one of <literal>"md5"</literal>, <literal>"sha1"</literal> or
-    <literal>"sha256"</literal>.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.head</function>
-  <replaceable>list</replaceable></term>
-
-    <listitem><para>Return the first element of a list; abort
-    evaluation if the argument isn’t a list or is an empty list.  You
-    can test whether a list is empty by comparing it with
-    <literal>[]</literal>.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>import</function>
-  <replaceable>path</replaceable></term>
-
-    <listitem><para>Load, parse and return the Nix expression in the
-    file <replaceable>path</replaceable>.  If <replaceable>path
-    </replaceable> is a directory, the file <filename>default.nix
-    </filename> in that directory is loaded.  Evaluation aborts if the
-    file doesn’t exist or contains an incorrect Nix expression.
-    <function>import</function> implements Nix’s module system: you
-    can put any Nix expression (such as a set or a function) in a
-    separate file, and use it from Nix expressions in other
-    files.</para>
-
-    <para>A Nix expression loaded by <function>import</function> must
-    not contain any <emphasis>free variables</emphasis> (identifiers
-    that are not defined in the Nix expression itself and are not
-    built-in).  Therefore, it cannot refer to variables that are in
-    scope at the call site.  For instance, if you have a calling
-    expression
-
-<programlisting>
-rec {
-  x = 123;
-  y = import ./foo.nix;
-}</programlisting>
-
-    then the following <filename>foo.nix</filename> will give an
-    error:
-
-<programlisting>
-x + 456</programlisting>
-
-    since <varname>x</varname> is not in scope in
-    <filename>foo.nix</filename>.  If you want <varname>x</varname>
-    to be available in <filename>foo.nix</filename>, you should pass
-    it as a function argument:
-
-<programlisting>
-rec {
-  x = 123;
-  y = import ./foo.nix x;
-}</programlisting>
-
-    and
-
-<programlisting>
-x: x + 456</programlisting>
-
-    (The function argument doesn’t have to be called
-    <varname>x</varname> in <filename>foo.nix</filename>; any name
-    would work.)</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.intersectAttrs</function>
-  <replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
-
-    <listitem><para>Return a set consisting of the attributes in the
-    set <replaceable>e2</replaceable> that also exist in the set
-    <replaceable>e1</replaceable>.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.isAttrs</function>
-  <replaceable>e</replaceable></term>
-
-    <listitem><para>Return <literal>true</literal> if
-    <replaceable>e</replaceable> evaluates to a set, and
-    <literal>false</literal> otherwise.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.isList</function>
-  <replaceable>e</replaceable></term>
-
-    <listitem><para>Return <literal>true</literal> if
-    <replaceable>e</replaceable> evaluates to a list, and
-    <literal>false</literal> otherwise.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.isFunction</function>
-  <replaceable>e</replaceable></term>
-
-    <listitem><para>Return <literal>true</literal> if
-    <replaceable>e</replaceable> evaluates to a function, and
-    <literal>false</literal> otherwise.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.isString</function>
-  <replaceable>e</replaceable></term>
-
-    <listitem><para>Return <literal>true</literal> if
-    <replaceable>e</replaceable> evaluates to a string, and
-    <literal>false</literal> otherwise.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.isInt</function>
-  <replaceable>e</replaceable></term>
-
-    <listitem><para>Return <literal>true</literal> if
-    <replaceable>e</replaceable> evaluates to an int, and
-    <literal>false</literal> otherwise.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.isBool</function>
-  <replaceable>e</replaceable></term>
-
-    <listitem><para>Return <literal>true</literal> if
-    <replaceable>e</replaceable> evaluates to a bool, and
-    <literal>false</literal> otherwise.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>isNull</function>
-  <replaceable>e</replaceable></term>
-
-    <listitem><para>Return <literal>true</literal> if
-    <replaceable>e</replaceable> evaluates to <literal>null</literal>,
-    and <literal>false</literal> otherwise.</para>
-
-    <warning><para>This function is <emphasis>deprecated</emphasis>;
-    just write <literal>e == null</literal> instead.</para></warning>
-
-    </listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.length</function>
-  <replaceable>e</replaceable></term>
-
-    <listitem><para>Return the length of the list
-    <replaceable>e</replaceable>.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.lessThan</function>
-  <replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
-
-    <listitem><para>Return <literal>true</literal> if the integer
-    <replaceable>e1</replaceable> is less than the integer
-    <replaceable>e2</replaceable>, and <literal>false</literal>
-    otherwise.  Evaluation aborts if either
-    <replaceable>e1</replaceable> or <replaceable>e2</replaceable>
-    does not evaluate to an integer.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.listToAttrs</function>
-  <replaceable>e</replaceable></term>
-
-    <listitem><para>Construct a set from a list specifying the names
-    and values of each attribute.  Each element of the list should be
-    a set consisting of a string-valued attribute
-    <varname>name</varname> specifying the name of the attribute, and
-    an attribute <varname>value</varname> specifying its value.
-    Example:
-
-<programlisting>
-builtins.listToAttrs
-  [ { name = "foo"; value = 123; }
-    { name = "bar"; value = 456; }
-  ]
-</programlisting>
-
-    evaluates to
-
-<programlisting>
-{ foo = 123; bar = 456; }
-</programlisting>
-
-    </para></listitem>
-
-  </varlistentry>
-
-  <varlistentry><term><function>map</function>
-  <replaceable>f</replaceable> <replaceable>list</replaceable></term>
-
-    <listitem><para>Apply the function <replaceable>f</replaceable> to
-    each element in the list <replaceable>list</replaceable>.  For
-    example,
-
-<programlisting>
-map (x: "foo" + x) [ "bar" "bla" "abc" ]</programlisting>
-
-    evaluates to <literal>[ "foobar" "foobla" "fooabc"
-    ]</literal>.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.mul</function>
-  <replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
-
-    <listitem><para>Return the product of the integers
-    <replaceable>e1</replaceable> and
-    <replaceable>e2</replaceable>.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.parseDrvName</function>
-  <replaceable>s</replaceable></term>
-
-    <listitem><para>Split the string <replaceable>s</replaceable> into
-    a package name and version.  The package name is everything up to
-    but not including the first dash followed by a digit, and the
-    version is everything following that dash.  The result is returned
-    in a set <literal>{ name, version }</literal>.  Thus,
-    <literal>builtins.parseDrvName "nix-0.12pre12876"</literal>
-    returns <literal>{ name = "nix"; version = "0.12pre12876";
-    }</literal>.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.pathExists</function>
-  <replaceable>path</replaceable></term>
-
-    <listitem><para>Return <literal>true</literal> if the path
-    <replaceable>path</replaceable> exists, and
-    <literal>false</literal> otherwise.  One application of this
-    function is to conditionally include a Nix expression containing
-    user configuration:
-
-<programlisting>
-let
-  fileName = builtins.getEnv "CONFIG_FILE";
-  config =
-    if fileName != "" &amp;&amp; builtins.pathExists (builtins.toPath fileName)
-    then import (builtins.toPath fileName)
-    else { someSetting = false; }; <lineannotation># default configuration</lineannotation>
-in config.someSetting</programlisting>
-
-    (Note that <envar>CONFIG_FILE</envar> must be an absolute path for
-    this to work.)</para></listitem>
-
-  </varlistentry>
-
-
-  <!--
-  <varlistentry><term><function>relativise</function></term>
-
-    <listitem><para>TODO</para></listitem>
-
-  </varlistentry>
-  -->
-
-
-  <varlistentry><term><function>builtins.readFile</function>
-  <replaceable>path</replaceable></term>
-
-    <listitem><para>Return the contents of the file
-    <replaceable>path</replaceable> as a string.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>removeAttrs</function>
-  <replaceable>set</replaceable> <replaceable>list</replaceable></term>
-
-    <listitem><para>Remove the attributes listed in
-    <replaceable>list</replaceable> from
-    <replaceable>set</replaceable>.  The attributes don’t have to
-    exist in <replaceable>set</replaceable>. For instance,
-
-<screen>
-removeAttrs { x = 1; y = 2; z = 3; } [ "a" "x" "z" ]</screen>
-
-    evaluates to <literal>{ y = 2; }</literal>.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.stringLength</function>
-  <replaceable>e</replaceable></term>
-
-    <listitem><para>Return the length of the string
-    <replaceable>e</replaceable>.  If <replaceable>e</replaceable> is
-    not a string, evaluation is aborted.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.sub</function>
-  <replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
-
-    <listitem><para>Return the difference between the integers
-    <replaceable>e1</replaceable> and
-    <replaceable>e2</replaceable>.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.substring</function>
-  <replaceable>start</replaceable> <replaceable>len</replaceable>
-  <replaceable>s</replaceable></term>
-
-    <listitem><para>Return the substring of
-    <replaceable>s</replaceable> from character position
-    <replaceable>start</replaceable> (zero-based) up to but not
-    including <replaceable>start + len</replaceable>.  If
-    <replaceable>start</replaceable> is greater than the length of the
-    string, an empty string is returned, and if <replaceable>start +
-    len</replaceable> lies beyond the end of the string, only the
-    substring up to the end of the string is returned.
-    <replaceable>start</replaceable> must be
-    non-negative.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.tail</function>
-  <replaceable>list</replaceable></term>
-
-    <listitem><para>Return the second to last elements of a list;
-    abort evaluation if the argument isn’t a list or is an empty
-    list.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>throw</function>
-  <replaceable>s</replaceable></term>
-
-    <listitem><para>Throw an error message
-    <replaceable>s</replaceable>.  This usually aborts Nix expression
-    evaluation, but in <command>nix-env -qa</command> and other
-    commands that try to evaluate a set of derivations to get
-    information about those derivations, a derivation that throws an
-    error is silently skipped (which is not the case for
-    <function>abort</function>).</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry
-  xml:id='builtin-toFile'><term><function>builtins.toFile</function>
-  <replaceable>name</replaceable> <replaceable>s</replaceable></term>
-
-    <listitem><para>Store the string <replaceable>s</replaceable> in a
-    file in the Nix store and return its path.  The file has suffix
-    <replaceable>name</replaceable>.  This file can be used as an
-    input to derivations.  One application is to write builders
-    “inline”.  For instance, the following Nix expression combines
-    <xref linkend='ex-hello-nix' /> and <xref
-    linkend='ex-hello-builder' /> into one file:
-
-<programlisting>
-{ stdenv, fetchurl, perl }:
-
-stdenv.mkDerivation {
-  name = "hello-2.1.1";
-
-  builder = builtins.toFile "builder.sh" "
-    source $stdenv/setup
-
-    PATH=$perl/bin:$PATH
-
-    tar xvfz $src
-    cd hello-*
-    ./configure --prefix=$out
-    make
-    make install
-  ";
-
-  src = fetchurl {
-    url = http://nix.cs.uu.nl/dist/tarballs/hello-2.1.1.tar.gz;
-    md5 = "70c9ccf9fac07f762c24f2df2290784d";
-  };
-  inherit perl;
-}</programlisting>
-
-    </para>
-
-    <para>It is even possible for one file to refer to another, e.g.,
-
-<programlisting>
-  builder = let
-    configFile = builtins.toFile "foo.conf" "
-      # This is some dummy configuration file.
-      <replaceable>...</replaceable>
-    ";
-  in builtins.toFile "builder.sh" "
-    source $stdenv/setup
-    <replaceable>...</replaceable>
-    cp ${configFile} $out/etc/foo.conf
-  ";</programlisting>
-
-    Note that <literal>${configFile}</literal> is an antiquotation
-    (see <xref linkend='ssec-values' />), so the result of the
-    expression <literal>configFile</literal> (i.e., a path like
-    <filename>/nix/store/m7p7jfny445k...-foo.conf</filename>) will be
-    spliced into the resulting string.</para>
-
-    <para>It is however <emphasis>not</emphasis> allowed to have files
-    mutually referring to each other, like so:
-
-<programlisting>
-let
-  foo = builtins.toFile "foo" "...${bar}...";
-  bar = builtins.toFile "bar" "...${foo}...";
-in foo</programlisting>
-
-    This is not allowed because it would cause a cyclic dependency in
-    the computation of the cryptographic hashes for
-    <varname>foo</varname> and <varname>bar</varname>.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.toJSON</function> <replaceable>e</replaceable></term>
-
-    <listitem><para>Return a string containing a JSON representation
-    of <replaceable>e</replaceable>.  Strings, integers, booleans,
-    nulls and lists are mapped to their JSON equivalents.  Sets
-    (except derivations) are represented as objects.  Derivations are
-    translated to a JSON string containing the derivation’s output
-    path.  Paths are copied to the store and represented as a JSON
-    string of the resulting store path.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.toPath</function> <replaceable>s</replaceable></term>
-
-    <listitem><para>Convert the string value
-    <replaceable>s</replaceable> into a path value.  The string
-    <replaceable>s</replaceable> must represent an absolute path
-    (i.e., must start with <literal>/</literal>).  The path need not
-    exist.  The resulting path is canonicalised, e.g.,
-    <literal>builtins.toPath "//foo/xyzzy/../bar/"</literal> returns
-    <literal>/foo/bar</literal>.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>toString</function> <replaceable>e</replaceable></term>
-
-    <listitem><para>Convert the expression
-    <replaceable>e</replaceable> to a string.
-    <replaceable>e</replaceable> can be a string (in which case
-    <function>toString</function> is a no-op) or a path (e.g.,
-    <literal>toString /foo/bar</literal> yields
-    <literal>"/foo/bar"</literal>.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry xml:id='builtin-toXML'><term><function>builtins.toXML</function> <replaceable>e</replaceable></term>
-
-    <listitem><para>Return a string containing an XML representation
-    of <replaceable>e</replaceable>.  The main application for
-    <function>toXML</function> is to communicate information with the
-    builder in a more structured format than plain environment
-    variables.</para>
-
-    <!-- TODO: more formally describe the schema of the XML
-    representation -->
-
-    <para><xref linkend='ex-toxml' /> shows an example where this is
-    the case.  The builder is supposed to generate the configuration
-    file for a <link xlink:href='http://jetty.mortbay.org/'>Jetty
-    servlet container</link>.  A servlet container contains a number
-    of servlets (<filename>*.war</filename> files) each exported under
-    a specific URI prefix.  So the servlet configuration is a list of
-    sets containing the <varname>path</varname> and
-    <varname>war</varname> of the servlet (<xref
-    linkend='ex-toxml-co-servlets' />).  This kind of information is
-    difficult to communicate with the normal method of passing
-    information through an environment variable, which just
-    concatenates everything together into a string (which might just
-    work in this case, but wouldn’t work if fields are optional or
-    contain lists themselves).  Instead the Nix expression is
-    converted to an XML representation with
-    <function>toXML</function>, which is unambiguous and can easily be
-    processed with the appropriate tools.  For instance, in the
-    example an XSLT stylesheet (<xref linkend='ex-toxml-co-stylesheet'
-    />) is applied to it (<xref linkend='ex-toxml-co-apply' />) to
-    generate the XML configuration file for the Jetty server.  The XML
-    representation produced from <xref linkend='ex-toxml-co-servlets'
-    /> by <function>toXML</function> is shown in <xref
-    linkend='ex-toxml-result' />.</para>
-
-    <para>Note that <xref linkend='ex-toxml' /> uses the <function
-    linkend='builtin-toFile'>toFile</function> built-in to write the
-    builder and the stylesheet “inline” in the Nix expression.  The
-    path of the stylesheet is spliced into the builder at
-    <literal>xsltproc ${stylesheet}
-    <replaceable>...</replaceable></literal>.</para>
-
-    <example xml:id='ex-toxml'><title>Passing information to a builder
-    using <function>toXML</function></title>
-
-<programlisting><![CDATA[
-{ stdenv, fetchurl, libxslt, jira, uberwiki }:
-
-stdenv.mkDerivation (rec {
-  name = "web-server";
-
-  buildInputs = [ libxslt ];
-
-  builder = builtins.toFile "builder.sh" "
-    source $stdenv/setup
-    mkdir $out
-    echo $servlets | xsltproc ${stylesheet} - > $out/server-conf.xml]]> <co xml:id='ex-toxml-co-apply' /> <![CDATA[
-  ";
-
-  stylesheet = builtins.toFile "stylesheet.xsl"]]> <co xml:id='ex-toxml-co-stylesheet' /> <![CDATA[
-   "<?xml version='1.0' encoding='UTF-8'?>
-    <xsl:stylesheet xmlns:xsl='http://www.w3.org/1999/XSL/Transform' version='1.0'>
-      <xsl:template match='/'>
-        <Configure>
-          <xsl:for-each select='/expr/list/attrs'>
-            <Call name='addWebApplication'>
-              <Arg><xsl:value-of select=\"attr[@name = 'path']/string/@value\" /></Arg>
-              <Arg><xsl:value-of select=\"attr[@name = 'war']/path/@value\" /></Arg>
-            </Call>
-          </xsl:for-each>
-        </Configure>
-      </xsl:template>
-    </xsl:stylesheet>
-  ";
-
-  servlets = builtins.toXML []]> <co xml:id='ex-toxml-co-servlets' /> <![CDATA[
-    { path = "/bugtracker"; war = jira + "/lib/atlassian-jira.war"; }
-    { path = "/wiki"; war = uberwiki + "/uberwiki.war"; }
-  ];
-})]]></programlisting>
-
-    </example>
-
-    <example xml:id='ex-toxml-result'><title>XML representation produced by
-    <function>toXML</function></title>
-
-<programlisting><![CDATA[<?xml version='1.0' encoding='utf-8'?>
-<expr>
-  <list>
-    <attrs>
-      <attr name="path">
-        <string value="/bugtracker" />
-      </attr>
-      <attr name="war">
-        <path value="/nix/store/d1jh9pasa7k2...-jira/lib/atlassian-jira.war" />
-      </attr>
-    </attrs>
-    <attrs>
-      <attr name="path">
-        <string value="/wiki" />
-      </attr>
-      <attr name="war">
-        <path value="/nix/store/y6423b1yi4sx...-uberwiki/uberwiki.war" />
-      </attr>
-    </attrs>
-  </list>
-</expr>]]></programlisting>
-
-    </example>
-
-    </listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.trace</function>
-  <replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
-
-    <listitem><para>Evaluate <replaceable>e1</replaceable> and print its
-    abstract syntax representation on standard error.  Then return
-    <replaceable>e2</replaceable>.  This function is useful for
-    debugging.</para></listitem>
-
-  </varlistentry>
-
-
-  <varlistentry><term><function>builtins.typeOf</function>
-  <replaceable>e</replaceable></term>
-
-    <listitem><para>Return a string representing the type of the value
-    <replaceable>e</replaceable>, namely <literal>"int"</literal>,
-    <literal>"bool"</literal>, <literal>"string"</literal>,
-    <literal>"path"</literal>, <literal>"null"</literal>,
-    <literal>"set"</literal>, <literal>"list"</literal> or
-    <literal>"lambda"</literal>.</para></listitem>
-
-  </varlistentry>
-
-
-</variablelist>
-
-
-</section>