lix/doc/manual/builtins.xml

<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 an attribute 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>attrs</replaceable></term>

    <listitem><para>Return the names of the attributes in the
    attribute set <replaceable>attrs</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 = attrs: map (name: builtins.getAttr name attrs) (builtins.attrNames attrs);</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 attribute 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.getAttr</function>
  <replaceable>s</replaceable> <replaceable>attrs</replaceable></term>

    <listitem><para><function>getAttr</function> returns the attribute
    named <replaceable>s</replaceable> from the attribute set
    <replaceable>attrs</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>attrs</replaceable></term>

    <listitem><para><function>hasAttr</function> returns
    <literal>true</literal> if the attribute set
    <replaceable>attrs</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 an attribute 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 an attribute 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 an attribute 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 an attribute set from a list specifying
    the names and values of each attribute.  Each element of the list
    should be an attribute 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 an attribute 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>attrs</replaceable> <replaceable>list</replaceable></term>

    <listitem><para>Remove the attributes listed in
    <replaceable>list</replaceable> from the attribute set
    <replaceable>attrs</replaceable>.  The attributes don’t have to
    exist in <replaceable>attrs</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.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
    attribute 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>


</variablelist>


</section>