lix/doc/manual/builtins.xml

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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 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 dont have the desired built-in function.
However, in that case you should not write
<programlisting>
if builtins ? getEnv then __getEnv "PATH" else ""</programlisting>
This Nix expression will trigger an “undefined variable” error on
older Nix versions since <function>__getEnv</function> doesnt
exist. <literal>builtins.getEnv</literal>, on the other hand, is
safe since <literal>builtins</literal> always exists and attribute
selection is lazy, so its only performed if the test
succeeds.</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.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 doesnt 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 doesnt 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 users 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.head</function>
<replaceable>list</replaceable></term>
<listitem><para>Return the first element of a list; abort
evaluation if the argument isnt 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>. Evaluation aborts if the
file doesnt exist or contains an incorrect Nix
expression. <function>import</function> implements Nixs 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 doesnt have to be called
<varname>x</varname> in <filename>foo.nix</filename>; any name
would work.)</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>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.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.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 dont 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.substr</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 isnt 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 wouldnt 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>