* Document the built-in functions.

This commit is contained in:
Eelco Dolstra 2006-10-03 15:19:05 +00:00
parent d20c3011a0
commit 3837fb233c
3 changed files with 504 additions and 33 deletions

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@ -190,9 +190,9 @@
<varlistentry><term><literal>escapes</literal></term>
<listitem><para>Indicate nesting using escape codes that can be
interpreted by the <command>log2xml</command> tool in the Nix
source distribution. The resulting XML file can be fed into the
<command>log2html.xsl</command> stylesheet to create an HTML
interpreted by the <command>nix-log2xml</command> tool in the
Nix source distribution. The resulting XML file can be fed into
the <command>log2html.xsl</command> stylesheet to create an HTML
file that can be browsed interactively, using Javascript to
expand and collapse parts of the output.</para></listitem>

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@ -41,17 +41,6 @@ div.section > div.titlepage h2 /* sections */
margin-top: 1.5em;
}
div.refnamediv h2, div.refsynopsisdiv h2, div.refsection h2 /* refentry parts */
{
margin-top: 1.4em;
font-size: 125%;
}
div.refsection h3
{
font-size: 110%;
}
h3 /* subsections */
{
font-size: 125%;
@ -63,6 +52,16 @@ div.appendix h3
margin-top: 1.5em;
}
div.refnamediv h2, div.refsynopsisdiv h2, div.refsection h2 /* refentry parts */
{
margin-top: 1.4em;
font-size: 125%;
}
div.refsection h3
{
font-size: 110%;
}
/***************************************************************************

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@ -586,14 +586,18 @@ components.</para>
language.</para>
<section xml:id='ssec-values'><title>Values</title>
<simplesect><title>Simple values</title>
<para>Nix has the following basic datatypes:
<itemizedlist>
<listitem><para><emphasis>Strings</emphasis>, enclosed between
double quotes, e.g., <literal>"foo bar"</literal>.</para></listitem>
<listitem><para><emphasis>Strings</emphasis>,
enclosed between double quotes, e.g., <literal>"foo bar"</literal>.
TODO: antiquotations, escaping.</para></listitem>
<listitem><para><emphasis>Integers</emphasis>, e.g.,
<literal>123</literal>.</para></listitem>
@ -681,6 +685,12 @@ evaluates to <literal>"Foo"</literal>.</para>
</simplesect>
</section>
<section><title>Language constructs</title>
<simplesect><title>Recursive attribute sets</title>
<para>Recursive attribute sets are just normal attribute sets, but the
@ -836,7 +846,7 @@ allows partial parameterisation (i.e., only filling some of the
arguments of a function); e.g.,
<programlisting>
map (concat "foo") ["bar", "bla", "abc"]</programlisting>
map (concat "foo") ["bar" "bla" "abc"]</programlisting>
evaluates to <literal>["foobar" "foobla" "fooabc"]</literal>.</para>
@ -977,7 +987,19 @@ locally in a <literal>rec</literal>-expression.</para>
</simplesect>
<simplesect><title>Operators</title>
<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
@ -994,13 +1016,28 @@ weakest binding).</para>
</row>
</thead>
<tbody>
<row>
<entry><replaceable>e</replaceable> .
<replaceable>id</replaceable></entry>
<entry>none</entry>
<entry>Select attribute named <replaceable>id</replaceable>
from attribute set <replaceable>e</replaceable>. Abort
evaluation if the attribute doesnt exist.</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> ?
<replaceable>id</replaceable></entry>
<entry>none</entry>
<entry>Test whether attribute set <replaceable>e</replaceable>
contains an attribute named
<replaceable>id</replaceable>.</entry>
contains an attribute named <replaceable>id</replaceable>;
return <literal>true</literal> or
<literal>false</literal>.</entry>
</row>
<row>
<entry><replaceable>e1</replaceable> ++ <replaceable>e2</replaceable></entry>
@ -1062,10 +1099,10 @@ weakest binding).</para>
</tgroup>
</table>
</simplesect>
</section>
<simplesect xml:id="ssec-derivation"><title>Derivations</title>
<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
@ -1230,23 +1267,457 @@ command-line argument. See <xref linkend='sec-standard-environment'
</para>
</simplesect>
</section>
<simplesect><title>Other built-in functions</title>
<section><title>Built-in functions</title>
<para>TODO</para>
</simplesect>
<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>Add integers <replaceable>e1</replaceable> and
<replaceable>e2</replaceable>..</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><term><varname>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>
<simplesect><title>Comments</title>
<!--
<varlistentry><term><function>currentTime</function></term>
<para>Comments can be single-line, started with a <literal>#</literal>
character, or inline/multi-line, enclosed within <literal>/*
... */</literal>.</para>
<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>
</simplesect>
</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.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.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>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>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>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.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>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><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></listitem>
</varlistentry>
</variablelist>
</section>
</section>
@ -1294,9 +1765,10 @@ of the following components:
<listitem><para>GNU Make. It has been patched to provide
<quote>nested</quote> output that can be fed into the
<command>log2xml</command> command and <command>log2html</command>
stylesheet to create a structured, readable output of the build
steps performed by Make.</para></listitem>
<command>nix-log2xml</command> command and
<command>log2html</command> stylesheet to create a structured,
readable output of the build steps performed by
Make.</para></listitem>
<listitem><para>Bash. This is the shell used for all builders in
the Nix Packages collection. Not using <command>/bin/sh</command>