* Start of language reference.

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Eelco Dolstra 2004-11-05 15:39:30 +00:00
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@ -499,6 +499,234 @@ run in parallel. Typically this should be the number of CPUs.</para>
</sect2>
<sect2><title>The generic builder</title>
<para>TODO</para>
</sect2>
</sect1>
<sect1><title>The Nix expression language</title>
<para>The Nix expression language is a pure, lazy, functional
language. Purity means that operations in the language don't have
side-effects (for instance, there is no variable assignment).
Laziness means that arguments to functions are evaluated only when
they are needed. Functional means that functions are
<quote>normal</quote> values that can be passed around and
manipulated in interesting ways.</para>
<para>The language is not a full-featured, general purpose language.
It's main job is to describe components, compositions of components,
and the variability within components. For this a functional language
is perfectly suited.</para>
<para>This section presents the various features of the
language.</para>
<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>Integers</emphasis>, e.g.,
<literal>123</literal>.</para></listitem>
<listitem><para><emphasis>URIs</emphasis> as defined in appendix B
of <ulink url='http://www.ietf.org/rfc/rfc2396.txt'>RFC
2396</ulink>, e.g.,
<literal>https://svn.cs.uu.nl:12443/dist/trace/trace-nix-trunk.tar.bz2</literal>.</para></listitem>
<listitem><para><emphasis>Paths</emphasis>, e.g.,
<filename>/bin/sh</filename> or <filename>./builder.sh</filename>.
A path must contain at least one slash to be recognised as such; for
instance, <filename>builder.sh</filename> is not a
path<footnote><para>It's parsed as an expression that selects the
attribute <varname>sh</varname> from the variable
<varname>builder</varname>.</para></footnote>. If the filename is
relative, i.e., if it does not begin with a slash, it is made
absolute at parse time relative to the directory of the Nix
expression that contained it. For instance, if a Nix expression in
<filename>/foo/bar/bla.nix</filename> refers to
<filename>../xyzzy/fnord.nix</filename>, the absolutised path is
<filename>/foo/xyzzy/fnord.nix</filename>.</para></listitem>
</itemizedlist>
</para>
</simplesect>
<simplesect><title>Lists</title>
<para>Lists are formed by enclosing a whitespace-separated list of
values between square bracktes. For example,
<programlisting>
[ 123 ./foo.nix "abc" (f {x=y;}) ]</programlisting>
defines a list of four elements, the last being the result of a call
to the function <varname>f</varname>. Note that function calls have
to be enclosed in parentheses. If they had been omitted, e.g.,
<programlisting>
[ 123 ./foo.nix "abc" f {x=y;} ]</programlisting>
the result would be a list of five elements, the fourth one being a
function and the fifth being an attribute set.</para>
</simplesect>
<simplesect><title>Attribute sets</title>
<para>Attribute sets are really the core of the language, since
ultimately it's all about creating derivations, which are really just
sets of attributes to be passed to build scripts.</para>
<para>Attribute sets are just a list of name/value pairs enclosed in
curly brackets, where each value is an arbitrary expression terminated
by a semicolon. For example:
<programlisting>
{ x = 123;
text = "Hello";
y = f { bla = 456; };
}</programlisting>
This defines an attribute set with attributes named
<varname>x</varname>, <varname>test</varname>, <varname>y</varname>.
The order of the attributes is irrelevant. An attribute name may only
occur once.</para>
<para>Attributes can be selected from an attribute set using the
<literal>.</literal> operator. For instance,
<programlisting>
{ a = "Foo"; b = "Bar"; }.a</programlisting>
evaluates to <literal>"Foo"</literal>.</para>
</simplesect>
<simplesect><title>Recursive attribute sets</title>
<para>Recursive attribute sets are just normal attribute sets, but the
attributes can refer to each other. For example,
<programlisting>
rec {
x = y;
y = 123;
}.x
</programlisting>
evaluates to <literal>123</literal>. Note that without
<literal>rec</literal> the binding <literal>x = y;</literal> would
refer to the variable <varname>y</varname> in the surrounding scope,
if one exists, and would be invalid if no such variable exists. That
is, in a normal (non-recursive) attribute set, attributes are not
added to the lexical scope; in a recursive set, they are.</para>
<para>Recursive attribute sets of course introduce the danger of
infinite recursion. For example,
<programlisting>
rec {
x = y;
y = x;
}.x</programlisting>
does not terminate<footnote><para>Actually, Nix detects infinite
recursion in this case and aborts (<quote>infinite recursion
encountered</quote>).</para></footnote>.</para>
</simplesect>
<!--
<para>It is often convenient to copy variables from the surrounding
scope (e.g., when you want to propagate attributes). This can be
shortened using the <literal>inherit</literal> keyword. For instance,
-->
<simplesect><title>Lets</title>
<para>TODO</para>
</simplesect>
<simplesect><title>Inheriting attributes</title>
<para>TODO</para>
</simplesect>
<simplesect><title>Functions</title>
<para>TODO</para>
<para>Higher-order functions; map</para>
</simplesect>
<simplesect><title>Conditionals</title>
<para>TODO</para>
</simplesect>
<simplesect><title><quote>With</quote> expressions</title>
<para>TODO</para>
</simplesect>
<simplesect><title>Operators</title>
<para>TODO</para>
</simplesect>
<simplesect><title>Derivations</title>
<para>TODO</para>
</simplesect>
<simplesect><title>Miscelleneous built-in functions</title>
<para>TODO</para>
</simplesect>
</sect1>
<sect1><title>The standard environment</title>
<para>TODO</para>
</sect1>