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<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-nix-syntax-summary">
<title>Syntax Summary</title>
<para>
Below is a summary of the most important syntactic constructs in the
Nix expression language. It’s not complete. In particular, there are
many other built-in functions. See the
<link xlink:href="https://nixos.org/nix/manual/#chap-writing-nix-expressions">Nix
manual</link> for the rest.
</para>
<informaltable>
<tgroup cols="2">
<colspec align="left" />
<colspec align="left" />
<thead>
<row>
<entry>
Example
</entry>
<entry>
Description
</entry>
</row>
</thead>
<tbody>
<row>
<entry>
<emphasis>Basic values</emphasis>
</entry>
<entry>
</entry>
</row>
<row>
<entry>
<literal>"Hello world"</literal>
</entry>
<entry>
A string
</entry>
</row>
<row>
<entry>
<literal>"${pkgs.bash}/bin/sh"</literal>
</entry>
<entry>
A string containing an expression (expands to
<literal>"/nix/store/hash-bash-version/bin/sh"</literal>)
</entry>
</row>
<row>
<entry>
<literal>true</literal>, <literal>false</literal>
</entry>
<entry>
Booleans
</entry>
</row>
<row>
<entry>
<literal>123</literal>
</entry>
<entry>
An integer
</entry>
</row>
<row>
<entry>
<literal>./foo.png</literal>
</entry>
<entry>
A path (relative to the containing Nix expression)
</entry>
</row>
<row>
<entry>
<emphasis>Compound values</emphasis>
</entry>
<entry>
</entry>
</row>
<row>
<entry>
<literal>{ x = 1; y = 2; }</literal>
</entry>
<entry>
A set with attributes named <literal>x</literal> and
<literal>y</literal>
</entry>
</row>
<row>
<entry>
<literal>{ foo.bar = 1; }</literal>
</entry>
<entry>
A nested set, equivalent to
<literal>{ foo = { bar = 1; }; }</literal>
</entry>
</row>
<row>
<entry>
<literal>rec { x = "foo"; y = x + "bar"; }</literal>
</entry>
<entry>
A recursive set, equivalent to
<literal>{ x = "foo"; y = "foobar"; }</literal>
</entry>
</row>
<row>
<entry>
<literal>[ "foo" "bar" ]</literal>
</entry>
<entry>
A list with two elements
</entry>
</row>
<row>
<entry>
<emphasis>Operators</emphasis>
</entry>
<entry>
</entry>
</row>
<row>
<entry>
<literal>"foo" + "bar"</literal>
</entry>
<entry>
String concatenation
</entry>
</row>
<row>
<entry>
<literal>1 + 2</literal>
</entry>
<entry>
Integer addition
</entry>
</row>
<row>
<entry>
<literal>"foo" == "f" + "oo"</literal>
</entry>
<entry>
Equality test (evaluates to <literal>true</literal>)
</entry>
</row>
<row>
<entry>
<literal>"foo" != "bar"</literal>
</entry>
<entry>
Inequality test (evaluates to <literal>true</literal>)
</entry>
</row>
<row>
<entry>
<literal>!true</literal>
</entry>
<entry>
Boolean negation
</entry>
</row>
<row>
<entry>
<literal>{ x = 1; y = 2; }.x</literal>
</entry>
<entry>
Attribute selection (evaluates to <literal>1</literal>)
</entry>
</row>
<row>
<entry>
<literal>{ x = 1; y = 2; }.z or 3</literal>
</entry>
<entry>
Attribute selection with default (evaluates to
<literal>3</literal>)
</entry>
</row>
<row>
<entry>
<literal>{ x = 1; y = 2; } // { z = 3; }</literal>
</entry>
<entry>
Merge two sets (attributes in the right-hand set taking
precedence)
</entry>
</row>
<row>
<entry>
<emphasis>Control structures</emphasis>
</entry>
<entry>
</entry>
</row>
<row>
<entry>
<literal>if 1 + 1 == 2 then "yes!" else "no!"</literal>
</entry>
<entry>
Conditional expression
</entry>
</row>
<row>
<entry>
<literal>assert 1 + 1 == 2; "yes!"</literal>
</entry>
<entry>
Assertion check (evaluates to
<literal>"yes!"</literal>). See
<xref linkend="sec-assertions" /> for using assertions in
modules
</entry>
</row>
<row>
<entry>
<literal>let x = "foo"; y = "bar"; in x + y</literal>
</entry>
<entry>
Variable definition
</entry>
</row>
<row>
<entry>
<literal>with pkgs.lib; head [ 1 2 3 ]</literal>
</entry>
<entry>
Add all attributes from the given set to the scope
(evaluates to <literal>1</literal>)
</entry>
</row>
<row>
<entry>
<emphasis>Functions (lambdas)</emphasis>
</entry>
<entry>
</entry>
</row>
<row>
<entry>
<literal>x: x + 1</literal>
</entry>
<entry>
A function that expects an integer and returns it increased
by 1
</entry>
</row>
<row>
<entry>
<literal>(x: x + 1) 100</literal>
</entry>
<entry>
A function call (evaluates to 101)
</entry>
</row>
<row>
<entry>
<literal>let inc = x: x + 1; in inc (inc (inc 100))</literal>
</entry>
<entry>
A function bound to a variable and subsequently called by
name (evaluates to 103)
</entry>
</row>
<row>
<entry>
<literal>{ x, y }: x + y</literal>
</entry>
<entry>
A function that expects a set with required attributes
<literal>x</literal> and <literal>y</literal> and
concatenates them
</entry>
</row>
<row>
<entry>
<literal>{ x, y ? "bar" }: x + y</literal>
</entry>
<entry>
A function that expects a set with required attribute
<literal>x</literal> and optional <literal>y</literal>,
using <literal>"bar"</literal> as default value
for <literal>y</literal>
</entry>
</row>
<row>
<entry>
<literal>{ x, y, ... }: x + y</literal>
</entry>
<entry>
A function that expects a set with required attributes
<literal>x</literal> and <literal>y</literal> and ignores
any other attributes
</entry>
</row>
<row>
<entry>
<literal>{ x, y } @ args: x + y</literal>
</entry>
<entry>
A function that expects a set with required attributes
<literal>x</literal> and <literal>y</literal>, and binds the
whole set to <literal>args</literal>
</entry>
</row>
<row>
<entry>
<emphasis>Built-in functions</emphasis>
</entry>
<entry>
</entry>
</row>
<row>
<entry>
<literal>import ./foo.nix</literal>
</entry>
<entry>
Load and return Nix expression in given file
</entry>
</row>
<row>
<entry>
<literal>map (x: x + x) [ 1 2 3 ]</literal>
</entry>
<entry>
Apply a function to every element of a list (evaluates to
<literal>[ 2 4 6 ]</literal>)
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</section>
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