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<section xmlns= "http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xi="http://www.w3.org/2001/XInclude"
version="5.0"
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’
built-in functions. See the
<link
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xlink:href="https://nixos.org/nix/manual/#chap-writing-nix-expressions">Nix
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manual</link> for the rest.
</para>
<informaltable frame= 'none' >
<tgroup cols= '2' >
<colspec colname= 'c1' rowsep= '1' colsep= '1' />
<colspec colname= 'c2' rowsep= '1' />
<thead >
<row >
<entry > Example</entry>
<entry > Description</entry>
</row>
</thead>
<tbody >
<row >
<entry namest= "c1" nameend= "c2" > <emphasis > Basic values</emphasis>
</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/<replaceable > hash</replaceable> -bash-<replaceable > version</replaceable> /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 namest= "c1" nameend= "c2" > <emphasis > Compound values</emphasis>
</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 namest= "c1" nameend= "c2" > <emphasis > Operators</emphasis>
</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 namest= "c1" nameend= "c2" > <emphasis > Control structures</emphasis>
</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 namest= "c1" nameend= "c2" > <emphasis > Functions (lambdas)</emphasis>
</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 namest= "c1" nameend= "c2" > <emphasis > Built-in functions</emphasis>
</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>
<!--
<row >
<entry > <literal > throw "Urgh"</literal> </entry>
<entry > Raise an error condition</entry>
</row>
-->
</tbody>
</tgroup>
</informaltable>
</section>
