nixpkgs/nixos/doc/manual/configuration/summary.xml

<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’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 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>