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

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    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="http://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>An set with attributes names <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 = "bla"; 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>)</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>