lix/doc/manual/expressions/language-operators.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-language-operators">

<title>Operators</title>

<para><xref linkend='table-operators' /> lists the operators in the
Nix expression language, in order of precedence (from strongest to
weakest binding).</para>

<table xml:id='table-operators'>
  <title>Operators</title>
  <tgroup cols='3'>
    <thead>
      <row>
        <entry>Syntax</entry>
        <entry>Associativity</entry>
        <entry>Description</entry>
      </row>
    </thead>
    <tbody>
      <row>
        <entry><replaceable>e</replaceable> <literal>.</literal>
        <replaceable>attrpath</replaceable>
        [ <literal>or</literal> <replaceable>def</replaceable> ]
        </entry>
        <entry>none</entry>
        <entry>Select attribute denoted by the attribute path
        <replaceable>attrpath</replaceable> from set
        <replaceable>e</replaceable>.  (An attribute path is a
        dot-separated list of attribute names.)  If the attribute
        doesn’t exist, return <replaceable>def</replaceable> if
        provided, otherwise abort evaluation.</entry>
      </row>
      <row>
        <entry><replaceable>e1</replaceable> <replaceable>e2</replaceable></entry>
        <entry>left</entry>
        <entry>Call function <replaceable>e1</replaceable> with
        argument <replaceable>e2</replaceable>.</entry>
      </row>
      <row>
        <entry><literal>-</literal> <replaceable>e</replaceable></entry>
        <entry>none</entry>
        <entry>Arithmetic negation.</entry>
      </row>
      <row>
        <entry><replaceable>e</replaceable> <literal>?</literal>
        <replaceable>attrpath</replaceable></entry>
        <entry>none</entry>
        <entry>Test whether set <replaceable>e</replaceable> contains
        the attribute denoted by <replaceable>attrpath</replaceable>;
        return <literal>true</literal> or
        <literal>false</literal>.</entry>
      </row>
      <row>
        <entry><replaceable>e1</replaceable> <literal>++</literal> <replaceable>e2</replaceable></entry>
        <entry>right</entry>
        <entry>List concatenation.</entry>
      </row>
      <row>
        <entry>
          <replaceable>e1</replaceable> <literal>*</literal> <replaceable>e2</replaceable>,
          <replaceable>e1</replaceable> <literal>/</literal> <replaceable>e2</replaceable>
        </entry>
        <entry>left</entry>
        <entry>Arithmetic multiplication and division.</entry>
      </row>
      <row>
        <entry>
          <replaceable>e1</replaceable> <literal>+</literal> <replaceable>e2</replaceable>,
          <replaceable>e1</replaceable> <literal>-</literal> <replaceable>e2</replaceable>
        </entry>
        <entry>left</entry>
        <entry>Arithmetic addition and subtraction.  String or path concatenation (only by <literal>+</literal>).</entry>
      </row>
      <row>
        <entry><literal>!</literal> <replaceable>e</replaceable></entry>
        <entry>none</entry>
        <entry>Boolean negation.</entry>
      </row>
      <row>
        <entry><replaceable>e1</replaceable> <literal>//</literal>
        <replaceable>e2</replaceable></entry>
        <entry>right</entry>
        <entry>Return a set consisting of the attributes in
        <replaceable>e1</replaceable> and
        <replaceable>e2</replaceable> (with the latter taking
        precedence over the former in case of equally named
        attributes).</entry>
      </row>
      <row>
        <entry>
          <replaceable>e1</replaceable> <literal>&lt;</literal> <replaceable>e2</replaceable>,
          <replaceable>e1</replaceable> <literal>&gt;</literal> <replaceable>e2</replaceable>,
          <replaceable>e1</replaceable> <literal>&lt;=</literal> <replaceable>e2</replaceable>,
          <replaceable>e1</replaceable> <literal>&gt;=</literal> <replaceable>e2</replaceable>
        </entry>
        <entry>none</entry>
        <entry>Arithmetic comparison.</entry>
      </row>
      <row>
        <entry>
          <replaceable>e1</replaceable> <literal>==</literal> <replaceable>e2</replaceable>,
          <replaceable>e1</replaceable> <literal>!=</literal> <replaceable>e2</replaceable>
        </entry>
        <entry>none</entry>
        <entry>Equality and inequality.</entry>
      </row>
      <row>
        <entry><replaceable>e1</replaceable> <literal>&amp;&amp;</literal>
        <replaceable>e2</replaceable></entry>
        <entry>left</entry>
        <entry>Logical AND.</entry>
      </row>
      <row>
        <entry><replaceable>e1</replaceable> <literal>||</literal>
        <replaceable>e2</replaceable></entry>
        <entry>left</entry>
        <entry>Logical OR.</entry>
      </row>
      <row>
        <entry><replaceable>e1</replaceable> <literal>-></literal>
        <replaceable>e2</replaceable></entry>
        <entry>none</entry>
        <entry>Logical implication (equivalent to
        <literal>!<replaceable>e1</replaceable> ||
        <replaceable>e2</replaceable></literal>).</entry>
      </row>
    </tbody>
  </tgroup>
</table>

</section>