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>Name</entry>
        <entry>Syntax</entry>
        <entry>Associativity</entry>
        <entry>Description</entry>
        <entry>Precedence</entry>
      </row>
    </thead>
    <tbody>
      <row>
        <entry>Select</entry>
        <entry><emphasis>e</emphasis> <literal>.</literal>
        <emphasis>attrpath</emphasis>
        [ <literal>or</literal> <emphasis>def</emphasis> ]
        </entry>
        <entry>none</entry>
        <entry>Select attribute denoted by the attribute path
        <emphasis>attrpath</emphasis> from set
        <emphasis>e</emphasis>.  (An attribute path is a
        dot-separated list of attribute names.)  If the attribute
        doesn’t exist, return <emphasis>def</emphasis> if
        provided, otherwise abort evaluation.</entry>
        <entry>1</entry>
      </row>
      <row>
        <entry>Application</entry>
        <entry><emphasis>e1</emphasis> <emphasis>e2</emphasis></entry>
        <entry>left</entry>
        <entry>Call function <emphasis>e1</emphasis> with
        argument <emphasis>e2</emphasis>.</entry>
        <entry>2</entry>
      </row>
      <row>
        <entry>Arithmetic Negation</entry>
        <entry><literal>-</literal> <emphasis>e</emphasis></entry>
        <entry>none</entry>
        <entry>Arithmetic negation.</entry>
        <entry>3</entry>
      </row>
      <row>
        <entry>Has Attribute</entry>
        <entry><emphasis>e</emphasis> <literal>?</literal>
        <emphasis>attrpath</emphasis></entry>
        <entry>none</entry>
        <entry>Test whether set <emphasis>e</emphasis> contains
        the attribute denoted by <emphasis>attrpath</emphasis>;
        return <literal>true</literal> or
        <literal>false</literal>.</entry>
        <entry>4</entry>
      </row>
      <row>
        <entry>List Concatenation</entry>
        <entry><emphasis>e1</emphasis> <literal>++</literal> <emphasis>e2</emphasis></entry>
        <entry>right</entry>
        <entry>List concatenation.</entry>
        <entry>5</entry>
      </row>
      <row>
        <entry>Multiplication</entry>
        <entry>
          <emphasis>e1</emphasis> <literal>*</literal> <emphasis>e2</emphasis>,
        </entry>
        <entry>left</entry>
        <entry>Arithmetic multiplication.</entry>
        <entry>6</entry>
      </row>
      <row>
        <entry>Division</entry>
        <entry>
          <emphasis>e1</emphasis> <literal>/</literal> <emphasis>e2</emphasis>
        </entry>
        <entry>left</entry>
        <entry>Arithmetic division.</entry>
        <entry>6</entry>
      </row>
      <row>
        <entry>Addition</entry>
        <entry>
          <emphasis>e1</emphasis> <literal>+</literal> <emphasis>e2</emphasis>
        </entry>
        <entry>left</entry>
        <entry>Arithmetic addition.</entry>
        <entry>7</entry>
      </row>
      <row>
        <entry>Subtraction</entry>
        <entry>
          <emphasis>e1</emphasis> <literal>-</literal> <emphasis>e2</emphasis>
        </entry>
        <entry>left</entry>
        <entry>Arithmetic subtraction.</entry>
        <entry>7</entry>
      </row>
      <row>
        <entry>String Concatenation</entry>
        <entry>
          <emphasis>string1</emphasis> <literal>+</literal> <emphasis>string2</emphasis>
        </entry>
        <entry>left</entry>
        <entry>String concatenation.</entry>
        <entry>7</entry>
      </row>
      <row>
        <entry>Not</entry>
        <entry><literal>!</literal> <emphasis>e</emphasis></entry>
        <entry>none</entry>
        <entry>Boolean negation.</entry>
        <entry>8</entry>
      </row>
      <row>
        <entry>Update</entry>
        <entry><emphasis>e1</emphasis> <literal>//</literal>
        <emphasis>e2</emphasis></entry>
        <entry>right</entry>
        <entry>Return a set consisting of the attributes in
        <emphasis>e1</emphasis> and
        <emphasis>e2</emphasis> (with the latter taking
        precedence over the former in case of equally named
        attributes).</entry>
        <entry>9</entry>
      </row>
      <row>
        <entry>Less Than</entry>
        <entry>
          <emphasis>e1</emphasis> <literal>&lt;</literal> <emphasis>e2</emphasis>,
        </entry>
        <entry>none</entry>
        <entry>Arithmetic comparison.</entry>
        <entry>10</entry>
      </row>
      <row>
        <entry>Less Than or Equal To</entry>
        <entry>
          <emphasis>e1</emphasis> <literal>&lt;=</literal> <emphasis>e2</emphasis>
        </entry>
        <entry>none</entry>
        <entry>Arithmetic comparison.</entry>
        <entry>10</entry>
      </row>
      <row>
        <entry>Greater Than</entry>
        <entry>
          <emphasis>e1</emphasis> <literal>&gt;</literal> <emphasis>e2</emphasis>
        </entry>
        <entry>none</entry>
        <entry>Arithmetic comparison.</entry>
        <entry>10</entry>
      </row>
      <row>
        <entry>Greater Than or Equal To</entry>
        <entry>
          <emphasis>e1</emphasis> <literal>&gt;=</literal> <emphasis>e2</emphasis>
        </entry>
        <entry>none</entry>
        <entry>Arithmetic comparison.</entry>
        <entry>10</entry>
      </row>
      <row>
        <entry>Equality</entry>
        <entry>
          <emphasis>e1</emphasis> <literal>==</literal> <emphasis>e2</emphasis>
        </entry>
        <entry>none</entry>
        <entry>Equality.</entry>
        <entry>11</entry>
      </row>
      <row>
        <entry>Inequality</entry>
        <entry>
          <emphasis>e1</emphasis> <literal>!=</literal> <emphasis>e2</emphasis>
        </entry>
        <entry>none</entry>
        <entry>Inequality.</entry>
        <entry>11</entry>
      </row>
      <row>
        <entry>Logical AND</entry>
        <entry><emphasis>e1</emphasis> <literal>&amp;&amp;</literal>
        <emphasis>e2</emphasis></entry>
        <entry>left</entry>
        <entry>Logical AND.</entry>
        <entry>12</entry>
      </row>
      <row>
        <entry>Logical OR</entry>
        <entry><emphasis>e1</emphasis> <literal>||</literal>
        <emphasis>e2</emphasis></entry>
        <entry>left</entry>
        <entry>Logical OR.</entry>
        <entry>13</entry>
      </row>
      <row>
        <entry>Logical Implication</entry>
        <entry><emphasis>e1</emphasis> <literal>-></literal>
        <emphasis>e2</emphasis></entry>
        <entry>none</entry>
        <entry>Logical implication (equivalent to
        <literal>!<emphasis>e1</emphasis> ||
        <emphasis>e2</emphasis></literal>).</entry>
        <entry>14</entry>
      </row>
    </tbody>
  </tgroup>
</table>

</section>