path: root/lib/erl_interface/doc/src/erl_format.xml

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  <header>
    <copyright>
      <year>1996</year><year>2013</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
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      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
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    <title>erl_format</title>
    <prepared>Torbj&ouml;rn T&ouml;rnkvist</prepared>
    <responsible>Torbj&ouml;rn T&ouml;rnkvist</responsible>
    <docno></docno>
    <approved>Bjarne D&auml;cker</approved>
    <checked>Torbj&ouml;rn T&ouml;rnkvist</checked>
    <date>961016</date>
    <rev>A</rev>
    <file>erl_format.sgml</file>
  </header>
  <lib>erl_format</lib>
  <libsummary>Create and Match Erlang Terms</libsummary>
  <description>
    <p>This module contains two routines - one general function for
      creating Erlang terms and one for pattern matching Erlang terms.</p>
  </description>
  <funcs>
    <func>
      <name><ret>ETERM *</ret><nametext>erl_format(FormatStr, ... )</nametext></name>
      <fsummary>Creates an Erlang term</fsummary>
      <type>
        <v>char *FormatStr;</v>
      </type>
      <desc>
        <p>This is a general function for creating Erlang terms using
          a format specifier and a corresponding set of arguments, much
          in the way <c><![CDATA[printf()]]></c> works.</p>
        <p><c><![CDATA[FormatStr]]></c> is a format specification string. The set
          of valid format specifiers is as follows:</p>
        <list type="bulleted">
          <item>
            <p>~i - Integer</p>
          </item>
          <item>
            <p>~f - Floating point</p>
          </item>
          <item>
            <p>~a - Atom</p>
          </item>
          <item>
            <p>~s - String</p>
          </item>
          <item>
            <p>~w - Arbitrary Erlang term</p>
          </item>
        </list>
        <p>For each format specifier that appears in <c><![CDATA[FormatStr]]></c>,
          there must be a corresponding argument following
          <c><![CDATA[FormatStr]]></c>. An Erlang term is built according to the
          <c><![CDATA[FormatStr]]></c> with values and Erlang terms substituted from
          the corresponding arguments and according to the individual
          format specifiers. For example:</p>
        <code type="none"><![CDATA[
erl_format("[{name,~a},{age,~i},{data,~w}]",
           "madonna",
           21,
           erl_format("[{adr,~s,~i}]","E-street",42));
        ]]></code>
        <p>This will create an <c><![CDATA[(ETERM *)]]></c> structure corresponding
          to the Erlang term: 
          <c><![CDATA[[{name,madonna},{age,21},{data,[{adr,"E-street",42}]}]]]></c></p>
        <p>The function returns an Erlang term, or NULL if
          <c><![CDATA[FormatStr]]></c> does not describe a valid Erlang term.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>erl_match(Pattern, Term)</nametext></name>
      <fsummary>Performs pattern matching</fsummary>
      <type>
        <v>ETERM *Pattern,*Term;</v>
      </type>
      <desc>
        <p>This function is used to perform pattern matching similar
          to that done in Erlang. Refer to an Erlang manual for matching
          rules and more examples.</p>
        <p><c><![CDATA[Pattern]]></c> is an Erlang term, possibly containing unbound
          variables. </p>
        <p><c><![CDATA[Term]]></c> is an Erlang term that we wish to match against
          <c><![CDATA[Pattern]]></c>.</p>
        <p><c><![CDATA[Term]]></c> and <c><![CDATA[Pattern]]></c> are compared, and any
          unbound variables in <c><![CDATA[Pattern]]></c> are bound to corresponding
          values in <c><![CDATA[Term]]></c>. </p>
        <p>If <c><![CDATA[Term]]></c> and <c><![CDATA[Pattern]]></c> can be matched, the
          function returns a non-zero value and binds any unbound
          variables in <c><![CDATA[Pattern]]></c>. If <c><![CDATA[Term]]></c> <c><![CDATA[Pattern]]></c> do
          not match, the function returns 0. For example:</p>
        <code type="none"><![CDATA[
ETERM *term, *pattern, *pattern2;
term1    = erl_format("{14,21}");
term2    = erl_format("{19,19}");
pattern1 = erl_format("{A,B}");
pattern2 = erl_format("{F,F}");
if (erl_match(pattern1, term1)) {
  /* match succeeds:
   * A gets bound to 14, 
   * B gets bound to 21 
   */
  ...  
}
if (erl_match(pattern2, term1)) {
  /* match fails because F cannot be 
   * bound to two separate values, 14 and 21
   */
  ...
}
if (erl_match(pattern2, term2)) {
  /* match succeeds and F gets bound to 19 */
  ...
}
        ]]></code>
        <p><c><![CDATA[erl_var_content()]]></c> can be used to retrieve the
          content of any variables bound as a result of a call to
          <c><![CDATA[erl_match()]]></c>.</p>
      </desc>
    </func>
  </funcs>
</cref>