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<chapter>
  <header>
    <copyright>
      <year>2003</year><year>2017</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      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>Errors and Error Handling</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>errors.xml</file>
  </header>

  <section>
    <title>Terminology</title>
    <p>Errors can roughly be divided into four different types:</p>
    <list type="bulleted">
      <item>Compile-time errors</item>
      <item>Logical errors</item>
      <item>Run-time errors</item>
      <item>Generated errors</item>
    </list>
    <p>A compile-time error, for example a syntax error, does not
      cause much trouble as it is caught by the compiler.</p>
    <p>A logical error is when a program does not behave as intended,
      but does not crash. An example is that nothing happens when
      a button in a graphical user interface is clicked.</p>
    <p>A run-time error is when a crash occurs. An example is
      when an operator is applied to arguments of the wrong type.
      The Erlang programming language has built-in features for
      handling of run-time errors.</p>
    <p>A run-time error can also be emulated by calling
      <c>erlang:error(Reason)</c> or <c>erlang:error(Reason, Args)</c>.</p>
    <p>A run-time error is another name for an exception
      of class <c>error</c>.
      </p>
    <p>A generated error is when the code itself calls
      <c>exit/1</c> or <c>throw/1</c>. Notice that emulated run-time
      errors are not denoted as generated errors here.
      </p>
    <p>Generated errors are exceptions of classes <c>exit</c> and
      <c>throw</c>.
      </p>
    <p>When a run-time error or generated error occurs in Erlang, 
      execution for the process that evaluated
      the erroneous expression is stopped.
      This is referred to as a <em>failure</em>, that execution or
      evaluation <em>fails</em>, or that the process <em>fails</em>,
      <em>terminates</em>, or <em>exits</em>. Notice that a process can
      terminate/exit for other reasons than a failure.</p>
    <p>A process that terminates emits an <em>exit signal</em> with
      an <em>exit reason</em> that says something about which error
      has occurred. Normally, some information about the error is
      printed to the terminal.</p>
  </section>

  <section>
    <title>Exceptions</title>
    <p>Exceptions are run-time errors or generated errors and 
      are of three different classes, with different origins. The
      <seealso marker="expressions#try">try</seealso> expression 
      can distinguish between the different classes, whereas the
      <seealso marker="expressions#catch">catch</seealso>
      expression cannot. They are described in
      <seealso marker="expressions">Expressions
      </seealso>.</p>
    <table>
      <row>
        <cell align="left" valign="middle"><em>Class</em></cell>
        <cell align="left" valign="middle"><em>Origin</em></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>error</c></cell>
        <cell align="left" valign="middle">Run-time error,
        for example, <c>1+a</c>, or the process called
        <c>erlang:error/1,2</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>exit</c></cell>
        <cell align="left" valign="middle">The process called <c>exit/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>throw</c></cell>
        <cell align="left" valign="middle">The process called <c>throw/1</c></cell>
      </row>
      <tcaption>Exception Classes.</tcaption>
    </table>
    <p>An exception consists of its class, an exit reason
      (see <seealso marker="#exit_reasons">Exit Reason</seealso>),
      and a stack trace (which aids in finding the code location of
      the exception).</p>
    <p>The stack trace can be retrieved using
      <c>erlang:get_stacktrace/0</c>
      from within a <c>try</c> expression, and is returned for 
      exceptions of class <c>error</c> from a <c>catch</c> expression.</p>
    <p>An exception of class <c>error</c> is also known as a run-time 
      error.</p>
  </section>

  <section>
    <title>Handling of Run-time Errors in Erlang</title>

    <section>
      <title>Error Handling Within Processes</title>
      <p>It is possible to prevent run-time errors and other
        exceptions from causing
        the process to terminate by using <c>catch</c> or
        <c>try</c>, see <seealso marker="expressions">
        Expressions</seealso> about
        <seealso marker="expressions#catch">catch</seealso>
        and <seealso marker="expressions#try">try</seealso>.</p>
    </section>

    <section>
      <title>Error Handling Between Processes</title>
      <p>Processes can monitor other processes and detect process
        terminations, see
        <seealso marker="processes#errors">Processes</seealso>.</p>
    </section>
  </section>

  <section>
    <marker id="exit_reasons"></marker>
    <title>Exit Reasons</title>
    <p>When a run-time error occurs,
      that is an exception of class <c>error</c>.
      The exit reason is a tuple <c>{Reason,Stack}</c>, where
      <c>Reason</c> is a term indicating the type of error:</p>
    <table>
      <row>
        <cell align="left" valign="middle"><em>Reason</em></cell>
        <cell align="left" valign="middle"><em>Type of Error</em></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>badarg</c></cell>
        <cell align="left" valign="middle">Bad argument. The argument is of wrong data type, or is otherwise badly formed.</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>badarith</c></cell>
        <cell align="left" valign="middle">Bad argument in an arithmetic expression.</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>{badmatch,V}</c></cell>
        <cell align="left" valign="middle">Evaluation of a match expression failed. The value <c>V</c> did not match.</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>function_clause</c></cell>
        <cell align="left" valign="middle">No matching function clause is found when evaluating a function call.</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>{case_clause,V}</c></cell>
        <cell align="left" valign="middle">No matching branch is found when evaluating a <c>case</c> expression. The value <c>V</c> did not match.</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>if_clause</c></cell>
        <cell align="left" valign="middle">No true branch is found when evaluating an <c>if</c> expression.</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>{try_clause,V}</c></cell>
        <cell align="left" valign="middle">No matching branch is found when evaluating the of-section of a <c>try</c> expression. The value <c>V</c> did not match.</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>undef</c></cell>
        <cell align="left" valign="middle">The function cannot be found when evaluating a function call.</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>{badfun,F}</c></cell>
        <cell align="left" valign="middle">Something is wrong with a fun <c>F</c>.</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>{badarity,F}</c></cell>
        <cell align="left" valign="middle">A fun is applied to the wrong number of arguments. <c>F</c> describes the fun and the arguments.</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>timeout_value</c></cell>
        <cell align="left" valign="middle">The timeout value in a <c>receive..after</c> expression is evaluated to something else than an integer or <c>infinity</c>.</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>noproc</c></cell>
        <cell align="left" valign="middle">Trying to link to a non-existing process.</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>{nocatch,V}</c></cell>
        <cell align="left" valign="middle">Trying to evaluate a <c>throw </c>outside a <c>catch</c>. <c>V</c> is the thrown term.</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>system_limit</c></cell>
        <cell align="left" valign="middle">A system limit has been reached.
        See <seealso marker="doc/efficiency_guide:advanced">
        Efficiency Guide</seealso> for information about system limits.
        </cell>
      </row>
      <tcaption>Exit Reasons</tcaption>
    </table>
    <p><c>Stack</c> is the stack of function calls being evaluated
      when the error occurred, given as a list of tuples
      <c>{Module,Name,Arity}</c> with the most recent function call
      first. The most recent function call tuple can in some
      cases be <c>{Module,Name,[Arg]}</c>.</p>
  </section>
</chapter>

