path: root/lib/stdlib/doc/src/assert_hrl.xml

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  <header>
    <copyright>
      <year>2012</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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          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
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      See the License for the specific language governing permissions and
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    </legalnotice>

    <title>assert.hrl</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <file>assert.hrl</file>
  <filesummary>Assert macros.</filesummary>
  <description>
    <p>The include file <c>assert.hrl</c> provides macros for inserting
      assertions in your program code.</p>

    <p>Include the following directive in the module from which the function is
      called:</p>

    <code type="none">
-include_lib("stdlib/include/assert.hrl").</code>

    <p>When an assertion succeeds, the assert macro yields the atom <c>ok</c>.
      When an assertion fails, an exception of type <c>error</c> is generated.
      The associated error term has the form <c>{Macro, Info}</c>. <c>Macro</c>
      is the macro name, for example, <c>assertEqual</c>. <c>Info</c> is a list
      of tagged values, such as <c>[{module, M}, {line, L}, ...]</c>, which
      gives more information about the location and cause of the exception. All
      entries in the <c>Info</c> list are optional; do not rely programatically
      on any of them being present.</p>

    <p>Each assert macro has a corresponding version with an extra argument,
      for adding comments to assertions. These can for example be printed as
      part of error reports, to clarify the meaning of the check that
      failed. For example, <c>?assertEqual(0, fib(0), "Fibonacci is defined
      for zero")</c>. The comment text can be any character data (string,
      UTF8-binary, or deep list of such data), and will be included in the
      error term as <c>{comment, Text}</c>.</p>

    <p>If the macro <c>NOASSERT</c> is defined when <c>assert.hrl</c> is read
      by the compiler, the macros are defined as equivalent to the atom
      <c>ok</c>. The test will not be performed and there is no cost at runtime.</p>

    <p>For example, using <c>erlc</c> to compile your modules, the following
      disables all assertions:</p>

    <code type="none">
erlc -DNOASSERT=true *.erl</code>

    <p>(The value of <c>NOASSERT</c> does not matter, only the fact that it is
      defined.)</p>

    <p>A few other macros also have effect on the enabling or disabling of
      assertions:</p>

    <list type="bulleted">
      <item><p>If <c>NODEBUG</c> is defined, it implies <c>NOASSERT</c> (unless
        <c>DEBUG</c> is also defined, which overrides <c>NODEBUG</c>).</p>
      </item>
      <item><p>If <c>ASSERT</c> is defined, it overrides <c>NOASSERT</c>, that
        is, the assertions remain enabled.</p></item>
    </list>

    <p>If you prefer, you can thus use only <c>DEBUG</c>/<c>NODEBUG</c> as the
      main flags to control the behavior of the assertions (which is useful if
      you have other compiler conditionals or debugging macros controlled by
      those flags), or you can use <c>ASSERT</c>/<c>NOASSERT</c> to control only
      the assert macros.</p>
  </description>

  <section>
    <title>Macros</title>
    <taglist>
      <tag><c>assert(BoolExpr)</c></tag>
      <item></item>
      <tag><c>assert(BoolExpr, Comment)</c></tag>
      <item>
        <p>Tests that <c>BoolExpr</c> completes normally returning
          <c>true</c>.</p>
      </item>
      <tag><c>assertNot(BoolExpr)</c></tag>
      <item></item>
      <tag><c>assertNot(BoolExpr, Comment)</c></tag>
      <item>
        <p>Tests that <c>BoolExpr</c> completes normally returning
          <c>false</c>.</p>
      </item>
      <tag><c>assertMatch(GuardedPattern, Expr)</c></tag>
      <item></item>
      <tag><c>assertMatch(GuardedPattern, Expr, Comment)</c></tag>
      <item>
        <p>Tests that <c>Expr</c> completes normally yielding a value that
          matches <c>GuardedPattern</c>, for example:</p>
        <code type="none">
?assertMatch({bork, _}, f())</code>
        <p>Notice that a guard <c>when ...</c> can be included:</p>
        <code type="none">
?assertMatch({bork, X} when X > 0, f())</code>
      </item>
      <tag><c>assertNotMatch(GuardedPattern, Expr)</c></tag>
      <item></item>
      <tag><c>assertNotMatch(GuardedPattern, Expr, Comment)</c></tag>
      <item>
        <p>Tests that <c>Expr</c> completes normally yielding a value that does
          not match <c>GuardedPattern</c>.</p>
        <p>As in <c>assertMatch</c>, <c>GuardedPattern</c> can have a
          <c>when</c> part.</p>
      </item>
      <tag><c>assertEqual(ExpectedValue, Expr)</c></tag>
      <item></item>
      <tag><c>assertEqual(ExpectedValue, Expr, Comment)</c></tag>
      <item>
         <p>Tests that <c>Expr</c> completes normally yielding a value that is
           exactly equal to <c>ExpectedValue</c>.</p>
      </item>
      <tag><c>assertNotEqual(ExpectedValue, Expr)</c></tag>
      <item></item>
      <tag><c>assertNotEqual(ExpectedValue, Expr, Comment)</c></tag>
      <item>
        <p>Tests that <c>Expr</c> completes normally yielding a value that is
          not exactly equal to <c>ExpectedValue</c>.</p>
      </item>
      <tag><c>assertException(Class, Term, Expr)</c></tag>
      <item></item>
      <tag><c>assertException(Class, Term, Expr, Comment)</c></tag>
      <item>
        <p>Tests that <c>Expr</c> completes abnormally with an exception of type
          <c>Class</c> and with the associated <c>Term</c>. The assertion fails
          if <c>Expr</c> raises a different exception or if it completes
          normally returning any value.</p>
        <p>Notice that both <c>Class</c> and <c>Term</c> can be guarded
          patterns, as in <c>assertMatch</c>.</p>
      </item>
      <tag><c>assertNotException(Class, Term, Expr)</c></tag>
      <item></item>
      <tag><c>assertNotException(Class, Term, Expr, Comment)</c></tag>
      <item>
        <p>Tests that <c>Expr</c> does not evaluate abnormally with an
          exception of type <c>Class</c> and with the associated <c>Term</c>.
          The assertion succeeds if <c>Expr</c> raises a different exception or
          if it completes normally returning any value.</p>
        <p>As in <c>assertException</c>, both <c>Class</c> and <c>Term</c> can
          be guarded patterns.</p>
      </item>
      <tag><c>assertError(Term, Expr)</c></tag>
      <item></item>
      <tag><c>assertError(Term, Expr, Comment)</c></tag>
      <item>
        <p>Equivalent to <c>assertException(error, Term, Expr)</c></p>
      </item>
      <tag><c>assertExit(Term, Expr)</c></tag>
      <item></item>
      <tag><c>assertExit(Term, Expr, Comment)</c></tag>
      <item>
        <p>Equivalent to <c>assertException(exit, Term, Expr)</c></p>
      </item>
      <tag><c>assertThrow(Term, Expr)</c></tag>
      <item></item>
      <tag><c>assertThrow(Term, Expr, Comment)</c></tag>
      <item>
        <p>Equivalent to <c>assertException(throw, Term, Expr)</c></p>
      </item>
    </taglist>
  </section>

  <section>
    <title>See Also</title>
    <p><seealso marker="compiler:compile"><c>compile(3)</c></seealso>,
      <seealso marker="erts:erlc"><c>erlc(3)</c></seealso></p>
  </section>
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