path: root/erts/doc/src/match_spec.xml

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<chapter>
  <header>
    <copyright>
      <year>1999</year><year>2010</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.

      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.

    </legalnotice>

    <title>Match specifications in Erlang</title>
    <prepared>Patrik Nyblom</prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date>1999-06-01</date>
    <rev>PA1</rev>
    <file>match_spec.xml</file>
  </header>
  <p>A "match specification" (match_spec) is an Erlang term describing a
    small "program" that will try to match something (either the
    parameters to a function as used in the <c><![CDATA[erlang:trace_pattern/2]]></c>
    BIF, or the objects in an ETS table.). 
    The match_spec in many ways works like a small function in Erlang, but is
    interpreted/compiled by the Erlang runtime system to something much more
    efficient than calling an Erlang function. The match_spec is also
    very limited compared to the expressiveness of real Erlang functions.</p>
  <p>Match specifications are given to the BIF <c><![CDATA[erlang:trace_pattern/2]]></c> to
    execute matching of function arguments as well as to define some actions
    to be taken when the match succeeds (the <c><![CDATA[MatchBody]]></c> part). Match
    specifications can also be used in ETS, to specify objects to be
    returned from an <c><![CDATA[ets:select/2]]></c> call (or other select
    calls). The semantics and restrictions differ slightly when using
    match specifications for tracing and in ETS, the differences are
    defined in a separate paragraph below.</p>
  <p>The most notable difference between a match_spec and an Erlang fun is
    of course the syntax. Match specifications are Erlang terms, not
    Erlang code.  A match_spec also has a somewhat strange concept of
    exceptions. An exception (e.g., <c><![CDATA[badarg]]></c>) in the <c><![CDATA[MatchCondition]]></c>
    part,
    which resembles an Erlang guard, will generate immediate failure,
    while an exception in the <c><![CDATA[MatchBody]]></c> part, which resembles the body of an
    Erlang function, is implicitly caught and results in the single atom
    <c><![CDATA['EXIT']]></c>.
    </p>

  <section>
    <title>Grammar</title>
    <p>A match_spec can be described in this <em>informal</em> grammar:</p>
    <list type="bulleted">
      <item>MatchExpression ::= [ MatchFunction, ... ]
      </item>
      <item>MatchFunction ::= { MatchHead, MatchConditions, MatchBody }
      </item>
      <item>MatchHead ::=  MatchVariable | <c><![CDATA['_']]></c> | [ MatchHeadPart, ... ]
      </item>
      <item>MatchHeadPart ::= term() | MatchVariable | <c><![CDATA['_']]></c></item>
      <item>MatchVariable ::= '$&lt;number&gt;'
      </item>
      <item>MatchConditions ::= [ MatchCondition, ...] | <c><![CDATA[[]]]></c></item>
      <item>MatchCondition ::= { GuardFunction } | 
       { GuardFunction, ConditionExpression, ... }
      </item>
      <item>BoolFunction ::= <c><![CDATA[is_atom]]></c> | <c><![CDATA[is_constant]]></c> |
      <c><![CDATA[is_float]]></c> | <c><![CDATA[is_integer]]></c> | <c><![CDATA[is_list]]></c> |
      <c><![CDATA[is_number]]></c> | <c><![CDATA[is_pid]]></c> | <c><![CDATA[is_port]]></c> |
      <c><![CDATA[is_reference]]></c> | <c><![CDATA[is_tuple]]></c> | <c><![CDATA[is_binary]]></c> |
      <c><![CDATA[is_function]]></c> | <c><![CDATA[is_record]]></c> | <c><![CDATA[is_seq_trace]]></c> |
      <c><![CDATA['and']]></c> | <c><![CDATA['or']]></c> | <c><![CDATA['not']]></c> | <c><![CDATA['xor']]></c> | 
      <c><![CDATA[andalso]]></c> | <c><![CDATA[orelse]]></c></item>
      <item>ConditionExpression ::= ExprMatchVariable | { GuardFunction } |
       { GuardFunction, ConditionExpression, ... } | TermConstruct
      </item>
      <item>ExprMatchVariable ::= MatchVariable (bound in the MatchHead) | 
      <c><![CDATA['$_']]></c> | <c><![CDATA['$$']]></c></item>
      <item>TermConstruct = {{}} | {{ ConditionExpression, ... }} | 
      <c><![CDATA[[]]]></c> | [ConditionExpression, ...] | NonCompositeTerm | Constant
      </item>
      <item>NonCompositeTerm ::= term() (not list or tuple)
      </item>
      <item>Constant ::= {<c><![CDATA[const]]></c>, term()}
      </item>
      <item>GuardFunction ::= BoolFunction | <c><![CDATA[abs]]></c> |
      <c><![CDATA[element]]></c> | <c><![CDATA[hd]]></c> | <c><![CDATA[length]]></c> | <c><![CDATA[node]]></c> |
      <c><![CDATA[round]]></c> | <c><![CDATA[size]]></c> | <c><![CDATA[tl]]></c> | <c><![CDATA[trunc]]></c> |
      <c><![CDATA['+']]></c> | <c><![CDATA['-']]></c> | <c><![CDATA['*']]></c> | <c><![CDATA['div']]></c> |
      <c><![CDATA['rem']]></c> | <c><![CDATA['band']]></c> | <c><![CDATA['bor']]></c> | <c><![CDATA['bxor']]></c> |
      <c><![CDATA['bnot']]></c> | <c><![CDATA['bsl']]></c> | <c><![CDATA['bsr']]></c> | <c><![CDATA['>']]></c> | 
      <c><![CDATA['>=']]></c> | <c><![CDATA['<']]></c> | <c><![CDATA['=<']]></c> | <c><![CDATA['=:=']]></c> | 
      <c><![CDATA['==']]></c> | <c><![CDATA['=/=']]></c> | <c><![CDATA['/=']]></c> | <c><![CDATA[self]]></c> |
      <c><![CDATA[get_tcw]]></c></item>
      <item>MatchBody ::= [ ActionTerm ]
      </item>
      <item>ActionTerm ::= ConditionExpression | ActionCall
      </item>
      <item>ActionCall ::= {ActionFunction} | 
       {ActionFunction, ActionTerm, ...}
      </item>
      <item>ActionFunction ::= <c><![CDATA[set_seq_token]]></c> |
      <c><![CDATA[get_seq_token]]></c> | <c><![CDATA[message]]></c> |
      <c><![CDATA[return_trace]]></c> | <c><![CDATA[exception_trace]]></c> | <c><![CDATA[process_dump]]></c> |
      <c><![CDATA[enable_trace]]></c> | <c><![CDATA[disable_trace]]></c> | <c><![CDATA[trace]]></c> |
      <c><![CDATA[display]]></c> | <c><![CDATA[caller]]></c> | <c><![CDATA[set_tcw]]></c> |
      <c><![CDATA[silent]]></c></item>
    </list>
  </section>

  <section>
    <title>Function descriptions</title>

    <section>
      <title>Functions allowed in all types of match specifications</title>
      <p>The different functions allowed in <c><![CDATA[match_spec]]></c> work like this:
        </p>
      <p><em>is_atom, is_constant, is_float, is_integer, is_list, is_number, is_pid, is_port, is_reference, is_tuple, is_binary, is_function: </em> Like the corresponding guard tests in
        Erlang, return <c><![CDATA[true]]></c> or <c><![CDATA[false]]></c>.
        </p>
      <p><em>is_record: </em>Takes an additional parameter, which SHALL
        be the result of <c><![CDATA[record_info(size, <record_type>)]]></c>,
        like in <c><![CDATA[{is_record, '$1', rectype, record_info(size, rectype)}]]></c>.
        </p>
      <p><em>'not': </em>Negates its single argument (anything other
        than <c><![CDATA[false]]></c> gives <c><![CDATA[false]]></c>).
        </p>
      <p><em>'and': </em>Returns <c><![CDATA[true]]></c> if all its arguments
        (variable length argument list) evaluate to <c><![CDATA[true]]></c>, else
        <c><![CDATA[false]]></c>. Evaluation order is undefined.
        </p>
      <p><em>'or': </em>Returns <c><![CDATA[true]]></c> if any of its arguments
        evaluates to <c><![CDATA[true]]></c>. Variable length argument
        list. Evaluation order is undefined.
        </p>
      <p><em>andalso: </em>Like <c><![CDATA['and']]></c>, but quits evaluating its
        arguments as soon as one argument evaluates to something else
        than true. Arguments are evaluated left to right.
        </p>
      <p><em>orelse: </em>Like <c><![CDATA['or']]></c>, but quits evaluating as soon
        as one of its arguments evaluates to <c><![CDATA[true]]></c>. Arguments are
        evaluated left to right.
        </p>
      <p><em>'xor': </em>Only two arguments, of which one has to be true
        and the other false to return <c><![CDATA[true]]></c>; otherwise
        <c><![CDATA['xor']]></c> returns false.
        </p>
      <p><em>abs, element, hd, length, node, round, size, tl, trunc, '+', '-', '*', 'div', 'rem', 'band', 'bor', 'bxor', 'bnot', 'bsl', 'bsr', '>', '>=', '&lt;', '=&lt;', '=:=', '==', '=/=', '/=', self: </em>Work as the corresponding Erlang bif's (or
        operators). In case of bad arguments, the result depends on
        the context. In the <c><![CDATA[MatchConditions]]></c> part of the
        expression, the test fails immediately (like in an Erlang
        guard), but in the <c><![CDATA[MatchBody]]></c>, exceptions are implicitly
        caught and the call results in the atom <c><![CDATA['EXIT']]></c>.</p>
    </section>

    <section>
      <title>Functions allowed only for tracing</title>
      <p><em>is_seq_trace: </em>Returns <c><![CDATA[true]]></c> if a sequential
        trace token is set for the current process, otherwise <c><![CDATA[false]]></c>.
        </p>
      <p><em>set_seq_token:</em> Works like
        <c><![CDATA[seq_trace:set_token/2]]></c>, but returns <c><![CDATA[true]]></c> on success
        and <c><![CDATA['EXIT']]></c> on error or bad argument. Only allowed in the
        <c><![CDATA[MatchBody]]></c> part and only allowed when tracing.
        </p>
      <p><em>get_seq_token:</em> Works just like 
        <c><![CDATA[seq_trace:get_token/0]]></c>, and is only allowed in the 
        <c><![CDATA[MatchBody]]></c> part when tracing.
        </p>
      <p><em>message:</em> Sets an additional message appended to the
        trace message sent. One can only set one additional message in
        the body; subsequent calls will replace the appended message. As
        a special case, <c><![CDATA[{message, false}]]></c> disables sending of
        trace messages ('call' and 'return_to') 
        for this function call, just like if the match_spec had not matched,
        which can be useful if only the side effects of 
        the <c><![CDATA[MatchBody]]></c> are desired. 
        Another special case is <c><![CDATA[{message, true}]]></c> which
        sets the default behavior, as if the function had no match_spec, 
        trace message is sent with no extra
        information (if no other calls to <c><![CDATA[message]]></c> are placed
        before <c><![CDATA[{message, true}]]></c>, it is in fact a "noop"). 
        </p>
      <p>Takes one argument, the message. Returns <c><![CDATA[true]]></c> and can
        only be used in the <c><![CDATA[MatchBody]]></c> part and when tracing.
        </p>
      <p><em>return_trace:</em> Causes a <c><![CDATA[return_from]]></c> trace 
        message to be sent upon return from the current function.
        Takes no arguments, returns <c><![CDATA[true]]></c> and can only be used 
        in the <c><![CDATA[MatchBody]]></c> part when tracing.
        If the process trace flag <c><![CDATA[silent]]></c>
        is active the <c><![CDATA[return_from]]></c> trace message is inhibited.
        </p>
      <p>NOTE! If the traced function is tail recursive, this match 
        spec function destroys that property. 
        Hence, if a match spec executing this function is used on a 
        perpetual server process, it may only be active for a limited 
        time, or the emulator will eventually use all memory in the host 
        machine and crash. If this match_spec function is inhibited
        using the <c><![CDATA[silent]]></c> process trace flag 
        tail recursiveness still remains.
        </p>
      <p><em>exception_trace:</em> Same as <em>return_trace</em>,
        plus; if the traced function exits due to an exception,
        an <c><![CDATA[exception_from]]></c> trace message is generated,
        whether the exception is caught or not.
        </p>
      <p><em>process_dump:</em> Returns some textual information about
        the current process as a binary. Takes no arguments and is only
        allowed in the <c><![CDATA[MatchBody]]></c> part when tracing.
        </p>
      <p><em>enable_trace:</em> With one parameter this function turns
        on tracing like the Erlang call <c><![CDATA[erlang:trace(self(), true, [P2])]]></c>, where <c><![CDATA[P2]]></c> is the parameter to
        <c><![CDATA[enable_trace]]></c>. With two parameters, the first parameter
        should be either a process identifier or the registered name of
        a process. In this case tracing is turned on for the designated
        process in the same way as in the Erlang call  <c><![CDATA[erlang:trace(P1, true, [P2])]]></c>, where P1 is the first and P2 is the second
        argument. The process <c><![CDATA[P1]]></c> gets its trace messages sent to the same
        tracer as the process executing the statement uses. <c><![CDATA[P1]]></c>
        can <em>not</em> be one of the atoms <c><![CDATA[all]]></c>, <c><![CDATA[new]]></c> or 
        <c><![CDATA[existing]]></c> (unless, of course, they are registered names).
        <c><![CDATA[P2]]></c> can <em>not</em> be <c><![CDATA[cpu_timestamp]]></c> nor
        <c><![CDATA[{tracer,_}]]></c>.
        Returns <c><![CDATA[true]]></c> and may only be used in
        the <c><![CDATA[MatchBody]]></c> part when tracing.
        </p>
      <p><em>disable_trace:</em> With one parameter this function
        disables tracing like the Erlang call <c><![CDATA[erlang:trace(self(), false, [P2])]]></c>, where <c><![CDATA[P2]]></c> is the parameter to
        <c><![CDATA[disable_trace]]></c>. With two parameters it works like the
        Erlang call <c><![CDATA[erlang:trace(P1, false, [P2])]]></c>, where P1 can
        be either a process identifier or a registered name and is given
        as the first argument to the match_spec function. 
        <c><![CDATA[P2]]></c> can <em>not</em> be <c><![CDATA[cpu_timestamp]]></c> nor
        <c><![CDATA[{tracer,_}]]></c>. Returns
        <c><![CDATA[true]]></c> and may only be used in the <c><![CDATA[MatchBody]]></c> part
        when tracing.
        </p>
      <p><em>trace:</em> With two parameters this function takes a list
        of trace flags to disable as first parameter and a list
        of trace flags to enable as second parameter. Logically, the
        disable list is applied first, but effectively all changes
        are applied atomically. The trace flags
        are the same as for <c><![CDATA[erlang:trace/3]]></c> not including
        <c><![CDATA[cpu_timestamp]]></c> but including <c><![CDATA[{tracer,_}]]></c>. If a
        tracer is specified in both lists, the tracer in the 
        enable list takes precedence. If no tracer is specified the
        same tracer as the process executing the match spec is 
        used. With three parameters to this function the first is 
        either a process identifier or the registered name of a
        process to set trace flags on, the second is the disable
        list, and the third is the enable list. Returns
        <c><![CDATA[true]]></c> if any trace property was changed for the
        trace target process or <c><![CDATA[false]]></c> if not. It may only
        be used in the <c><![CDATA[MatchBody]]></c> part when tracing.
        </p>
      <p><em>caller:</em> 
        Returns the calling function as a tuple {Module,
        Function, Arity} or the atom <c><![CDATA[undefined]]></c> if the calling
        function cannot be determined. May only be used in the 
        <c><![CDATA[MatchBody]]></c> part when tracing.
        </p>
      <p>Note that if a "technically built in function" (i.e. a
        function not written in Erlang) is traced, the <c><![CDATA[caller]]></c>
        function will sometimes return the atom <c><![CDATA[undefined]]></c>. The calling
        Erlang function is not available during such calls.
        </p>
      <p><em>display:</em> For debugging purposes only; displays the
        single argument as an Erlang term on stdout, which is seldom
        what is wanted. Returns <c><![CDATA[true]]></c> and may only be used in the
        <c><![CDATA[MatchBody]]></c> part when tracing.
        </p>
      <p>        <marker id="get_tcw"></marker>
<em>get_tcw:</em> 
        Takes no argument and returns the value of the node's trace
        control word. The same is done by
        <c><![CDATA[erlang:system_info(trace_control_word)]]></c>. 
        </p>
      <p>The trace control word is a 32-bit unsigned integer intended for
        generic trace control. The trace control word can be tested and 
        set both from within trace match specifications and with BIFs. 
        This call is only allowed when tracing. 
        </p>
      <p>        <marker id="set_tcw"></marker>
<em>set_tcw:</em>
        Takes one unsigned integer argument, sets the value of 
        the node's trace control word to the value of the argument 
        and returns the previous value. The same is done by 
        <c><![CDATA[erlang:system_flag(trace_control_word, Value)]]></c>. It is only
        allowed to use <c><![CDATA[set_tcw]]></c> in the <c><![CDATA[MatchBody]]></c> part 
        when tracing.
        </p>
      <p><em>silent:</em>
        Takes one argument. If the argument is <c><![CDATA[true]]></c>, the call
        trace message mode for the current process is set to silent
        for this call and all subsequent, i.e call trace messages
        are inhibited even if <c><![CDATA[{message, true}]]></c> is called in the
        <c><![CDATA[MatchBody]]></c> part for a traced function. 
        </p>
      <p>This mode can also be activated with the <c><![CDATA[silent]]></c> flag
        to <c><![CDATA[erlang:trace/3]]></c>.
        </p>
      <p>If the argument is <c><![CDATA[false]]></c>, the call trace message mode
        for the current process is set to normal (non-silent) for
        this call and all subsequent.
        </p>
      <p>If the argument is neither <c><![CDATA[true]]></c> nor <c><![CDATA[false]]></c>, 
        the call trace message mode is unaffected.</p>
    </section>
    <p><em>Note</em> that all "function calls" have to be tuples, 
      even if they take no arguments. The value of <c><![CDATA[self]]></c> is 
      the atom() <c><![CDATA[self]]></c>, but the value of <c><![CDATA[{self}]]></c> is 
      the pid() of the current process.</p>
  </section>

  <section>
    <title>Variables and literals</title>
    <p>Variables take the form <c><![CDATA['$<number>']]></c> where
      <c><![CDATA[<number>]]></c> is an integer between 0 (zero) and
      100000000 (1e+8), the behavior if the number is outside these
      limits is <em>undefined</em>.  In the <c><![CDATA[MatchHead]]></c> part, the special
      variable <c><![CDATA['_']]></c> matches anything, and never gets bound (like
      <c><![CDATA[_]]></c> in Erlang). In the <c><![CDATA[MatchCondition/MatchBody]]></c>
      parts, no unbound variables are allowed, why <c><![CDATA['_']]></c> is
      interpreted as itself (an atom). Variables can only be bound in
      the <c><![CDATA[MatchHead]]></c> part. In the <c><![CDATA[MatchBody]]></c> and
      <c><![CDATA[MatchCondition]]></c> parts, only variables bound previously may
      be used. As a special case, in the
      <c><![CDATA[MatchCondition/MatchBody]]></c> parts, the variable <c><![CDATA['$_']]></c>
      expands to the whole expression which matched the
      <c><![CDATA[MatchHead]]></c> (i.e., the whole parameter list to the possibly
      traced function or the whole matching object in the ets table)
      and the variable <c><![CDATA['$$']]></c> expands to a list
      of the values of all bound variables in order 
      (i.e. <c><![CDATA[['$1','$2', ...]]]></c>).
      </p>
    <p>In the <c><![CDATA[MatchHead]]></c> part, all literals (except the variables
      noted above) are interpreted as is. In the
      <c><![CDATA[MatchCondition/MatchBody]]></c> parts, however, the
      interpretation is in some ways different.  Literals in the
      <c><![CDATA[MatchCondition/MatchBody]]></c> can either be written as is,
      which works for all literals except tuples, or by using the
      special form <c><![CDATA[{const, T}]]></c>, where <c><![CDATA[T]]></c> is any Erlang
      term. For tuple literals in the match_spec, one can also use
      double tuple parentheses, i.e., construct them as a tuple of
      arity one containing a single tuple, which is the one to be
      constructed. The "double tuple parenthesis" syntax is useful to
      construct tuples from already bound variables, like in
      <c><![CDATA[{{'$1', [a,b,'$2']}}]]></c>. Some examples may be needed:
      </p>
    <table>
      <row>
        <cell align="left" valign="middle">Expression		</cell>
        <cell align="left" valign="middle">Variable bindings		</cell>
        <cell align="left" valign="middle">Result	</cell>
      </row>
      <row>
        <cell align="left" valign="middle">{{'$1','$2'}}		</cell>
        <cell align="left" valign="middle">'$1' = a, '$2' = b</cell>
        <cell align="left" valign="middle">{a,b}</cell>
      </row>
      <row>
        <cell align="left" valign="middle">{const, {'$1', '$2'}}	</cell>
        <cell align="left" valign="middle">doesn't matter</cell>
        <cell align="left" valign="middle">{'$1', '$2'}</cell>
      </row>
      <row>
        <cell align="left" valign="middle">a			</cell>
        <cell align="left" valign="middle">doesn't matter			</cell>
        <cell align="left" valign="middle">a</cell>
      </row>
      <row>
        <cell align="left" valign="middle">'$1'			</cell>
        <cell align="left" valign="middle">'$1' = []			</cell>
        <cell align="left" valign="middle">[]</cell>
      </row>
      <row>
        <cell align="left" valign="middle">['$1']			</cell>
        <cell align="left" valign="middle">'$1' = []			</cell>
        <cell align="left" valign="middle">[[]]</cell>
      </row>
      <row>
        <cell align="left" valign="middle">[{{a}}]			</cell>
        <cell align="left" valign="middle">doesn't matter</cell>
        <cell align="left" valign="middle">[{a}]</cell>
      </row>
      <row>
        <cell align="left" valign="middle">42			</cell>
        <cell align="left" valign="middle">doesn't matter</cell>
        <cell align="left" valign="middle">42</cell>
      </row>
      <row>
        <cell align="left" valign="middle">"hello"			</cell>
        <cell align="left" valign="middle">doesn't matter</cell>
        <cell align="left" valign="middle">"hello"</cell>
      </row>
      <row>
        <cell align="left" valign="middle">$1			</cell>
        <cell align="left" valign="middle">doesn't matter</cell>
        <cell align="left" valign="middle">49 (the ASCII value for the character '1')</cell>
      </row>
      <tcaption>Literals in the MatchCondition/MatchBody parts of a match_spec</tcaption>
    </table>
  </section>

  <section>
    <title>Execution of the match</title>
    <p>The execution of the match expression, when the runtime system
      decides whether a trace message should be sent, goes as follows:
      </p>
    <p>For each tuple in the <c><![CDATA[MatchExpression]]></c> list and while no
      match has succeeded:</p>
    <list type="bulleted">
      <item>Match the <c><![CDATA[MatchHead]]></c> part against the arguments to the
       function, 
       binding the <c><![CDATA['$<number>']]></c> variables (much like in
      <c><![CDATA[ets:match/2]]></c>).
       If the <c><![CDATA[MatchHead]]></c> cannot match the arguments, the match fails.
      </item>
      <item>Evaluate each <c><![CDATA[MatchCondition]]></c> (where only
      <c><![CDATA['$<number>']]></c> variables previously bound in the
      <c><![CDATA[MatchHead]]></c> can occur) and expect it to return the atom
      <c><![CDATA[true]]></c>. As soon as a condition does not evaluate to
      <c><![CDATA[true]]></c>, the match fails. If any BIF call generates an
       exception, also fail.
      </item>
      <item>
        <list type="bulleted">
          <item><em>If the match_spec is executing when tracing:</em><br></br>
           Evaluate each <c><![CDATA[ActionTerm]]></c> in the same way as the
          <c><![CDATA[MatchConditions]]></c>, but completely ignore the return
           values. Regardless of what happens in this part, the match has
           succeeded.</item>
          <item><em>If the match_spec is executed when selecting objects from an ETS table:</em><br></br>
           Evaluate the expressions in order and return the value of
           the last expression (typically there is only one expression
           in this context)</item>
        </list>
      </item>
    </list>
  </section>

  <section>
    <title>Differences between match specifications in ETS and tracing</title>
    <p>ETS match specifications are there to produce a return
      value. Usually the expression contains one single
      <c><![CDATA[ActionTerm]]></c> which defines the return value without having
      any side effects. Calls with side effects are not allowed in the
      ETS context.</p>
    <p>When tracing there is no return value to produce, the
      match specification either matches or doesn't. The effect when the
      expression matches is a trace message rather then a returned
      term. The <c><![CDATA[ActionTerm]]></c>'s are executed as in an imperative
      language, i.e. for their side effects. Functions with side effects
      are also allowed when tracing.</p>
    <p>In ETS the match head is a <c><![CDATA[tuple()]]></c> (or a single match
      variable) while it is a list (or a single match variable) when
      tracing.</p>
  </section>

  <section>
    <title>Examples</title>
    <p>Match an argument list of three where the first and third arguments 
      are equal:</p>
    <code type="none"><![CDATA[
[{['$1', '_', '$1'],
  [],
  []}]
    ]]></code>
    <p>Match an argument list of three where the second argument is 
      a number greater than three:</p>
    <code type="none"><![CDATA[
[{['_', '$1', '_'],
  [{ '>', '$1', 3}],
  []}]
    ]]></code>
    <p>Match an argument list of three, where the third argument
      is a tuple containing argument one and two <em>or</em> a list
      beginning with argument one and two (i. e. <c><![CDATA[[a,b,[a,b,c]]]]></c> or 
      <c><![CDATA[[a,b,{a,b}]]]></c>):
      </p>
    <code type="none"><![CDATA[
[{['$1', '$2', '$3'],
  [{orelse, 
      {'=:=', '$3', {{'$1','$2'}}},
      {'and', 
        {'=:=', '$1', {hd, '$3'}},
        {'=:=', '$2', {hd, {tl, '$3'}}}}}],
  []}]
    ]]></code>
    <p>The above problem may also be solved like this:</p>
    <code type="none"><![CDATA[
[{['$1', '$2', {'$1', '$2}], [], []},
 {['$1', '$2', ['$1', '$2' | '_']], [], []}]
    ]]></code>
    <p>Match two arguments where the first is a tuple beginning with
      a list which in turn begins with the second argument times
      two (i. e. [{[4,x],y},2] or [{[8], y, z},4])</p>
    <code type="none"><![CDATA[
[{['$1', '$2'],[{'=:=', {'*', 2, '$2'}, {hd, {element, 1, '$1'}}}],
  []}]
    ]]></code>
    <p>Match three arguments. When all three are equal and are
      numbers, append the process dump to the trace message, else
      let the trace message be as is, but set the sequential trace
      token label to 4711.</p>
    <code type="none"><![CDATA[
[{['$1', '$1', '$1'],
  [{is_number, '$1'}],
  [{message, {process_dump}}]},
 {'_', [], [{set_seq_token, label, 4711}]}]
    ]]></code>
    <p>As can be noted above, the parameter list can be matched
      against a single <c><![CDATA[MatchVariable]]></c> or an <c><![CDATA['_']]></c>. To replace the
      whole
      parameter list with a single variable is a special case. In all 
      other cases the <c><![CDATA[MatchHead]]></c> has to be a <em>proper</em> list.
      </p>
    <p>Match all objects in an ets table where the first element is 
      the atom 'strider' and the tuple arity is 3 and return the whole
      object.</p>
    <code type="none"><![CDATA[
[{{strider,'_','_'},
  [],
  ['$_']}]
    ]]></code>
    <p>Match all objects in an ets table with arity &gt; 1 and the first 
      element is 'gandalf', return element 2.</p>
    <code type="none"><![CDATA[
[{'$1',
  [{'==', gandalf, {element, 1, '$1'}},{'>=',{size, '$1'},2}],
  [{element,2,'$1'}]}]
    ]]></code>
    <p>In the above example, if the first element had been the key,
      it's much more efficient to match that key in the <c><![CDATA[MatchHead]]></c>
      part than in the <c><![CDATA[MatchConditions]]></c> part. The search space of
      the tables is restricted with regards to the <c><![CDATA[MatchHead]]></c> so
      that only objects with the matching key are searched. 
      </p>
    <p>Match tuples of 3 elements where the second element is either
      'merry' or 'pippin', return the whole objects.</p>
    <code type="none"><![CDATA[
[{{'_',merry,'_'},
  [],
  ['$_']},
 {{'_',pippin,'_'},
  [],
  ['$_']}]
    ]]></code>
    <p>The function <c><![CDATA[ets:test_ms/2]]></c> can be useful for testing
      complicated ets matches.</p>
  </section>
</chapter>