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<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2013</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.
      You may obtain a copy of the License at
 
          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,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
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    </legalnotice>

    <title>erl_eval</title>
    <prepared>Robert Virding</prepared>
    <responsible>Bjarne D&auml;cker</responsible>
    <docno>1</docno>
    <approved>Bjarne D&auml;cker</approved>
    <checked></checked>
    <date>97-01-21</date>
    <rev>B</rev>
    <file>erl_eval.sgml</file>
  </header>
  <module>erl_eval</module>
  <modulesummary>The Erlang Meta Interpreter</modulesummary>
  <description>
    <p>This module provides an interpreter for Erlang expressions. The
      expressions are in the abstract syntax as returned by
      <seealso marker="erl_parse"><c>erl_parse</c></seealso>,
      the Erlang parser, or <seealso marker="io">
      <c>io</c></seealso>.</p>
  </description>
  <datatypes>
    <datatype>
      <name name="bindings"/>
    </datatype>
    <datatype>
      <name name="binding_struct"/>
      <desc><p>A binding structure.</p></desc>
    </datatype>
    <datatype>
      <name name="expression"/>
    </datatype>
    <datatype>
      <name name="expressions"/>
      <desc><p>As returned by <seealso marker="erl_parse#parse_exprs/1">
        <c>erl_parse:parse_exprs/1</c></seealso> or
        <seealso marker="io#parse_erl_exprs/2">
        <c>io:parse_erl_exprs/2</c></seealso>.</p></desc>
    </datatype>
    <datatype>
      <name name="expression_list"/>
    </datatype>
    <datatype>
      <name name="func_spec"/>
    </datatype>
    <datatype>
      <name name="lfun_eval_handler"/>
    </datatype>
    <datatype>
      <name name="lfun_value_handler"/>
    </datatype>
    <datatype>
      <name name="local_function_handler"/>
      <desc><p>Further described
        <seealso marker="#local_function_handler">below.</seealso></p>
      </desc>
    </datatype>
    <datatype>
      <name name="name"/>
    </datatype>
    <datatype>
      <name name="nlfun_handler"/>
    </datatype>
    <datatype>
      <name name="non_local_function_handler"/>
      <desc><p>Further described
        <seealso marker="#non_local_function_handler">below.</seealso></p>
      </desc>
    </datatype>
    <datatype>
      <name name="value"/>
    </datatype>
  </datatypes>
  <funcs>
    <func>
      <name name="exprs" arity="2"/>
      <name name="exprs" arity="3"/>
      <name name="exprs" arity="4"/>
      <fsummary>Evaluate expressions</fsummary>
      <desc>
        <p>Evaluates <c><anno>Expressions</anno></c> with the set of bindings
          <c><anno>Bindings</anno></c>, where <c><anno>Expressions</anno></c>
          is a sequence of
          expressions (in abstract syntax) of a type which may be
          returned by <seealso marker="io#parse_erl_exprs/2">
        <c>io:parse_erl_exprs/2</c></seealso>. See below for an
          explanation of how and when to use the arguments
          <c><anno>LocalFunctionHandler</anno></c> and
          <c><anno>NonLocalFunctionHandler</anno></c>.
          </p>
        <p>Returns <c>{value, <anno>Value</anno>, <anno>NewBindings</anno>}</c>
        </p>
      </desc>
    </func>
    <func>
      <name name="expr" arity="2"/>
      <name name="expr" arity="3"/>
      <name name="expr" arity="4"/>
      <name name="expr" arity="5"/>
      <fsummary>Evaluate expression</fsummary>
      <desc>
        <p>Evaluates <c><anno>Expression</anno></c> with the set of bindings
          <c><anno>Bindings</anno></c>. <c><anno>Expression</anno></c>
          is an expression in
          abstract syntax. See below for an explanation of
          how and when to use the arguments
          <c><anno>LocalFunctionHandler</anno></c> and
          <c><anno>NonLocalFunctionHandler</anno></c>.
          </p>
        <p>Returns <c>{value, <anno>Value</anno>,
        <anno>NewBindings</anno>}</c> by default. But if the
        <c><anno>ReturnFormat</anno></c> is <c>value</c> only
        the <c><anno>Value</anno></c> is returned.</p>
      </desc>
    </func>
    <func>
      <name name="expr_list" arity="2"/>
      <name name="expr_list" arity="3"/>
      <name name="expr_list" arity="4"/>
      <fsummary>Evaluate a list of expressions</fsummary>
      <desc>
        <p>Evaluates a list of expressions in parallel, using the same
          initial bindings for each expression. Attempts are made to
          merge the bindings returned from each evaluation.  This
          function is useful in the <c>LocalFunctionHandler</c>. See below.
          </p>
        <p>Returns <c>{<anno>ValueList</anno>, <anno>NewBindings</anno>}</c>.
        </p>
      </desc>
    </func>
    <func>
      <name name="new_bindings" arity="0"/>
      <fsummary>Return a bindings structure</fsummary>
      <desc>
        <p>Returns an empty binding structure.</p>
      </desc>
    </func>
    <func>
      <name name="bindings" arity="1"/>
      <fsummary>Return bindings</fsummary>
      <desc>
        <p>Returns the list of bindings contained in the binding
          structure.</p>
      </desc>
    </func>
    <func>
      <name name="binding" arity="2"/>
      <fsummary>Return bindings</fsummary>
      <desc>
        <p>Returns the binding of <c><anno>Name</anno></c>
          in <c><anno>BindingStruct</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="add_binding" arity="3"/>
      <fsummary>Add a binding</fsummary>
      <desc>
        <p>Adds the binding <c><anno>Name</anno> = <anno>Value</anno></c>
          to <c><anno>BindingStruct</anno></c>.
          Returns an updated binding structure.</p>
      </desc>
    </func>
    <func>
      <name name="del_binding" arity="2"/>
      <fsummary>Delete a binding</fsummary>
      <desc>
        <p>Removes the binding of <c><anno>Name</anno></c>
          in <c><anno>BindingStruct</anno></c>.
          Returns an updated binding structure.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Local Function Handler</title>
    <p><marker id="local_function_handler"></marker>
      During evaluation of a function, no calls can be made to local
      functions. An undefined function error would be
      generated. However, the optional argument
      <c>LocalFunctionHandler</c> may be used to define a function
      which is called when there is a call to a local function. The
      argument can have the following formats:</p>
    <taglist>
      <tag><c>{value,Func}</c></tag>
      <item>
        <p>This defines a local function handler which is called with:</p>
        <code type="none">
Func(Name, Arguments)        </code>
        <p><c>Name</c> is the name of the local function (an atom) and
          <c>Arguments</c> is a list of the <em>evaluated</em>
          arguments. The function handler returns the value of the
          local function. In this case, it is not possible to access
          the current bindings. To signal an error, the function
          handler just calls <c>exit/1</c> with a suitable exit value.</p>
      </item>
      <tag><c>{eval,Func}</c></tag>
      <item>
        <p>This defines a local function handler which is called with:</p>
        <code type="none">
Func(Name, Arguments, Bindings)        </code>
        <p><c>Name</c> is the name of the local function (an atom),
          <c>Arguments</c> is a list of the <em>unevaluated</em>
          arguments, and <c>Bindings</c> are the current variable
          bindings. The function handler returns:</p>
        <code type="none">
{value,Value,NewBindings}        </code>
        <p><c>Value</c> is the value of the local function and
          <c>NewBindings</c> are the updated variable bindings. In
          this case, the function handler must itself evaluate all the
          function arguments and manage the bindings. To signal an
          error, the function handler just calls <c>exit/1</c> with a
          suitable exit value.</p>
      </item>
      <tag><c>none</c></tag>
      <item>
        <p>There is no local function handler.</p>
      </item>
    </taglist>
  </section>

  <section>
    <title>Non-local Function Handler</title>
    <p><marker id="non_local_function_handler"></marker>
      The optional argument <c>NonlocalFunctionHandler</c> may be
      used to define a function which is called in the following
      cases: a functional object (fun) is called; a built-in function
      is called; a function is called using the M:F syntax, where M
      and F are atoms or expressions; an operator Op/A is called
      (this is handled as a call to the function <c>erlang:Op/A</c>).
      Exceptions are calls to <c>erlang:apply/2,3</c>; neither of the
      function handlers will be called for such calls. 
      The argument can have the following formats:</p>
    <taglist>
      <tag><c>{value,Func}</c></tag>
      <item>
        <p>This defines an nonlocal function handler which is called with:</p>
        <code type="none">
Func(FuncSpec, Arguments)        </code>
        <p><c>FuncSpec</c> is the name of the function on the form
          <c>{Module,Function}</c> or a fun, and <c>Arguments</c> is a
          list of the <em>evaluated</em> arguments. The function
          handler returns the value of the function. To
          signal an error, the function handler just calls
          <c>exit/1</c> with a suitable exit value.</p>
      </item>
      <tag><c>none</c></tag>
      <item>
        <p>There is no nonlocal function handler.</p>
      </item>
    </taglist>
    <note>
      <p>For calls such as <c>erlang:apply(Fun, Args)</c> or
        <c>erlang:apply(Module, Function, Args)</c> the call of the
        non-local function handler corresponding to the call to
        <c>erlang:apply/2,3</c> itself--<c>Func({erlang, apply}, [Fun, Args])</c> or <c>Func({erlang, apply}, [Module, Function, Args])</c>--will never take place. The non-local function
        handler <em>will</em> however be called with the evaluated
        arguments of the call to <c>erlang:apply/2,3</c>: <c>Func(Fun, Args)</c> or <c>Func({Module, Function}, Args)</c> (assuming
        that <c>{Module, Function}</c> is not <c>{erlang, apply}</c>).</p>
      <p>Calls to functions defined by evaluating fun expressions
        <c>"fun ... end"</c> are also hidden from non-local function
        handlers.</p> </note>
    <p>The nonlocal function handler argument is probably not used as
      frequently as the local function handler argument. A possible
      use is to call <c>exit/1</c> on calls to functions that for some
      reason are not allowed to be called.</p>
  </section>

  <section>
    <title>Bugs</title>
    <p>Undocumented functions in <c>erl_eval</c> should not be used.</p>
  </section>
</erlref>

