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<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2020</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
      limitations under the License.
    
    </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>1997-01-21</date>
    <rev>B</rev>
    <file>erl_eval.xml</file>
  </header>
  <module since="">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
      <seeerl marker="erl_parse"><c>erl_parse</c></seeerl>,
      the Erlang parser, or <seeerl marker="io"><c>io</c></seeerl>.</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 <seemfa marker="erl_parse#parse_exprs/1">
        <c>erl_parse:parse_exprs/1</c></seemfa> or
        <seemfa marker="io#parse_erl_exprs/2">
        <c>io:parse_erl_exprs/2</c></seemfa>.</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 in section
        <seeerl marker="#local_function_handler">
        Local Function Handler</seeerl> in this module</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 in section
        <seeerl marker="#non_local_function_handler">
        Non-Local Function Handler</seeerl> in this module.</p></desc>
    </datatype>
    <datatype>
      <name name="value"/>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="add_binding" arity="3" since=""/>
      <fsummary>Add a binding.</fsummary>
      <desc>
        <p>Adds 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="binding" arity="2" since=""/>
      <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="bindings" arity="1" since=""/>
      <fsummary>Return bindings.</fsummary>
      <desc>
        <p>Returns the list of bindings contained in the binding
          structure.</p>
      </desc>
    </func>

    <func>
      <name name="del_binding" arity="2" since=""/>
      <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>

    <func>
      <name name="expr" arity="2" since=""/>
      <name name="expr" arity="3" since=""/>
      <name name="expr" arity="4" since=""/>
      <name name="expr" arity="5" since=""/>
      <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.
          For an explanation of when and how to use arguments
          <c><anno>LocalFunctionHandler</anno></c> and
          <c><anno>NonLocalFunctionHandler</anno></c>, see sections
          <seeerl marker="#local_function_handler">
          Local Function Handler</seeerl> and
          <seeerl marker="#non_local_function_handler">
          Non-Local Function Handler</seeerl> in this module.</p>
        <p>Returns <c>{value, <anno>Value</anno>, <anno>NewBindings</anno>}</c>
          by default. If <c><anno>ReturnFormat</anno></c> is <c>value</c>,
          only <c><anno>Value</anno></c> is returned.</p>
      </desc>
    </func>

    <func>
      <name name="expr_list" arity="2" since=""/>
      <name name="expr_list" arity="3" since=""/>
      <name name="expr_list" arity="4" since=""/>
      <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 <c>LocalFunctionHandler</c>, see section
          <seeerl marker="#local_function_handler">
          Local Function Handler</seeerl> in this module.</p>
        <p>Returns <c>{<anno>ValueList</anno>, <anno>NewBindings</anno>}</c>.
        </p>
      </desc>
    </func>

    <func>
      <name name="exprs" arity="2" since=""/>
      <name name="exprs" arity="3" since=""/>
      <name name="exprs" arity="4" since=""/>
      <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 that can
          be returned by <seemfa marker="io#parse_erl_exprs/2">
          <c>io:parse_erl_exprs/2</c></seemfa>.
          For an explanation of when and how to use arguments
          <c><anno>LocalFunctionHandler</anno></c> and
          <c><anno>NonLocalFunctionHandler</anno></c>, see sections
          <seeerl marker="#local_function_handler">
          Local Function Handler</seeerl> and
          <seeerl marker="#non_local_function_handler">
          Non-Local Function Handler</seeerl> in this module.</p>
        <p>Returns <c>{value, <anno>Value</anno>, <anno>NewBindings</anno>}</c>
        </p>
      </desc>
    </func>

    <func>
      <name name="new_bindings" arity="0" since=""/>
      <fsummary>Return a bindings structure.</fsummary>
      <desc>
        <p>Returns an empty binding structure.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <marker id="local_function_handler"></marker>
    <title>Local Function Handler</title>
    <p>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> can be used to define a function
      that 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 that 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, the current bindings cannot be
          accessed. To signal an error, the function
          handler 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 that 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 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>
    <marker id="non_local_function_handler"></marker>
    <title>Non-Local Function Handler</title>
    <p>The optional argument <c>NonLocalFunctionHandler</c> can be
      used to define a function that is called in the following
      cases:</p>
    <list type="bulleted">
      <item><p>A functional object (fun) is called.</p></item>
      <item><p>A built-in function is called.</p></item>
      <item><p>A function is called using the <c>M:F</c> syntax, where <c>M</c>
        and <c>F</c> are atoms or expressions.</p></item>
      <item><p>An operator <c>Op/A</c> is called (this is handled as a call to
        function <c>erlang:Op/A</c>).</p></item>
    </list>
    <p>Exceptions are calls to <c>erlang:apply/2,3</c>; neither of the
      function handlers are called for such calls.
      The argument can have the following formats:</p>
    <taglist>
      <tag><c>{value,Func}</c></tag>
      <item>
        <p>This defines a non-local function handler that 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 calls
          <c>exit/1</c> with a suitable exit value.</p>
      </item>
      <tag><c>none</c></tag>
      <item>
        <p>There is no non-local 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>)
        never takes place.</p>
      <p>The non-local function handler <em>is</em> however 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 non-local 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>Known Limitation</title>
    <p>Undocumented functions in this module are not to be used.</p>
  </section>
</erlref>

