<?xml version="1.0" encoding="latin1" ?> <!DOCTYPE erlref SYSTEM "erlref.dtd"> <erlref> <header> <copyright> <year>1996</year><year>2011</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>erl_eval</title> <prepared>Robert Virding</prepared> <responsible>Bjarne Däcker</responsible> <docno>1</docno> <approved>Bjarne Däcker</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>The evaluator is not complete. <c>receive</c> cannot be handled properly. </p> <p>Any undocumented functions in <c>erl_eval</c> should not be used.</p> </section> </erlref>