19962016 Ericsson AB. All Rights Reserved. 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. erl_eval Robert Virding Bjarne Däcker 1 Bjarne Däcker 1997-01-21 B erl_eval.xml
erl_eval The Erlang meta interpreter.

This module provides an interpreter for Erlang expressions. The expressions are in the abstract syntax as returned by erl_parse, the Erlang parser, or io.

A binding structure.

As returned by erl_parse:parse_exprs/1 or io:parse_erl_exprs/2.

Further described in section Local Function Handler in this module

Further described in section Non-Local Function Handler in this module.

 Add a binding.

Adds binding Name=Value to BindingStruct. Returns an updated binding structure.

 Return bindings.

Returns the binding of Name in BindingStruct.

 Return bindings.

Returns the list of bindings contained in the binding structure.

 Delete a binding.

Removes the binding of Name in BindingStruct. Returns an updated binding structure.

 Evaluate expression.

Evaluates Expression with the set of bindings Bindings. Expression is an expression in abstract syntax. For an explanation of when and how to use arguments LocalFunctionHandler and NonLocalFunctionHandler, see sections Local Function Handler and Non-Local Function Handler in this module.

Returns {value, Value, NewBindings} by default. If ReturnFormat is value, only Value is returned.

 Evaluate a list of expressions.

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 LocalFunctionHandler, see section Local Function Handler in this module.

Returns {ValueList, NewBindings}.

Evaluate expressions.

Evaluates Expressions with the set of bindings Bindings, where Expressions is a sequence of expressions (in abstract syntax) of a type that can be returned by io:parse_erl_exprs/2. For an explanation of when and how to use arguments LocalFunctionHandler and NonLocalFunctionHandler, see sections Local Function Handler and Non-Local Function Handler in this module.

Returns {value, Value, NewBindings}

Return a bindings structure.

Returns an empty binding structure.
Local Function Handler

During evaluation of a function, no calls can be made to local functions. An undefined function error would be generated. However, the optional argument LocalFunctionHandler 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:

{value,Func}

This defines a local function handler that is called with:

Func(Name, Arguments)

Name is the name of the local function (an atom) and Arguments is a list of the evaluated 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 exit/1 with a suitable exit value.

 {eval,Func}

This defines a local function handler that is called with:

Func(Name, Arguments, Bindings)

Name is the name of the local function (an atom), Arguments is a list of the unevaluated arguments, and Bindings are the current variable bindings. The function handler returns:

{value,Value,NewBindings}

Value is the value of the local function and NewBindings 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 exit/1 with a suitable exit value.

 none

There is no local function handler.
Non-Local Function Handler

The optional argument NonLocalFunctionHandler can be used to define a function that 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 function erlang:Op/A).

Exceptions are calls to erlang:apply/2,3; neither of the function handlers are called for such calls. The argument can have the following formats:

{value,Func}

This defines a non-local function handler that is called with:

Func(FuncSpec, Arguments)

FuncSpec is the name of the function on the form {Module,Function} or a fun, and Arguments is a list of the evaluated arguments. The function handler returns the value of the function. To signal an error, the function handler calls exit/1 with a suitable exit value.

 none

There is no non-local function handler.

For calls such as erlang:apply(Fun, Args) or erlang:apply(Module, Function, Args), the call of the non-local function handler corresponding to the call to erlang:apply/2,3 itself (Func({erlang, apply}, [Fun, Args]) or Func({erlang, apply}, [Module, Function, Args])) never takes place.

The non-local function handler is however called with the evaluated arguments of the call to erlang:apply/2,3: Func(Fun, Args) or Func({Module, Function}, Args) (assuming that {Module, Function} is not {erlang, apply}).

Calls to functions defined by evaluating fun expressions "fun ... end" are also hidden from non-local function handlers.

The non-local function handler argument is probably not used as frequently as the local function handler argument. A possible use is to call exit/1 on calls to functions that for some reason are not allowed to be called.

Known Limitation

Undocumented functions in this module are not to be used.