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# erl_eval

### MODULE

erl_eval

### MODULE SUMMARY

The Erlang Meta Interpreter

### DESCRIPTION

This module provides an interpreter for Erlang expressions. The expressions are in the abstract syntax as returned by erl_parse, the Erlang parser, or a call to io:parse_erl_exprs/2.

### EXPORTS

exprs(Expressions, Bindings) -> {value, Value, NewBindings}
exprs(Expressions, Bindings, LocalFunctionHandler) -> {value, Value, NewBindings}
exprs(Expressions, Bindings, LocalFunctionHandler, NonlocalFunctionHandler) -> {value, Value, NewBindings}

Types:

Expressions = as returned by erl_parse or io:parse_erl_exprs/2
Bindings = as returned by bindings/1
LocalFunctionHandler = {value, Func} | {eval, Func} | none
NonlocalFunctionHandler = {value, Func} | none

Evaluates Expressions with the set of bindings Bindings, where Expressions is a sequence of expressions (in abstract syntax) of a type which may be returned by io:parse_erl_exprs/2. See below for an explanation of how and when to use the arguments LocalFunctionHandler and NonlocalFunctionHandler.

Returns {value, Value, NewBindings}

expr(Expression, Bindings) -> { value, Value, NewBindings }
expr(Expression, Bindings, LocalFunctionHandler) -> { value, Value, NewBindings }
expr(Expression, Bindings, LocalFunctionHandler, NonlocalFunctionHandler) -> { value, Value, NewBindings }
expr(Expression, Bindings, LocalFunctionHandler, NonlocalFunctionHandler, ReturnFormat) -> { value, Value, NewBindings } | Value

Types:

Expression = as returned by io:parse_erl_form/2, for example
Bindings = as returned by bindings/1
LocalFunctionHandler = {value, Func} | {eval, Func} | none
NonlocalFunctionHandler = {value, Func} | none
ReturnFormat = value | none

Evaluates Expression with the set of bindings Bindings. Expression is an expression (in abstract syntax) of a type which may be returned by io:parse_erl_form/2. See below for an explanation of how and when to use the arguments LocalFunctionHandler and NonlocalFunctionHandler.

Returns {value, Value, NewBindings} by default. But if the ReturnFormat is value only the Value is returned.

expr_list(ExpressionList, Bindings) -> {ValueList, NewBindings}
expr_list(ExpressionList, Bindings, LocalFunctionHandler) -> {ValueList, NewBindings}
expr_list(ExpressionList, Bindings, LocalFunctionHandler, NonlocalFunctionHandler) -> {ValueList, NewBindings}

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 LocalFunctionHandler. See below.

Returns {ValueList, NewBindings}.

new_bindings() -> BindingStruct

Returns an empty binding structure.

bindings(BindingStruct) -> Bindings

Returns the list of bindings contained in the binding structure.

binding(Name, BindingStruct) -> Binding

Returns the binding of Name in BindingStruct.

add_binding(Name, Value, Bindings) -> BindingStruct

Adds the binding Name = Value to Bindings. Returns an updated binding structure.

del_binding(Name, Bindings) -> BindingStruct

Removes the binding of Name in Bindings. Returns an updated 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 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:

{value,Func}

  This defines a local function handler which 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, it is not possible to access the current bindings. To signal an error, the function handler just calls exit/1 with a suitable exit value.

{eval,Func}

  This defines a local function handler which 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 just calls exit/1 with a suitable exit value.

none

  There is no local function handler.

### Non-local Function Handler

The optional argument NonlocalFunctionHandler 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 erlang:Op/A). Exceptions are calls to erlang:apply/2,3; neither of the function handlers will be called for such calls. The argument can have the following formats:

{value,Func}

  This defines an nonlocal function handler which 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 just calls exit/1 with a suitable exit value.

none

  There is no nonlocal function handler.

Note

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])--will never take place. The non-local function handler will however be 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 nonlocal 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.

### Bugs

The evaluator is not complete. receive cannot be handled properly.

Any undocumented functions in erl_eval should not be used.