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 }
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
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}.
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:
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}).
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.
stdlib 1.16.1
Copyright © 1991-2009
Ericsson AB