[Caml-list] Identifying the code part of a closure

[Caml-list] Identifying the code part of a closure

[Alain Frisch]

Hello,

for some reasons(*), I need to associate to a runtime closure an integer
that identifies its code pointer. Is the following a "safe" solution
for ocamlc and ocamlopt ("safe" means here that it works in all cases,
with the current implementation, under all the architectures where
ocamlopt exists) ?

let addr (h : 'a -> 'b) : int = fst (Obj.magic h);;

Do you see a better way to achieve this ?


-- Alain


(*) for those who are interested, what I'm trying to do is to implement a
functional language by compiling it to OCaml; the closures of the language
map directly to OCaml closures. It turns out that the language has
explicit typing for functions (for instance:  fun (t1 -> t2) x -> ...) and
the semantics of the language has to access the type information (t1 ->
t2) at runtime; all the closures for a function share this piece of
information, so I'd like to store it during program initialization in a
structure indexed by some integer computed from a sample closure of the
function.

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Re: [Caml-list] Identifying the code part of a closure

[Xavier Leroy]

> for some reasons(*), I need to associate to a runtime closure an integer
> that identifies its code pointer. Is the following a "safe" solution
> for ocamlc and ocamlopt ("safe" means here that it works in all cases,
> with the current implementation, under all the architectures where
> ocamlopt exists) ?
> 
> let addr (h : 'a -> 'b) : int = fst (Obj.magic h);;

It will not work as you expect in the case of curried functions
compiled with ocamlopt:

  let f x y = x + y
  let g x y = x - y

  let _ =
    if addr f = addr g
    then print_string "equal"
    else print_string "different";
    print_newline()

This will print "different" when compiled with ocamlc, and "equal"
when compiled with ocamlopt.  The reason is that ocamlopt represents
closures for curried functions as follows: field 0 contains a pointer
to a generic, shared "un-currying" combinator that handles partial
applications; the real code pointer is stored elsewhere (in field 2).

- Xavier Leroy
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