Re: [Caml-list] Obj.magic, Obj.t etc.

[Jacques Garrigue <garrigue@kurims.kyoto-u.ac.jp>]

From: Christopher Dutchyn <cdutchyn@cs.ubc.ca>

> In a few instances I've encountered code with these special types.
> The former appears to be a way to "hint" the typechecker with
> additional information, and the latter seems to be some sort of
> catchall.
> 
> Where are these documented?  Are there any others?

Related functions are all in the Obj module.
They are not documented as ML function because almost all of them can
break type safety when used incorrectly.

On the other hand, they are just equivalent to C macros described in
the "interfacing C" part of the manual, so you can consider this as
documentation.

As an interesting aside, I've discovered recently that even Obj.repr
is dangerous:

# Obj.repr;;
- : 'a -> Obj.t = <fun>
# let arr = Array.create 1 (Obj.repr 1.0);;
val arr : Obj.t array = [|<abstr>|]
# arr.(0) <- Obj.repr 1;;
Segmentation fault

>From a subtyping point of view, this basically means that there cannot
be any "top" element in the ocaml type system.

Jacques Garrigue

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