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

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

From: "Florian Douetteau" <Florian.Douetteau@ens.fr>

> > 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.
> 
> To be fully accurate, it only means that the current implementation of the
> code generator would
> not support it directly.
> If subtyping was to be supported, an unboxed float array would be created
> iff
> its the compile-time type "float array" (it would be created boxed if typed
> "Top array")  .

Not so simple: it would also mean that all polymorphic functions on
arrays would create only boxed arrays, since their compile time
type is not float array.
As a result, the representation of float arrays would not be uniform,
requiring a check before access, which would probably neutralize any
performance advantage of having a special representation.

Sure, this is an implementation problem. But not a simple one.
The only simple solution would be to have a special type for float
arrays, just like strings... highly incompatible.

Jacques Garrigue

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