Re: [Caml-list] getting the type of a polymorphic data ?

[Damien Doligez <damien.doligez@inria.fr>]

On Aug 12, 2004, at 14:09, Diego Olivier Fernandez Pons wrote:

> It seems that Obj.is_int does exactly that (determining if a type is
> isomorphic to a subset of integers).

No, not exactly:

         Objective Caml version 3.09+dev0 (2004-07-13)

# type x = A | B of int;;
type x = A | B of int
# Obj.is_int (Obj.repr A);;
- : bool = true
# Obj.is_int (Obj.repr (B 1));;
- : bool = false

All Obj.is_int tells you about the _type_ of its argument is that,
if it returns true then at least one of the type's values is
represented as an integer.


> Is it reasonable to admit that if Obj.is_int is true for x and y then
> Hashtbl.hash is injective ?
>
> hash x = hash y => x = y (for structural polymorphic equality)

Not quite.

# Hashtbl.hash 0x4000_0000;;
- : int = 0
# Hashtbl.hash 0;;
- : int = 0

-- Damien

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