RE: [Caml-list] Magic with fun (type t) ... ?

[David Allsopp <dra-news@metastack.com>]

Jim Pryor wrote:
> I expect this is bad coding style, and should not be relied on. However,
> I was surprised at the behavior, and wondered whether it was intended:
> 
> # let f = fun (type t) x ->
>     let module M = struct exception E of t end in
>     M.E x;;
> val f : 'a -> exn = <fun>
> 
> f is polymorphic, as we expect:
> 
> # f 1;;
> - : exn = E 1
> # f "s";;
> - : exn = E "s"
> 
> But now notice:
> 
> # f ();;
> - : exn = E 0
> # f None;;
> - : exn = E 0
> 
> It appears that non-heap values are always getting magicked into ints.

I think there are other situations (I can't remember how you can force them) where the toploop displays the "wrong" things. As I understand it, the compiler/interpreter really has to jump through hoops sometimes to display the correct version of a value because the value representation throws away so much (virtually) of the typing information.

> Has this been noted before?

However, this is weird since () and the constructor None are not equal in OCaml-land (they happen to have equivalent representation, but that's not the same thing):

# f () = f None;;
- : bool = true


David