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

[Jim Pryor <lists+caml@jimpryor.net>]

On Wed, Jan 26, 2011 at 05:17:56PM -0500, 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.
> 
> Has this been noted before?

I expect it's probably harmless, because what could you do with the
exception value in question? Since it's leaked outside the scope of its
type declaration, I can't see any (unmagicky) way to
deconstruct it. But as I said, this behavior struck me as odd. I
would more have expected an error.

-- 
Jim Pryor
profjim@jimpryor.net