2007/10/2, skaller <skaller@users.sourceforge.net>:
>
> On Tue, 2007-10-02 at 20:02 +0200, kirillkh wrote:
> > Replying to a private mail from Brian:
>
> > (* I couldn't figure out, how to declare a polymorphic exception
> > properly *)
> > exception Done of 'a
>
> That's easy -- you can't: even if you could, how could
> you possibly use it?
>
> This compiles fine:
>
> type t = { field : 'a. 'a }
> exception Done of t
>
> but 'field' is useless. This is not at all the same as
>
>         let f (x:'a) (g:'a -> int) =
>         match g x with
>         | 0 -> ..
>         | ..
>
> because *inside* the function, 'a is not a type variable,
> and the code is not polymorphic, it is simply a sole
> unknown type, sometimes said to be monomorphised.
>
> The problem with exceptions is that they're not captured,
> so they cannot be polymorphic. Exceptions SUCK because
> their context is not delimited -- you can throw all the way
> out of the mainline .. :)
>
> [This happens to me regularly and it can takes days to figure
> out what is Not_found ..]


Is there a way to instantiate an exception with a value of unspecified type
and then do explicit casting on catch?

Is it a deficiency in the language? I suppose OCaml could support
polymorphic exceptions, if they were checked, like in Java, and appeared in
function signatures in a similar way to parameters and return values.