Re: [Caml-list] GADT existential escape

[Jeremy Yallop <yallop@gmail.com>]

Daniel Bünzli wrote:
>> N.B. As it stands this is slightly less efficient than using open variants, since you'd have to allocate an exn value for checking equality.

Milan Stanojević:
> Exceptions and open variants are actually exactly the same.

More precisely, the exn type is a particular extensible variant with
some additional syntactic sugar.  The exn type doesn't support GADT
constructors (since it doesn't have type parameters), so it can't be
used to implement the typed keys under discussion -- at least, not in
the same way as Core's extensible-variant-based implementation.

The point here, though, is that Daniel's code allocates:

    match R.E W.w with

while the version using open variants does not allocate:

    match A.Key with

Jeremy.