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Date: Thu, 19 Mar 2015 17:41:55 +0000
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From: Jeremy Yallop <yallop@gmail.com>
To: =?UTF-8?Q?Milan_Stanojevi=C4=87?= <milanst@gmail.com>
Cc: =?UTF-8?Q?Daniel_B=C3=BCnzli?= <daniel.buenzli@erratique.ch>, 
	Jeremie Dimino <jdimino@janestreet.com>, Caml-list <caml-list@inria.fr>
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Subject: Re: [Caml-list] GADT existential escape

Daniel B=C3=BCnzli wrote:
>> N.B. As it stands this is slightly less efficient than using open varian=
ts, since you'd have to allocate an exn value for checking equality.

Milan Stanojevi=C4=87:
> 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.