From: David Monniaux <David.Monniaux@ens.fr>
To: Liste CAML <caml-list@inria.fr>
Subject: [Caml-list] restricting polymorphic variants
Date: Thu, 31 Jan 2002 07:38:51 +0100 (MET) [thread overview]
Message-ID: <Pine.GSO.4.03.10201310727520.15113-100000@basilic.ens.fr> (raw)
Let us say I have a module M as follows:
module M = struct type op=[`A|`B] let f: op->int=function `A->0|`B->1 end;
I cannot just sat that f has type [< `A|`B]->int because M has to fit
within a signature:
module type MT=sig type op val f: op->int end;;
I would like to obtain another module where f only accepts `A:
# module N = struct type op=[`A] let f: op->int=M.f end;;
^^^
This expression has type M.op -> int but is here used with type op -> int
Let us try eta-expansion:
# module N = struct type op=[`A] let f: op->int=fun x->M.f x end;;
^
This expression has type op = [ `A] but is here used with type
M.op = [ `A | `B]
Well, let us try enumerating all the constructors in the closed type op!
# module N = struct type op=[`A] let f: op->int=function (`A as x)->M.f x
end;;
module N : sig type op = [ `A] val f : op -> int end
Ok, it now works, but it is a bit clumsy: the typer knows all the
constructors of type N.op, which all fit into type M.op, why should I have
to enumerate them again?
This would not be so bad if this code was optimized away (after all, this
is a tail call with the same argument). Is it the case?
David Monniaux http://www.di.ens.fr/~monniaux
Laboratoire d'informatique de l'École Normale Supérieure,
Paris, France
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next reply other threads:[~2002-01-31 6:38 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-01-31 6:38 David Monniaux [this message]
2002-01-31 9:19 ` Laurent Vibert
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