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Date: Fri, 26 Jul 2013 23:25:11 +0200
From: Alain Frisch <alain@frisch.fr>
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To: Gabriel Scherer <gabriel.scherer@gmail.com>
CC: Mathieu Barbin <mathieu.barbin@gmail.com>, caml-list@inria.fr
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Subject: Re: [Caml-list] GADT: question about inference

On 7/26/2013 10:27 PM, Gabriel Scherer wrote:
> This is explained in the manual (emphasis mine)
>    http://caml.inria.fr/pub/docs/manual-ocaml/manual021.html#toc85
>
>> The constraints associated to each constructor can be recovered through pattern-matching.
>> Namely, **if the type of the scrutinee of a pattern-matching contains a locally abstract type**,
>> this type can be refined according to the constructor used.
>
> The only types that can be refined by the type equalities introduced
> by GADT are locally abstract types, the variable-like constructor "a"
> introduced by the "(type a)" and "foo : type a . bar" syntaxes. Your
> first two examples have no locally abstract type, only type members of
> modules, so there was no GADT refinement happening.

A feature request related to allowing more kinds of types to be refined 
by GADT patterns (abstract types in the current environment, introduced 
by unpacking a module -- but the same would be useful for abstract types 
in a functor's argument):

http://caml.inria.fr/mantis/view.php?id=5713

-- Alain