Got it. I missed that somehow until now. Indeed using eta expansion to
enforce introducing variable-like constructors works well, and looks like a
very reasonable work around for the cases I have in mind.
Thank you very much for the relevant links and the quick answers.


2013/7/26 Alain Frisch <alain@frisch.fr>

> 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<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<http://caml.inria.fr/mantis/view.php?id=5713>
>
> -- Alain
>