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From: Ivan Gotovchits <ivg@ieee.org>
To: Gabriel Scherer <gabriel.scherer@gmail.com>
Cc: Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>,  caml-list@inria.fr
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Date: Wed, 28 Nov 2012 14:32:30 +0400
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	(Gabriel Scherer's message of "Wed, 28 Nov 2012 09:12:35 +0100")
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Subject: Re: [Caml-list] phantom types and identity function

Gabriel Scherer <gabriel.scherer@gmail.com> writes:

> The reason for this behavior is that in the first case, t was inferred
> covariant, while the presence of a constraint disables variance
> inference (in the manual: "otherwise (ie. [...] for non-free
> parameters) the variance must be given explicitly)"). You can make the
> constrained version work with an explicit variance annotation:
>   type +'a t = {x : 'a} constraint 'a = [< `A | `B ];;
>   let f x = (x : [ `A ] t :> [ `A | `B ] t);;

Well, now I understand this. Thanks!

> While we're at it: are you sure you need to play with phantom
> polymorphic variant types now that we have GADTs? I have used phantom
> types in the past, but my personal use cases would be profitably
> rewritten using GADTs. They tend to produce hairy error messages, but
> phantom polymorphic variants are not better in this regard.

I wish I could, but I'm bond to Ocaml 3.11 =(

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