[Caml-list] phantom types and identity function

[Ivan Gotovchits <ivg@ieee.org>]

Hello,

These simple signature 

   module type T = sig
       type 'a t constraint 'a = [< `A | `B ]
       val init: [`A] t
       val f: [`A] t -> [`B] t
   end

can be used to constrain the following module

   module T : T = struct
       type 'a t = unit constraint 'a = [< `A | `B]
       let init = ()
       let f x = x
   end

where identity function successfully satisfies the constraint

   [`A] t -> [`B] t

but in the following module 

   module T : T = struct
       type 'a t = {x:int} constraint 'a = [< `A | `B]
       let init = {x=0}
       let f x = x
   end

the same identity function doesn't satisfy.

I'll be genuinelly happy, if somebody clarify this issue for me :) My
guess is that the problem lies in the field of higher order polymorpism,
and instead the identity function with signature 'a -> 'a, a more
polymorphic function 'a -> 'b should be used. Though this contradicts
with the fact that the identity function works fine in the first
example and brings up another problem, which is how to obtain such
polymorphic function...

Thanks in advance!

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