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From: Leo White <lpw25@cam.ac.uk>
To: Erkki Seppala <flux@modeemi.cs.tut.fi>
Cc: caml-list@yquem.inria.fr
References: <51A38A23.9000402@gmail.com> <m49txlnpq48.fsf@coffee.modeemi.fi>
Date: Tue, 28 May 2013 09:20:04 +0100
In-Reply-To: <m49txlnpq48.fsf@coffee.modeemi.fi> (Erkki Seppala's message of
	"Tue, 28 May 2013 09:36:55 +0300")
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Subject: Re: [Caml-list] Recursive fixed polymorphic variants

> let rec f = function `B x -> g x | `A -> ()
> and g = function `B x -> f x | `A -> ();;
>
> # f (M.b (M.a ()))
> - : unit = ()
>
> This gives f (and g) the inferred type ([< `A | `B of [< `A | `B of 'a
> ] ] as 'a) -> unit.
>
> I was under the impression I should be able to describe using the type
> variable syntax in function type, but I was unable to.

Note that you can use polymorphic recursion to give f a very similar
type that also works for this particular example:

  # let rec f : 'a. ([< `A | `B of ([< `A | `B of 'b ] as 'b) ] as 'a) -> unit = 
      function `B x -> f x | `A -> ();;
    val f : [< `A | `B of [< `A | `B of 'a ] as 'a ] -> unit = <fun>
  # f (M.b (M.a ()));;
  - : unit = ()

Regards,

Leo