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From: Ivan Gotovchits <ivg@ieee.org>
To: Gabriel Scherer <gabriel.scherer@gmail.com>
Cc: caml-list@inria.fr
References: <87ehciseni.fsf@golf.niidar.ru>
	<CAPFanBEau9FFNJAVF+9RyhfnbuLi+smcW3Hc6bk2BMxM3ysnnA@mail.gmail.com>
Date: Tue, 04 Jun 2013 11:23:23 +0400
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	(Gabriel Scherer's message of "Tue, 4 Jun 2013 07:47:06 +0200")
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Subject: Re: [Caml-list] GADT and locally ADT

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

> If the function knows (or infer) that the argument type is (ro), then
> there is only one case to handle. But there is also a function that is
> polymorphic over the second argument, and it must be callable with
> both (ro) and (rw) values : this function works "for any access mode".
> This is what the annotated function
>
>>      let f (type t)  (inp: ('a,t) access)  = match inp with
>>        | ReadWrite (ch) -> ()
>>        | Read (ch) -> ();;
>
> does.

Looks reasonable, but can you, please, explain the difference between
function:

    let f (type t)  (inp: ('a,t) access)  = match inp with
      | ReadWrite ch -> ()
      | Read ch -> ()

that have type 

    val f : ('a, 'b) access -> unit

and

    let f (inp: ('a,'b) access)  = match inp with
      | ReadWrite ch -> ()
      | Read ch -> ()

that fails to type check:

Error: This pattern matches values of type ('a, ro) access
       but a pattern was expected which matches values of type
         ('a, rw) access