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From: Jacques Garrigue <jacques.garrigue@gmail.com>
To: caml-list@inria.fr
Subject: Re: [Caml-list] phantom types and coercion
Date: Mon, 28 Oct 2019 16:10:52 +0100
Message-ID: <e130aea1-9aac-47ca-3f06-65b80e6f63fa@gmail.com> (raw)
In-Reply-To: <b070e40a-3ff9-4ede-8c95-6219fc76f723@www.fastmail.com>

On 28/10/2019 15:27, rixed@happyleptic.org wrote:
> Hello list.
>
> I'm trying to use several parameters in a phantom type ; everything works alright, but coercion.
> For instance, in the code below, how come s1 can be coerced into a string by s2 cannot ?
This is due to the presence of a free type variable in the type of s2.
In that case, a single coercion will just generate a generic supertype, 
and attempt to unify the type of s2 with it.
Your best bet is to write a double coercion.

    (s2 : _ M.t :> string)

Jacques Garrigue
> # module M : sig type +'a t = private string val m1 : string -> [`C1] t val m2 : string -> ([`C2] * 'a) t end = struct type 'a t = string let m1 s = s let m2 s = s end;;   module M :
>    sig
>      type +'a t = private string
>      val m1 : string -> [ `C1 ] t
>      val m2 : string -> ([ `C2 ] * 'a) t
>    end
> # let s1 = M.m1 "foo";;
> val s1 : [ `C1 ] M.t = "foo"
> # let s2 = M.m2 "bar";;
> val s2 : ([ `C2 ] * 'a) M.t = "bar"
> # print_string (s1 :> string);;
> foo- : unit = ()
> # print_string (s2 :> string);;
> Error: This expression cannot be coerced to type string; it has type
>           ([ `C2 ] * 'a) M.t
>         but is here used with type string
>


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Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-28 14:28 rixed
2019-10-28 15:11 ` Jacques Garrigue [this message]
2019-10-28 16:45   ` rixed

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