Re: [Caml-list] Calling a single function on every member of a GADT?

[Jacques Garrigue <jacques.garrigue@gmail.com>]

Actually, this is a rare case where using a polymorphic method may be 
handy too:

let rec fold : type a r. r -> <v : 'b. r -> 'b term -> r> -> a term -> r =
     fun i f -> function
     | Int _ as t -> f#v i t
     | Add -> f#v i Add
     | App (x, y) as t -> f#v (fold (fold i f x) f y) t

let v =
   object method v : type a. _ -> a Gadt.term -> _ =
     fun x -> function
       | Int n -> x+n
       | Add -> x+1
       | App _ -> x+2
   end

let r = Gadt.fold 0 v (App (App (Add, Int 3), Int 5))

The point being that to match on a Gadt you will anyway need to use the 
(type a) construct to allow refinement.

Jacques

On 08/01/2020 07:54, rixed@happyleptic.org wrote:
> Hello and thank you for the answer.
>
> On Tue, Jan 7, 2020, at 21:21, Ivan Gotovchits wrote:
>> It is the limitation of the let-bound polymorphism. (...)
>> In your case, I would define a visitor type, e.g.,
>>   type 'r visitor = {visit : 'a. 'a term -> 'r -> 'r}
> Oh I see. I've used this trick to force a function to be polymorphic, but I failed to see that this was the problem because to me `f` is not any more polymorphic when the `term` is a GADT than when it's not.
>
> So there is no lighter syntax to specify that `f` should accept any member of a GADT than the syntax to specify that `f` should accept any type at all?
>
> I wonder, is this just a limitation of the OCaml parser or is there some deep reason for these work-around (like is the case, from my understanding, for the value restriction)?
>
> Sorry for asking such simple questions but I've build a mental model of how OCaml type checking works mostly from trials and errors, which used to work great until some versions ago :)