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From: =?UTF-8?Q?Milan_Stanojevi=C4=87?= <milanst@gmail.com>
Date: Wed, 18 Mar 2015 21:27:09 -0400
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To: =?UTF-8?Q?Daniel_B=C3=BCnzli?= <daniel.buenzli@erratique.ch>
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Subject: Re: [Caml-list] GADT existential escape

This is doable in OCaml.
Core has one possible implementation.
https://github.com/janestreet/core_kernel/blob/master/lib/type_equal.mli#L1=
71

type 'a key =3D 'a Type_equal.Id.t

let lookup dict key =3D
   List.find_map disct ~f:(function
    | B (key', a) ->
      match Type_equal.Id.same_witness key key' with
      | None -> None
      | Some T -> Some a)
;;

You can also check out Univ and Univ_map modules in Core.

Note that Univ was possible in OCaml even before first-class
modules, GADTs and extensible types (which are all used in Type_equal)

https://blogs.janestreet.com/a-universal-type/


On Wed, Mar 18, 2015 at 9:07 PM, Daniel B=C3=BCnzli
<daniel.buenzli@erratique.ch> wrote:
> Le jeudi, 19 mars 2015 =C3=A0 01:39, Milan Stanojevi=C4=87 a =C3=A9crit :
>> Can you give some hypothetical example code ?
> type 'a key =3D ?
> type binding =3D B : 'a key * 'a -> binding
> type dict =3D binding list
> let lookup : dict -> 'a key -> 'a option =3D fun d k -> ?
>
> Is it possible to specify a `'a key` type that allows `lookup` (with the =
obvious semantics) to be written ? This is trivial to achieve with a univer=
sal type, see http://mlton.org/PropertyList
>
> Best,
>
> Daniel
>
>