poly.ml

(****** works ******)
type foo = [`A of int | `B of int]
module type K = sig
                  type t
                  val value: t -> int
                end
module J (K: K) = struct
  let value = K.value
end
module FooJ = J(struct
                  type t = foo
                  let value = function | `A r -> r | `B r -> r
                end)


(****** doesn't work ******)
type aa = {value: int}
type bb = {value: int}
type foo = [`A of aa | `B of bb]
module type K = sig
                  type t
                  val value: t -> int
                end
module J (K: K) = struct
  let value = K.value
end
module FooJ = J(struct
                  type t = foo
                  let value = function | `A r -> r.value | `B r -> r.value
                end)
                

(*
Error: Signature mismatch:
       Modules do not match:
         sig type t = foo val value : [< `A of bb | `B of bb ] -> int end
       is not included in
         K
       Values do not match:
         val value : [< `A of bb | `B of bb ] -> int
       is not included in
         val value : t -> int
*)