OCaml functor taking polymorphic variant type -

OCaml functor taking polymorphic variant type -

trying compile

module f (m : sig type t = [> `foo ] end) = struct type t = [ m.t | `bar ] end

gets me

error: type variable unbound in type declaration. in type [> `foo ] 'a variable 'a unbound

what doing wrong?

type t = [> `foo] invalid since [> `foo] open type , contains type variable implicitly. definition rejected next type definition rejected since rhs has type variable not quantified in lhs:

type t = 'a list

you have create closed:

type t = [ `foo ]

or quantify type variable:

type 'a t = [> `foo] 'a

which equivalent to

type 'a t = 'a constraint 'a = [> `foo]

ocaml