OCaml functor taking polymorphic variant type

type t = [> `Foo] is invalid since [> `Foo] is an open type and contains a type variable implicitly. The definition is rejected just as the following type definition is rejected since the RHS has a type variable which is not quantified in LHS:

type t = 'a list

You have to make it closed:

type t = [ `Foo ]

or quantify the type variable:

type 'a t = [> `Foo] as 'a

which is equivalent to

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