jobjo

// Functor interface.
type Functor<'F> = { map : ('T -> 'U) -> 'F<'T> -> 'F<'U> }
 
// Monad interface.
type Monad<'M> = {
    return : 'T -> 'M<'T>
    join : 'M<'M<'T>> -> 'M<'T>
}
 
// Monad "library" (operations over monads).

jobjo

(* General functor signature. *)
module type FUNCTOR = sig
  type 'a t
  val map : ('a -> 'b) -> 'a t -> 'b t
end

(* General def of catamorphism module, parameterized by a functor *)
module Cata (F : FUNCTOR) = struct

  type 'a t = 'a F.t

jobjo

(* Functors and modular implicits
 * opam switch 4.02.0+modular-implicits
 * eval `opam config env`
 *)
module type FUNCTOR = sig
  type 'a t
  val map : ('a -> 'b) -> 'a t -> 'b t
end
let map {F : FUNCTOR} = F.map