Identity.re

type identity('a) = Identity('a);
module Functor_class = {
  module type F = {
    type t(_);
    let map: ('a => 'b, t('a)) => t('b);
  };
};

module Applicative_class = {
  include Functor_class;
  module type A = {
    type t(_);
    let pure: 'a => t('a);
    let ap: (t('a => 'b), t('a)) => t('b);
  }
}

module Monad_class = {
  include Applicative_class;
  module type M = {
    type t(_);
    let return: 'a => t('a);
    let bind: ('a => t('b), t('a)) => t('b);
  }
}

module Identity_functor: Functor_class.F with type t('a) = identity('a) = {
  type t('a) = identity('a);
  let map = (f) =>
    fun | Identity(a) => Identity(f(a));
}

module Identity_applicative: Applicative_class.A with type t('a) = identity('a) = {
  type t('a) = identity('a);
  let pure = a => Identity(a);
  let ap = (fab, fa) => 
    switch(fab, fa) {
      | (Identity(f), Identity(g)) => Identity(f(g));
    }
};

module Identity_monad: Monad_class.M with type t('a) = identity('a) = {
  type t('a) = identity('a);
  let return = a => Identity(a);
  let bind = f => 
    fun | Identity(a) => f(a);
};

let functorResult = Identity(3) |> Identity_functor.map(n => n + 2);
let applicativeResult = Identity(3) |> Identity_applicative.ap(Identity(n => n + 2));
let monadResult = Identity(3) |> Identity_monad.bind(n => Identity(n + 2))

Js.log(functorResult); /* [ 5 ] */
Js.log(applicativeResult);
Js.log(monadResult);