Reader, I guess

Reader.re

let const = (a, _) => a;
let (<<) = (f, g, a) => f(g(a));

module type Type = {type t;};

module Function1 = {
  type t('input, 'output) = 'input => 'output;
};

module type Functor = {
  type t('a);
  let map: ('a => 'b, t('a)) => t('b);
};

module type Apply = {
  include Functor;
  let liftA2: (('a, 'b) => 'c, t('a), t('b)) => t('c);
};

module type Applicative = {
  include Apply;
  let pure: 'a => t('a);
};

module type Monad = {
  include Applicative;
  let flatMap: ('a => t('b), t('a)) => t('b);
};

module Reader = (Input: Type) : Monad => {
  type t('a) = Function1.t(Input.t, 'a);

  let map = (<<);

  let liftA2 = (f, a, b, input) => f(a(input), b(input));

  let pure = const;

  let flatMap = (f, a, input) => f(a(input), input);
};