Kleisli-category.js

class Kleisli {
  
  // given f :: x -> Monad y, constructs a category of type:
  // Cat (x ↣ y)
  constructor(f) {
    
    // this.run = f
    this.run = x => f(x)
    
    // this :: Cat (x ↣ y)
    // Cat (y ↣ z) -> Cat (x ↣ z)
    this.pipe = g => new Kleisli(x => this.run(x).then(g.run)) // then == bind
    
    // utility functon:
    // this :: Cat (x ↣ y)
    // (y -> Monad z) -> Cat (x ↣ z)
    this.pipeTo = g => new Kleisli(x => this.run(x).then(g)) // then == bind
  }
}

// Monad => Cat (x ↣ x)
Kleisli.id = monad => new Kleisli(x => monad.resolve(x))