Monad & Syntax

MonadAndSyntax.scala

object HKind {

  trait Monad[M[_]] {
    def flatMap[T, U](m: M[T], f: T => M[U]): M[U]

    def map[T, U](m: M[T], f: T => U): M[U]
  }

  implicit class MonadSyntax[X, M[_] : Monad](sx: M[X]) {
    def flatMap[U](f: X => M[U]): M[U] = implicitly[Monad[M]].flatMap(sx, f)

    def map[U](f: X => U): M[U] = implicitly[Monad[M]].map(sx, f)
  }

  def map2Monad[A, B, C, M[_] : Monad](sa: M[A], sb: => M[B])(f: (A, B) => C): M[C] = {
    val monad = implicitly[Monad[M]]
    // without syntax
    monad.flatMap(sa, (a: A) => monad.map(sb, (b: B) => f(a, b)))
    // with syntax
    for (a <- sa; b <- sb) yield f(a, b)
  }
}