gistfile1.scala

trait Functor[F[_]] {
  def map[T, U](function: T => U)(functor: F[T]): F[U]
}

trait Applicative[F[_]] extends Functor[F] {
  def pure[T](value: T): F[T]

  def seq[T, U](function: F[T => U])(functor: F[T]): F[U]

  override def map[T, U](function: T => U)(functor: F[T]): F[U] =
    seq(pure(function))(functor)
}

trait Monad[F[_]] extends Applicative[F] {
  def bind[T, U](functor: F[T])(function: T => F[U]): F[U]

  override def seq[T, U](function: F[T => U])(functor: F[T]): F[U] =
    bind(function) { bfunction => bind(functor) { bfunctor => pure(bfunction(bfunctor)) } }
}

object ListInstance extends Monad[List] {
  // only have to implement pure and bind; map and seq are derived
  override def pure[T](value: T) = List(value)

  override def bind[T, U](functor: List[T])(function: T => List[U]) =
    functor flatMap function
}