Scala higher kinded types

Scala Higher-Kinded Types

In Python, HKTs are simulated because Python's type system lacks them. In Scala, you can express them directly, but sometimes a simple generic class is sufficient depending on the spec.

Functor Example

A TokenFunctor[T] can be a generic class with map operations:

class TokenFunctor[T](val value: T) {
  def map[U](f: T => U): TokenFunctor[U] = new TokenFunctor(f(value))
  def flatMap[U](f: T => TokenFunctor[U]): TokenFunctor[U] = f(value)
  def getOrElse[U >: T](default: => U): U = if (value != null) value else default
  def get: T = value
}

Applicative/Monad Apply

For an Applicative's ap method (in a TokenMonad subclass):

class TokenMonad[T](value: T) extends TokenFunctor[T](value) {
  def ap[U](funcWrapped: TokenMonad[T => U]): TokenMonad[U] = 
    new TokenMonad(funcWrapped.value(this.value))
}
object TokenMonad {
  def pure[T](value: T): TokenMonad[T] = new TokenMonad(value)
}