CODEWITHSUNDEEP
scala
mariop
mariop

Reputation: 3216

Applicative Functor instance for functions with the same result type in Scala

I'm trying to implement the functor and applicative instances for functions with the same initial type in Scala. My current implementation of the two type classes is:

trait Functor[F[_]] {
  def fmap[A,B](f: A => B): F[A] => F[B]
}

trait Applicative[F[_]] extends Functor[F] {
  def pure[A](a: A): F[A]
  def apply[A,B](f: F[A => B]): F[A] => F[B]
}

The problem that I have is that Function1 has two type parameters and I think I cannot fix only one of them. Is it possible to do this in Scala?

Upvotes: 1

Views: 89

Answers (1)

Chirlo
Chirlo

Reputation: 6132

One way to do it is with a type alias: 

def FunctionFunctor[X] = {
  type F[Y] = Function[X,Y]

  new Functor[F] {
     def fmap[A,B](f: A => B)  = (fa: F[A]) => f.compose(fa)
  }
}

Notice the def, so you're creating a new functor instance for each specific X type.

Other way to do is with type lambdas, which have a funny syntax:

 def FunctionFunctor[X] = new Functor[({type F[Y] = Function[X,Y]})#F] {
  def fmap[A,B](f: A => B)  = (fa: Function[X,A]) => f.compose(fa)
 }

Try and read about type lambdas, can be an obscure concept if you're not familiar with it. Good read about it is Scala in Depth

Upvotes: 3

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