Scalaz Functor

Functor is a mapping from type F[A] hight kind to F[B]. Scalaz defines Functor[F[_]] trait with map abstract method.

def map[A, B](fa: F[A])(f: A ⇒ B): F[B]

Having a higher kind A and A to B mapping we can get higher kind B.

Having map function we can define lift fucntion that takes A ⇒ B mapping and it lifts it to F[A] ⇒ F[B] mapping.

  def lift[A, B](f: A ⇒ B): F[A] ⇒ F[B] = map(_)(f)

Since map is curried and first parameter list is F[A] and we pass f as second parameter list, we get back F[A] ⇒ F[B] function. Other operations scalaz derives from map function are:

• ∘, apply, map. Those are all aliases for map function. List(1, 2, 3) ∘ (_ + 1) = List(2, 3, 4)
• strengthL. Injects value to the left. List(1,2,3).strengthL("a") = List((a,1), (a,2), (a,3))
• strengthR. Injects value to the right. List(1,2,3).strengthR("a") = List((1,a), (2,a), (3,a))
• fpair. Pairs all values into tupples of two. List(1, 2, 3).fpair = List((1,1), (2,2), (3,3))
• fproduct. Pairs all values into tupples of two where first element is a and second element is f(a). List(1, 2, 3).fproduct(_ + 1) = List((1,2), (2,3), (3,4))
• void. Maps elements to void values. List(1, 2, 3).void = List((), (), ())
• fpoint. Points values into an applicative. List(1, 2, 3).fpoint(scalaz.std.option.optionInstance) = List(Some(1), Some(2), Some(3))
• >| and as. Changes all values to provided value. List(1, 2, 3) >| "a" = List(a, a, a)

Source

import scalaz.std.list._
import scalaz.syntax.functor._

display(List(1, 2, 3) ∘ (_ + 1), """ List(1,2,3) ∘ (_ + 1) """)

display(List(1, 2, 3).strengthL("a"), """ List(1,2,3).strengthL("a") """)
display(List(1, 2, 3).strengthR("a"), """ List(1,2,3).strengthR("a") """)
display(List(1, 2, 3).fpair, """ List(1, 2, 3).fpair """)
display(List(1, 2, 3).fproduct(_ + 1), """ List(1, 2, 3).fproduct(_ + 1) """)
display(List(1, 2, 3).void, """ List(1, 2, 3).void """)
display(
  List(1, 2, 3).fpoint(scalaz.std.option.optionInstance),
  """ List(1, 2, 3).fpoint(scalaz.std.option.optionInstance) """)
display(List(1, 2, 3) >| "a", """ List(1, 2, 3) >| "a" """)

Output

List(1,2,3) ∘ (_ + 1)                                      List(2, 3, 4)
List(1,2,3).strengthL("a")                                 List((a,1), (a,2), (a,3))
List(1,2,3).strengthR("a")                                 List((1,a), (2,a), (3,a))
List(1, 2, 3).fpair                                        List((1,1), (2,2), (3,3))
List(1, 2, 3).fproduct(_ + 1)                              List((1,2), (2,3), (3,4))
List(1, 2, 3).void                                         List((), (), ())
List(1, 2, 3).fpoint(scalaz.std.option.optionInstance)     List(Some(1), Some(2), Some(3))
List(1, 2, 3) >| "a"                                       List(a, a, a)