Fun with Functors

Karl Bielefeldt

Lists

• val in  = List(1, 2, 3)
• val out = List("1", "2", "3")
• in map {_.toString}

Options

• val in: Option[Int] = None
• val out = None
• val in  = Some(1)
• val out = Some("1")
• in map {_.toString}

Eithers

• val in: Either[Exception,Int] = Left(new Exception())
• val out = Left(new Exception())
• val in  = Right(1)
• val out = Right("1")
• in map {_.toString}

Functions

• val in = (x: Int) => x + 1
• val out =
  (x: Int) => (x + 1).toString
• in map {_.toString}

Referring to them together?

Mappable

Mappable

Functor

Tree[A]

• def map[B](f: (A) => B): Tree[B] = ???
• def map[B](f: (A) => B): Tree[B] = Tree.empty[B]

Functor Laws

• in map identity == in
• (f compose g).map ==
f.map compose g.map

What are they good for?

• val in = Future.successful(List(Some(1), None, Some(3)))
• val futListOpt = Functor[Future] compose Functor[List] compose Functor[Option]
• futListOpt.map(in)(_.toString)

Thank you