Haskell Functors for the programmer

Functor is a type class used for types that can be mapped over.

• Containers: [], Trees, Map, HashMap...
• "Feature Type":
  • Maybe a: help to handle absence of a.
    Ex: safeDiv x 0 ⇒ Nothing
  • Either String a: help to handle errors
    Ex: reportDiv x 0 ⇒ Left "Division by 0!"