Parametric_polymorphism.fold

--
-- Parametric polymorphism
-- http://rosettacode.org/wiki/Parametric_polymorphism#Metaphor
--


type<A> Tree A = Empty
               | Node (Tree A, A, Tree A)

map :: (A -> B) -> Tree A -> Tree B
map f = Empty => Empty
      | Node (l, x, r) => Node (map f l, f x, map f r)


-- // --

type<A> Tree A = {
    Empty
    Node (Tree A, A, Tree A)
}

map f = {
    Empty => Empty
    Node (l, x, r) => Node (map f l, f x, map f r)
}