CoffeeScript で 二分木構造 と fmap

tree = (l, r) ->
  node = (l, r) ->
    fmap: (f, g) ->
      f l.fmap(f, g), r.fmap(f, g)
  leaf = (a) ->
    fmap: (f, g) -> g a
  if l?.fmap and r?.fmap
    return node l, r
  return leaf l

showTree = (t) ->
  f = (l, r) -> "(#{l}, #{r})"
  g = (a) -> "#{a}"
  return t.fmap f, g

a = tree 1
b = tree 2
c = tree a, b
d = tree 3
e = tree c, d
console.log showTree e

# 出力
((1, 2), 3)