playing around with trees to solidify working with instance declarations

trees.hs

data Tree a = Branch (Tree a) (Tree a) | Leaf a deriving Show

instance Functor Tree where
  fmap fn (Branch l r) = Branch (fmap fn l) (fmap fn r)
  fmap fn (Leaf a)     = Leaf $ fn a

instance Applicative Tree where
  (<*>) (Leaf fn) t = fmap fn t
  (<*>) (Branch l r) t = Branch (l <*> t) (r <*> t)
  pure = Leaf

instance Monad Tree where
  (>>=) (Branch l r) fn = Branch (l >>= fn) (r >>= fn)
  (>>=) (Leaf a) fn     = fn a
  return = Leaf