{-# LANGUAGE BangPatterns, MagicHash #-}
module M (len) where

import GHC.Exts

data Tree a = Tip | Bin a !(Tree a) !(Tree a)

foldrTree :: (a -> b -> b) -> b -> Tree a -> b
foldrTree f z0 t = go t z0
  where
    go Tip z = z
    go (Bin x l r) z = go l (f x (go r z))

len :: Tree a -> Int
len t = foldrTree (\_ k -> oneShot (\ !acc -> k (acc + 1))) id t 0

-- lenAlt :: Tree a -> Int
-- lenAlt t = foldrTree (\_ k -> oneShot (\acc# -> k (acc# +# 1#))) I# t 0#