Expanding `>>=` on functions

expanding.hs

-- start with
(+) >>= id $ 2

-- Expand `>>=` using the Monad instance for functions
--     instance Monad ((->) r) where
--        f >>= k = \ r -> k (f r) r
(\r -> id ((+) r) r) $ 2

-- beta reduction
id ((+) 2) 2

-- expand `id`
((+) 2) 2

2 + 2

4