再帰ドリル http://d.hatena.ne.jp/kazu-yamamoto/20130330/1364610328 http://d.hatena.ne.jp/kazu-yamamoto/20130331

recursion.hs

plus :: Int -> Int -> Int
plus m 0 = m
plus m n
  | n > 0  = plus m (n - 1) + 1
  | n < 0  = plus m (n + 1) - 1

minus :: Int -> Int -> Int
minus m 0 = m
minus m n
  | n > 0 = minus m (n - 1) - 1
  | n < 0 = minus m (n + 1) + 1

power :: Float -> Float -> Float
power m 0.0 = 1
power m 1.0 = m
power m n
  | n > 0.0 = power m (n - 1) * m
  | n < 0.0 = power m (n + 1) / m

-- tail call optimization

plus' :: Int -> Int -> Int
plus' m n = plus'' m n m

plus'' :: Int -> Int -> Int -> Int
plus'' m 0 acc = acc
plus'' m n acc
  | n > 0 = plus'' m (n - 1) (acc + 1)
  | n < 0 = plus'' m (n + 1) (acc - 1)

minus' :: Int -> Int -> Int
minus' m n = minus'' m n m

minus'' :: Int -> Int -> Int -> Int
minus'' m 0 acc = acc
minus'' m n acc
  | n > 0 = minus'' m (n - 1) (acc - 1)
  | n < 0 = minus'' m (n + 1) (acc + 1)

power' :: Float -> Float -> Float
power' m n
  | n > 0.0  = power'' m n m
  | n <= 0.0 = power'' m n 1.0

power'' :: Float -> Float -> Float -> Float
power'' m 0.0 acc = acc
power'' m n acc
  | n > 0.0 = power'' m (n - 1.0) (acc * m)
  | n < 0.0 = power'' m (n + 1.0) (acc / m)