open number

_ ^ 0           =  1
x ^ n | n > 0   =  f x (n - 1) x
      |else = fail "Negative exponent"
  where f _ 0 y = y
        f a d y = g a d
          where g b i | even i  = g (b * b) (i `quot` 2)
                      | else = f b (i - 1) (b * y)

(12 ^ 4, 12 ** 4)