[英]How do you make this Haskell power function tail recursive?
How would I make this Haskell power function tail recursive? 我如何使这个Haskell幂函数尾递归?
turboPower a 0 = 1
turboPower a b
| even b = turboPower (a*a) (b `div` 2)
| otherwise = a * turboPower a (b-1)
turboPower a b = turboPower' 1 a b
where
turboPower' x a 0 = x
turboPower' x a b
| x `seq` a `seq` b `seq` False = undefined
| even b = turboPower' x (a*a) (b `div` 2)
| otherwise = turboPower' (x*a) a (b-1)
Basically, what you want to do is move the multiplication that you're doing in the " otherwise
" step (since that's what keeps this from being tail-recursive initially) to another parameter. 基本上,你想要做的是在“
otherwise
”步骤中移动你正在进行的乘法(因为这是使得这不能最初是尾递归的)到另一个参数。
Edited to add a line making all three parameters strictly evaluated, instead of lazy, since this is one of those well-known situations where laziness can hurt us. 编辑添加一行,使所有三个参数严格评估,而不是懒惰,因为这是众所周知的懒惰会伤害我们的情况之一。
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