loops - Haskell - for 循环

Question

如果我想表达[只是一个简单的例子]之类的东西:

int a = 0;
for (int x = 0; x < n; x += 1)
    a = 1 - a;

我应该在 Haskell 中做什么，因为它没有变量概念？ (也许错了，见: Does Haskell have variables?)

Answer 1

有几个选项。首先，您可以用朴素递归重写问题:

loop :: Int -> Int
loop n = loop' n 0
  where loop' 0 a = a
        loop' n a = loop' (n - 1) (1 - a)

接下来，您可以将递归重述为折叠:

loop :: Int -> Int
loop n = foldr (\a _ -> 1 - a) 0 [0..n]

或者您可以使用 State模拟 for 循环:

import Control.Monad
import Control.Monad.State

loop :: Int -> Int
loop n = execState (forM_ [0..n] 
                      (\_ -> modify (\a -> 1 - a))) 0