Haskell - for loop

Question

if I want to express something like [just a simple example]:

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

what should I do in Haskell, since it doesn't have variable concept? (maybe wrong, see: Does Haskell have variables?)

Answer 1

There are a few options. First, you can rewrite the problem with naive recursion:

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

Next, you can restate recursion as a fold:

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

Or you can use State to simulate a for loop:

import Control.Monad
import Control.Monad.State

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

Answer 2

Often, repetition that you would perform with a loop in a procedural language is accomplished with recursion in Haskell. In this case, you should think about what the result of the loop is. It appears to alternate between 0 and 1. There are several ways to do this in Haskell. One way is

alternatingList n = take n alternating0and1
alternating0and1 = 0 : alternating1and0
alternating1and0 = 1 : alternating0and1