Implement the functions using map and foldr, haskell

Question

I have two functions. The first one gives true if all elements of the list are zero

allZero :: [Int] -> Bool
allZero [] = False
allZero [0] = True
allZero (x:xs)
  | x == 0 && allZero xs = True
  |otherwise = False

The second function gives true if at least one element of the list is zero

oneZero :: [Int] -> Bool
oneZero [] = False
oneZero (x:xs)
   | x == 0 = True
   | otherwise = oneZero xs

Maybe there is another way to solve this problems. For example with map or foldr? Thank you

question from:https://stackoverflow.com/questions/65645698/implement-the-functions-using-map-and-foldr-haskell

Answer 1

foldr function works so:

Suppose, you have list [1, 2, 3]. Let's write this list as (:) 1 ((:) 2 ((:) 3 [])), where each element has type a. Function foldr takes function f of a -> b -> b type and starting element z of b type, and just replace [] to z and : to f. So, foldr f z ((:) 1 ((:) 2 ((:) 3 []))) == f 1 (f 2 (f 3 z)).

So, you can define your functions so:

allZero = foldr (x -> x == 0 &&) True

oneZero = foldr (x -> x == 0 ||) False