Take elements on even positions from the list

Question

Problem: using fold, take from the list elements which are on the even positions:

GHCi> evenOnly [1..10]
 [2,4,6,8,10]
GHCi> evenOnly [\'a\'..         


        

Answer 1

(.) is function composition but zip g xs is a list not a function. You can just apply the resulting list as the argument to foldr directly. Note you have the arguments g and xs in the wrong order:

evenOnly :: [a] -> [a]
evenOnly xs = let g = iterate (\x -> (x + 1) `mod` 2) 0
  in
  foldr (\ (x, n) s -> if n == 1 then x : s else s) [] (zip xs g)