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

One ought not to calculate anything that doesn't need calculating. The choice is positional, it is already known in advance. Calculating the modulos, comparing with Booleans, is all superfluous work.

Instead, do this, then do that, and go on switching like that; using foldr, as asked:

evenly :: [t] -> [t]
evenly xs = foldr c z xs f g
   where
   c x r f g = f x (r g f)

Next we finish up the definitions, according to how each is used:

   z _ _  = []
   f _ xs = xs
   g x xs = x : xs