extract :: [a] -> a
extract [] = error("Too few objects")
extract (x:xs) = x
a = extract [1,2,3,4,5]

addto :: (Num a) => [a] -> a -> [a]
addto [] _ = [] -- edge case (list empty)
addto (x:xs) y = (x+y) : addto xs y
b = [1..4]
-- c == [5,6,7,8]
c = addto b 4