This style of coding in which we define a function without reference to its arguments — in some sense saying what a function *is* rather than what it *does* — is known as “point-free” style.

point_free.hs

-- starting point
fun1 :: [Integer] -> Integer
fun1 []    = 1
fun1 (x:xs)
  | even x    = (x - 2) * fun1 xs
  | otherwise = fun1 xs

-- incorporating existing recursion patterns
fun1' :: [Integer] -> Integer
fun1' = foldr (\x acc -> (if even x then x - 2 else 1) * acc) 1

-- much better
fun1'' :: [Integer] -> Integer
fun1'' = product . map (subtract 2) . filter even