The code you gave has a few errors.

1. As @FramkSchmitt mentioned there is a parameter xs missing.
2. you try to map elem x over a list - which would need a list of lists to be correct.

here is what I would guess what you intended.

members :: Code -> Bool
members xs = and [ x `elem` symbols  | x <- xs ]

which can be written a bit more concise (I believe tools like hlint would even suggest this simplification).

members' :: Code -> Bool
members' = all (`elem` symbols)