I am getting a list of lists from chop9. I want to somehow filter each element in the list. How do I do that. I have encoded binary data eg [1,1,0,0,1,0] etc Each 9th bit is a parity bit which is checked using this function: type Bit = Int paritychecker :: [Bit] -> Bool paritychecker xs | length xs == 9 && ((sum (init xs)) `mod` 2) == (last xs) = True | otherwise = False In the stream (the list) I use chop to retrieve each block of 9 bits as in: chop9 :: [Bit] -> [[Bit]] chop9 [] = [] chop9 bits = take 8 bits : chop9 (drop 8 bits) I have been playing with this sort of thing: filter paritychecker ??? chop9 [1,0,0,0,1,1,1,0,0,...] But doesn't work. I want to end up with all the 9 bit chunks which pass the filter. So my end type should be [[Bit]] paritychecker requires a list - eg [Bit] . So I want to run paritychecker on each element returned from chop9. How do I do that? Example encoded data stream: [1,0,0,0,1,1,1,0,0,1,1,1,0,1,1,1,0,0,1,0,1,0,0,1,1,0,0,0,1,0,0,1,1,1,0,0,0,0,1,0,1,1,1,0,0,1,0,0,1,1,1,1,0,1] Angus