Question

I want something like

splitBy pred list = ( filter pred list, filter (not . pred) list )

but in one pass.

Answer 1

You are looking for the partition function from Data.List:

partition :: (a -> Bool) -> [a] -> ([a], [a])

It can be implemented nicely using a fold:

splitBy pred = foldr f ([], []) 
    where f x ~(yes, no) = if pred x then (x : yes, no) 
                                    else (yes, x : no)