http://stackoverflow.com/questions/18728448/why-does-this-list-comprehension-fail

nub_as_filter.hs

s = filter (`notElem`s) x   -- NOT

s = foldr (\(n,a) r-> if (a `notElem` take n s) then a:r else r) [] $ zip [0..] x  -- not quite, yet

s = let a = [ [e | e `notElem` take n s] | (n,e) <- zip c x]   -- here it is!
        b = map length a     --       \
        c = scanl (+) 0 b    --        \
        s = concat a         -- _this_ `s` 
    in s

-- or, as a function, 

filterNub xs = s
  where
    s = concat a
    a = [ [e | e `notElem` take n s] | (n,e) <- zip c xs]
    b = map length a
    c = scanl (+) 0 b

-- which is

filterNub xs = s 
  where 
    s = concat . fix $ map (\(e,n)-> [e | e `notElem` take n s]) . zip xs . scanl ((.length).(+)) 0 
    s = concat . fix $ zipWith (\e n-> [e | e `notElem` take n s]) xs . scanl ((.length).(+)) 0 
    s = concatMap fst . fix $ (([],0):) . zipWith (\e (_,n)-> 
                                            let r=[e | e `notElem` take n s] in (r,n+length r)) xs