hi everyone, i'm a complete beginner on programming and i've been solving the 99 haskell problems and i've come into a situation. just like all my problems with haskell, it's about types. i wrote a little thing that takes a list and puts all the equal characters together, like [1,1,3,2,3] will be [ [1,1], [3,3], [2] ]. here it is: pack xs = pack' xs where pack' :: [a] -> [[a]] pack' [] = [] pack' (x:y) = extra x xs ++ pack' y extra x xs = [sum' (map (remover x) (map ((==) x) xs))] remover :: a -> Bool -> [a] remover y x = if x then [y] else [] sum' :: [[a]] -> [a] sum' [] = [] sum' (x:xs) = x ++ sum' xs i know, i know, this code is probably terrible and i'm sure there are more clever ways to do this...but i wanna understand what's wrong. the "extra" function works perfectly, it takes a variable and then looks at how many times it's presented on a list and outputs a list with a list of that variable the amount of times that it was presented on the original list. however, pack does not work quite right. first of all, it'll use extra on repeated characters, creating repeated lists, but that doesn't matter because i have a function that fixes that issue and i'll use it after i figure out what's wrong with pack. now, the real problem is with types. here's what haskell says when i try to load it: noventa.hs:77:32: Couldn't match type `a' with `a1' `a' is a rigid type variable bound by the inferred type of pack :: [a] -> [[a]] at noventa.hs:73:1 `a1' is a rigid type variable bound by the type signature for pack' :: [a1] -> [[a1]] at noventa.hs:75:19 Expected type: [a1] Actual type: [a] In the second argument of `extra', namely `xs' In the first argument of `(++)', namely `extra x xs' In the expression: extra x xs ++ pack' y Failed, modules loaded: none. i don't understand this at all! if i replace [a] with String and [[a]] with [String], it works! but i want pack to work for lists of numbers too... types are so confusing. can anyone help me?