Haskell group function / First element always missing

Question

I want to write a function called group that takes a list of type [a] and returns a a list of list of type [[a]] containing identical characters. Here is a simple example of the expected behaviour:

group [1,1,2,1,3,3,3] ⇒+ [[1,1],[2],[1],[3,3,3]]

group "aaabbcdddd" ⇒+ ["aaa","bb","c","dddd"]

I wrote the following code but the problem is that this code always misses the first element from the original list

group [] = []
group (x:xs) = groupHelper xs x []

groupHelper :: Eq a => [a] -> a -> [[a]] -> [[a]]
groupHelper [] prev fs = fs
groupHelper (x:xs) prev [] = groupHelper xs prev [[x]]
groupHelper (x:xs) prev fs = if (x == prev) then groupHelper (xs) (x) ((init fs) ++ [(last(fs)++[x])]) 
     else groupHelper (xs) x (fs++[[x]])

The code would produce the following output for this input.

group [1,1,2,1,3,3,3] ⇒+ [[1],[2],[1],[3,3,3]]

=> Here a 1 at the beginning of the first list would be missing

---

group "aaabbcdddd" ⇒+ ["aa","bb","c","dddd"]

=> Here one 'a' at the beginning of the first list would be missing

I hope someone can help me to solve this bug. Thank you very much

Answer 1

When you call groupHelper the first time (from group) you’re passing the prev arg, but the helper uses it only for comparison. Solution could be to add the fist item as the first element of the first group on first call. Like this: group (x:xs) = groupHelper xs x [[x]]