I have written a flatten function from scratch that takes a nested list as input and outputs another.

def flatten[A] (list:List[List[A]]):List[A]

I used scala's ':::' to bind the two lists and it worked fine. However, my exercise prohibits me from using that or any built in method ( exercise from a personal scala book).

Now, i have written a helper method that merges those two list called merge.

How would merge fit in a new version of flatten?