[Haskell-beginners] Using stack inside a function without declaring it as input

Hi I have a function like this : myfunc :: [Char] -> [Char] It  is supposed to work pretty much like this : 1. Take a string 2. Put some elements of this input string to output string and put others to stack. 3. Pop elements to that output string too. 4. Do 2 and 3 recursively until stack is empty. 5. Print the output string when stack is empty. I couldn't figure out where to define stack and output string. Can you help me with that? I'm new to Haskell so I can't think in Haskell's logic very well.