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

[doaltan]

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.
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