[Haskell-beginners] Checking if a Stack is Empty

[doaltan]

Hello,
I have this stack data structure : 

data Stack = Empty | Element Char Stack deriving Show
I want to check if it is equal to "Empty"
When I try something like this : 

"a = Empty" or "a = (Empty)" in a haskell file and then write this on ghci : "a = Empty"
I get this : 

<interactive>:1:0:
    No instance for (Eq Stack)
      arising from a use of `==' at <interactive>:1:0-11
    Possible fix: add an instance declaration for (Eq Stack)
    In the expression: a == (Empty)
    In the definition of `it': it = a == (Empty)
I don't know how to fix this. Can you help me so that I can check if a stack is Empty without getting this error? 

Thank you.
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