[Haskell-beginners] Checking if a Stack is Empty

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.