[Haskell-beginners] Checking if a Stack is Empty
doaltan at yahoo.co.uk
Tue Mar 12 15:48:26 CET 2013
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 :
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?
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