This chapter of Real World Haskell exactly explains the problem you're having:
http://book.realworldhaskell.org/read/using-typeclasses.html
(The short answer is, you need an `Eq` instance for the data type
you've created (`Stack`). You can just use `deriving`.)
Tom
On Tue, Mar 12, 2013 at 7:48 AM, 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.
_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners