Hi, Dnia 2013-03-12, wto o godzinie 14:48 +0000, doaltan pisze:
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?
First, it looks like you should read 2 or 3 times this book: http://learnyouahaskell.com/ Second, you should use a function like this: isEmpty :: Stack -> Bool isEmpty Empty = True isEmpty _ = False Emanuel