[Haskell-beginners] Checking if a Stack is Empty
poczta at emanuelkoczwara.pl
Tue Mar 12 16:05:19 CET 2013
Dnia 2013-03-12, wto o godzinie 14:48 +0000, doaltan pisze:
> 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?
First, it looks like you should read 2 or 3 times this book:
Second, you should use a function like this:
isEmpty :: Stack -> Bool
isEmpty Empty = True
isEmpty _ = False
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