Hi,

I'm very, very new to Haskell. I am trying to understand the following error, but I'm having a hard time wrapping my head around the whole type system.


Stack 
                  
module Stack (Stack(..)) where
    import Prelude hiding (head, tail)
    class Stack s where
        empty :: s a
        isEmpty :: s a -> Bool
        cons :: s a -> a -> s a
        head :: s a -> a
        tail :: s a -> s a
    data ListStack a = LS [a] deriving (Show)
    instance Stack ListStack where
        empty = LS []
        isEmpty (LS s) = null s
        cons (LS s) x = LS(x:s)
        head(LS(x:_)) = x
        tail(LS(x:xs)) = LS xs
Mixmax Not using Mixmax yet?



The problem is that when I load this code in the interpreter and write
>empty
I get the following error

<interactive>:108:1:
    No instance for (Show (s0 a0)) arising from a use of ‘print’
    The type variables ‘s0’, ‘a0’ are ambiguous

I mean when I write "LS []" it works just fine. But something is wrong with empty.


Thanks,
Uneeb