[Haskell-cafe] Type classes... popular for newbies, isn't it?

[Arjun Guha]

This class definition is giving me a lot of problems with the successor 
function:

class (Ord st) => MinimaxState st where
   successors:: st -> [(action,st)]
   terminal:: st -> Bool

A trivial example would be:

instance MinimaxState Int where
   terminal i = i == 0
   successors i = [(1,i+1), (-1,i-1)]

However, I get this error in GHC:

     Could not deduce (Num action)
         from the context (MinimaxState Int, Ord Int)
       arising from the literal `1' at AbTest.hs:7
     Probable fix:
         Add (Num action) to the class or instance method `successors'
     In the first argument of `negate', namely `1'
     In the list element: (- 1, (- i) - 1)
     In the definition of `successors':
         successors i = [(1, i + 1), (- 1, (- i) - 1)]

I have the class definition and the instance definition in seperate 
files.  I don't understand where I'm supposed to put the "probable fix." 
  I don't want it to be in the class definition, since action should be 
fairly arbitrary.

In fact, no matter what I try, I get errors, for example:

instance MinimaxState Int where
   terminal i = i == 0
   successors i = [("action",i+1), ("action",i-1)]

     Cannot unify the type-signature variable `action'
         with the type `[Char]'
         Expected type: action
         Inferred type: [Char]
     In the list element: ("action", i + 1)
     In the definition of `successors':
         successors i = [("action", i + 1), ("action", (- i) - 1)]

Any suggestions?

-Arjun