[Haskell-cafe] Re: Overlapping instances

24 Aug 2005


      In article <200508110639.j7B6dSqP004876@mx02.kabsi.at>,
 "Frank"  wrote:
...
I would like to state that a class Sup is exhaustively broken down in two
subclasses Sub1 and Sub2 (meaning, for every instance of Sub1 and every
instance of Sub2, the methods in Sup apply).
I try to code this as:
instance Sub1 x => Sup x 
instance Sub2 x => Sup x
And get the (expected) error message
ExhaustiveClass.hs:22:0:
    Duplicate instance declarations:
      ExhaustiveClass.hs:22:0: instance (Sub1 x) => Sup x
      ExhaustiveClass.hs:25:0: instance (Sub2 x) => Sup x
(I have allowed overlapping instances, undecidable instances)
Is there another way of achieving this? 
Any help appreciated!
Andrew
----------------- complete code I tried with:
class Sub1 a where
    op1 :: a -> a
class Sub2 a where
    op2 :: a -> a -> a
class Sup a where
    op3 :: a -> Bool
instance Sub1 Int where
    op1 = id
instance Sub2 Float where
    op2 x y = y
instance Sub1 x => Sup x where
    op3 _ = True
instance Sub2 x => Sup x where
    op3 _ = False
Try this:

class Sup a where
    op3 :: a -> Bool
class (Sup a) => Sub1 a where
    op1 :: a -> a
class (Sup a) => Sub2 a where
    op2 :: a -> a -> a

instance Sup Int where
    op3 _ = True
instance Sup Float where
    op3 _ = False
instance Sub1 Int where
    op1 = id
instance Sub2 Float where
    op2 x y = y

-- 
Ashley Yakeley, Seattle WA