[Haskell] Instance declaration of classes with method type
johho590 at student.liu.se
Wed Jun 29 17:53:10 EDT 2005
How do you instantiate from classes with method type constraints, such as this
class C a where
m :: (Num b) => a -> b
I have been trying for some time now but everything I have tried fails.
In particular, what I want to do is something like this:
class Rect a where
width :: (Num b) => a -> b
height :: (Num b) => a -> b
data Num a => PRect a = PRect (a, a) (a, a) deriving (Eq, Show)
data IRect = IRect (Int, Int) (Int, Int) deriving (Eq, Show)
instance Rect IRect where
width ( IRect (x1, _ ) (x2, _ ) ) = abs(x2 - x1)
height ( IRect ( _, y1) ( _, y2) ) = abs(y2 - y1)
In this case, efforts to intantiate IRect from Rect fails with error messages
like: (using GHCI)
Cannot unify the type-signature variable `b' with the type `Int'
Expected type: b
Inferred type: Int
In the expression: x2 - x1
In the first argument of `abs', namely `(x2 - x1)'
Again, what should I do to make this work?
Also, how is the PRect type to be instantiated?
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