I think you're headed down a path that's only going to cause you confusion. You shouldn't think in OO terms like sub and super classes. Each class is its own thing, they aren't sub and super classes in the OO sense. A class my have a constraint applied to it such that any type that is an instance of that class, must also be an instance of one or more other classes. That constraint is what allows you to use functions that belong to other classes inside of default function definitions. Semantically this behaves much like inheritance in an OO language, however functionally it's very different.
Hi,
In the example below does the concept of inheritance only apply to classes and not instances?
By inheritance I mean that a method in a sub-class can use a method from the super-class but not vice versa.
Would the inclusion of instance contexts such as 'instance (Eq a, Show b) => MyClass a b' apply such inheritance semantics between LHS => RHS of instances?
My current misunderstandings are in the comments.
Thanks,
Pat
-- Any instance of SUPER must define the super method. The super method definition may use any function in scope on the RHS of equations.
-- If the default method is defined in the class it may use any method in scope except those from the sub-class
class SUPER a where
super :: a -> a
-- Any instance of SUB must define sub. The sub method definition may use super or any function in scope on the RHS of equations
class SUPER a => SUB a where
sub :: a -> a
-- This instance defines super and can use sub because sub it is defined and is in scope
instance SUPER Int where
super n = 1 + (sub n)
-- This instance defines sub
instance SUB Int where
sub n = 6 + n
-- The Char instances differs from the Int instances in that the SUB uses super.
instance SUPER Char where
super n = 's'
-- The use of super here because it is defined and is in scope.
instance SUB Char where
sub n = super n
_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://www.haskell.org/mailman/listinfo/beginners