On Sat, Feb 28, 2015 at 12:28 PM, Clinton Mead
Let's say for arguments sake I want to make a new finer grained numeric class hierarchy. But, I also want to have the following occur:
(1) Existing modules which have defined data types which have Num instances can be used with functions in my new hierarchy (2) New data types defined in the new hierarchy automatically get Num instances if appropriate so they can be used in existing modules.
I know this is somewhat of a vague question but roughly how would I go about this?
You could use overlapping & undecidable instances to get (1) and (2): class Plus a where plus :: a -> a -> a class NewNum a -- no methods -- a type using the "new" hierarchy data X = X Int instance Plus X where X a `plus` X b = X (a+b) instance NewNum X instance Num a => Plus a where plus = (Prelude.+) instance (Plus a, Minus a, ... , NewNum a) => Num a where (+) = plus Having an extra constraint NewNum stops "() + ()" from being an infinite loop where we can make an instance Num () if we have an instance Plus () which exists if we have Num () ... The usual objection to overlapping instances (that adding an import can change the result of the program), is probably less of a problem here than usual because + and plus have a chance of still doing the same thing when somebody defines "instance Num X" separately. Regards, Adam