Hello Dan, Thursday, June 21, 2007, 7:39:35 PM, you wrote:
class FooOp a b where foo :: a -> b -> IO ()
instance FooOp Int Double where foo x y = putStrLn $ (show x) ++ " Double " ++ (show y)
this is rather typical question :) unlike C++ which resolves any overloading at COMPILE TIME, selecting among CURRENTLY available overloaded definitions and complaining only when when this overloading is ambiguous, type classes are the RUN-TIME overloading mechanism
As I understood it, it was at COMPILE TIME (i.e. no type witness) whenever explicitly type-annotated, implicitly when not exported from a module, or when inlined at the call site, at least in GHC. Or did I get this wrong?
overloading rules are general and they should work in any situation. generally speaking, you define POLYMORPHIC function which will work with any instance of FooOp class. there is no way to force GHC to use ad-hoc overloading -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com