On 2006-08-14 at 12:03+0400 Bulat Ziganshin wrote:
Hello Taral,
Monday, August 14, 2006, 9:02:58 AM, you wrote:
In my opinion, an instance definition of a subclass should allow the superclass's methods to be defined as if they were part of the subclass, e.g.:
instance Monad [] where fmap = map return x = [x] join = concat
i support this idea. [...]
I'm not sure it's quite right. Surely it only makes sense if it defines all the (necessary) superclass methods -- in other words, what you are doing is defining an instance, just omitting the "instance Functor []" line, which doesn't seem like a great advantage. If we are going to play around with this stuff, here's another suggestion that solves my original problem more neatly: In a class declaration, a superclass context is a requirement that instances of the class have instances of the superclass; this is similar to the type declarations of the methods. We could have had class Monad m where instance Functor m (>>=):: ... instead of class Functor m => Monad m where (>>=):: ... of course, there's no reason to do that, but what I'm proposing is that we allow default instance declarations in class declarations in much the same way as default methods:
class Functor m => Monad m where (>>=):: ... return:: ... join:: ... instance Functor m where fmap f = (>>= return . f)
This shouldn't be hard to implement: all the compiler has to do when reading an instance declaration for Monad is to generate an instance declaration for Functor, substituting the espression for m that comes from the instance declaration for Monad. I don't know whether there's anything to be gained by adding the option of overriding the default inside an instance declaration:
instance Monad [] where return x = [x] join = concat instance Functor [] where fmap = map
but clearly a top-level instance declaration would override the default anyway. -- Jón Fairbairn Jon.Fairbairn at cl.cam.ac.uk