On 6 Dec 2010, at 15:47, Stephen Tetley wrote:
Me thinks there's more value that "the next language after Haskell" might have such a hierarchy than Haskell itself.
I should advise anyone designing the next language after Haskell to think about these issues and a great deal more, when it comes to organising functorial structure and programming with computational effects.
At Haskell 1.3 I think there was one text book (Davie), possibly none at Haskell 1.4 and many at Haskell 98. The value of invalidating 12 or so years of documentation for "logic" seems somewhat questionable from my perspective. If either proposal were limited to just adding class constraints to get the desired hierarchy rather than eliminating methods, they might have more legs...
Right. This issue is often mentioned as a motivating example for various language proposals to allow superclass method definitions in instance declarations and indeed default superclass instances in subclass declarations. I don't know the status of these proposals, and what else they're trying to achieve besides the telescoping of multiple instance declarations. I'd like to be able to say something like class Functor f where fmap :: (s -> t) -> f s -> f t class Functor f => Applicative f where return :: x -> f x (<*>) :: f (s -> t) -> f s -> f t instance Functor f where fmap = (<*>) . return class Applicative f => Monad f where (>>=) :: f s -> (s -> f t) -> f t instance Applicative f where ff <*> fs = ff >>= \ f -> fs >>= \ s -> return (f s) (possibly without the duplicated constraint) then declare Monad instances as before, and collect all three. The obvious dumb way to unpack these compound declarations would be to generate a superclass instance from each subclass instance according to the scheme supplied in the class (overriding defaults as demanded). I'm sure that's hopelessly naive for reasons I'm just too, er, hopelessly naive to see right now. It does, however, have the merit of requiring only the awareness of which methods belong to which class and a spot of textual rearrangement, to be delivered on top of Haskell as it stands. Does anybody have a Haskell preprocessor they could just hack this into for a laugh over Chrimbo? Cheers Conor