On Fri, Oct 4, 2013 at 10:31 PM, Wvv <vite...@rambler.ru> wrote: > Newclasses are something like instances, but out of scope. In a baggage. >

So under the hood of GHC, newclasses would be partially filled in dictionaries. We already have too many classes: (...) > > We can't divide all classes to atimic ones. > As you have seen, we can. As you also see, it is a little impractical. Main purpose of newclasses is to make instances as minimal as possible. In > many cases empty. > > About newclass and compose data, we can do next: > > newclass Foo [a] => FooList a where {containerMainipulation=...} > > newclass Foo (Set a) => FooSet a where {containerMainipulation=...} > > newclass Foo (Sequence a) => FooSeq a where {containerMainipulation=...} > > so now I can switch any container of my data, changing only name of > newclass: > > instance FooList MyData where {dataMainipulation=...} > You can already solve that in Haskell 98: class Foo2 f where { containerManipulation = ... } instance Foo2 [] where { ... } instance Foo2 Set where { ... } instance Foo2 Sequence where { ... } class (Foo2 f) => Foo1 f a where { dataManipulation = ... } Or even: class Foo' a where { dataManipulation' = ... } dataManipulation = dataManipulation' yourDefaultContainerManipulation Remember: the only special things about type classes is that they are types that can/must be implicit. You can (almost?) always replace them by explicit parameters. Or let I have an MyArrow data. And I need some semigroupoid manipulations. > I just write > > instance ArrSemigroupoid MyArrow --empty > > that's all, I plug-in, let's just use semigroupoids functions! > > Or I have MyMonad and I want some Functor, so I just plug-in: > > instance MFunctor MyMonad --empty > > that's all. > I also need some Applicative! Easy: > > instance MApplicative MyMonad --empty again > > done! > Let's see how many lines of code this costs in Haskell 98: instance Monad MyMonad where { ... } instance Functor MyMonad where fmap = liftM instance Applicative MyMonad where pure = return (<*>) = ap Only three lines more, and they're readable. I think newclasses are not solving the existing problems, as you're only removing three well-understood lines of code in the above example, while people have to look up what you mean by MFunctor and MApplicative. I think default superclass instances are a much better idea, or alternatively, the ConstraintSynonymInstances I previously mentioned (but not both -- they'll probably bite each other). -Stijn

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