### Re: [Haskell-cafe] Newclasses

Stijn van Drongelen wrote On Fri, Oct 4, 2013 at 10:31 PM, Wvv lt; vitea3v@ gt; wrote: 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 Yes, I agree, use newclasses for composite data is an additional, secondary feature. Haskell has huge infrastructure of data to use alternative ways instead using newclasses this way. Stijn van Drongelen wrote On Fri, Oct 4, 2013 at 10:31 PM, Wvv lt; vitea3v@ gt; wrote: 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 see, we are looking to the same situation from different angles. I try to show you why I think my point of view is important. 2 situations: first is more practical, second is more philosophical. (1) we have several libraries with lenses and lens-looking like libraries. Why is the main popularity going to Kmett's library? My answer: easy connection: just add one line makeLens MyRecord'' and we already could use all their abilities. Why Kmett's library of JSON is so popular. Sure, it more quicker, but what is the main reason? My answer: easy connection. Why Pipes, Streams, Conduit, are not as super-popular as they could be? They have very powerful abilities. You need time not only for studying how this library works, but also how to switch your data to library functions. If you need to work with ... RMonad or MonadDatabase or something like this, do you always know how to switch your data on? (2) Let I have a lamp and I wish to switch it on. So, I say, I want to have a plug. But you argue: this isn't necessary: you take 3 wires, green one you contact here, blue one contact here, and finally, brown one contact there! Easy! Ok, now I wish to connect computer with iPhone. You say: take 12 wires , pins-scheme, But I want USB-30pin cable. Newclasses are those connectors-plugs and adapters. Deriving are those plugs. Generic instances and Data instances are those plugs. Newclasses are something like deriving, but much-much flexible and more universal (sure, we can't replace deriving with newclasses). If I have a Data which has an instance of Foo and I want to switch it to class Bar, and I have 2 newclasses: Foo2Tmp and Tmp2Bar, I do the next: instance Foo2Tmp MyData instance Tmp2Bar MyData done! or much simpler if I have Foo2Bar newclass: instance Foo2Bar MyData I do not care how easy or complex those instances I could write without newclasses. My aim is not to connect, but use abilities, which I take after connection. And newclass is amazing tool for easy connection between classes. -- View this message in context: http://haskell.1045720.n5.nabble.com/Newclasses-tp5737596p5737833.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Newclasses

Newclasses are something like instances, but out of scope. In a baggage. We don't use them for interfere their functions. This why newclasses never overlap each other and between them and any instances. We use newclasses to plug-in/connect to any related class or combine data Replying to you question, yes, instance of newclass desugar to instance of class: instance BMonad MyBind where {return= ...} desugar into instance Monad MyBind where {return= ...; (=) = (-)} We already have too many classes: look at Edward Kmett http://hackage.haskell.org/package/semigroupoids 13 dependent classes (from Foldable to MonadPlus) http://hackage.haskell.org/package/category-extras 30-60 dependent class http://hackage.haskell.org/package/lens 11 dependent classes We can't divide all classes to atimic ones. I do not want to implement all depended class instances, even of atomic, if I want to work with hight class only. But I want easy connection with any related class! And newclasses solve this situation. Also in reality we have several realizations of same class/compose data and we want to mix them for better realizations. Newclasses allows switch them as engines! Easy. 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=...} 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! About conflicts, I don't understand a bit. Which ones? We catch Overlapped instances or even Incoherent instances at once we add both newclass instances of the same class. John Lato-2 wrote I meant to say, does it mean that by writing a BMonad instance a Monad instance would be automatically generated? If so, that seems like it would cause conflicts in many cases. Regardless, I think newclass needs to be better specified if you want other people to be able to support it. On Thu, Oct 3, 2013 at 7:53 PM, John Lato lt; jwlato@ gt; wrote: I don't really understand what a newclass is supposed to be. On Thu, Oct 3, 2013 at 2:15 PM, Wvv lt; vitea3v@ gt; wrote: newclass Bind a = Monad a = BMonad a where { (=) = (-) } I think this means that `BMonad` is supposed to be a new class that has both Bind and Monad in scope, the same as class (Bind a, Monad a) = BMonad a except that the Monad instance's (=) is replaced by (-). If that's what newclass means, it seems absolutely pointless. Does it instead mean that one could write instance Bind MyType where instance BMonad MyType ___ Haskell-Cafe mailing list Haskell-Cafe@ http://www.haskell.org/mailman/listinfo/haskell-cafe -- View this message in context: http://haskell.1045720.n5.nabble.com/Newclasses-tp5737596p5737792.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Newclasses

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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Newclasses

Your first two cases will be fixed in 7.10, as Applicative finally becomes a superclass of Monad. Sure, newclassses not about Applicative and Monads only. This question is more wider. Must Apply be a superclass of Bind? Must Bind be a superclass of Monad? So, must Monad has 2 superclasses at once: Bind and Applicative? Must Semigroupoids be a superclass of Category? Must Category be a superclass of Arrow? With newclasses we could write empty instances to provide correct functional dependencies: instance ArrCategory MyArrow instance CatSemigroupoids MyCategory instance MBind MyMonad instance MApply MyMonad instance MApplicative MyMonad instance MFunctor MyMonad Also, I don't see why it would be a misfeature to have Eq as a superclass of Ord, or Functor as a superclass of Applicative. I see 2 reasons: 1) class functions in reality don't depend of superclass functions 2) Haskell can't check if superclass instance is correspond with class laws -- View this message in context: http://haskell.1045720.n5.nabble.com/Newclasses-tp5737596p5737625.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Newclasses

On Thu, Oct 3, 2013 at 8:16 AM, Wvv vite...@rambler.ru wrote: Your first two cases will be fixed in 7.10, as Applicative finally becomes a superclass of Monad. Sure, newclassses not about Applicative and Monads only. This question is more wider. Must Apply be a superclass of Bind? Must Bind be a superclass of Monad? So, must Monad has 2 superclasses at once: Bind and Applicative? Must Semigroupoids be a superclass of Category? Must Category be a superclass of Arrow? There is no theoretical problem here, just a practical one. It would be resolved by solving your 4th problem, for which you don't need newclasses. Consider: {-# LANGUAGE ConstraintKinds #-} class Functor f where { fmap :: (a - b) - f a - f b } class Functor f = Apply f where { (*) :: f (a - b) - f a - f b } class Apply f = Applicative f where { pure :: a - f a } class Apply f = Bind f where { (=) :: (a - f b) - f a - f b } type Monad f = (Applicative f, Bind f) return :: Monad f = a - f a return = pure I might have made some mistakes in the exact hierarchy, but something like this should work. There are no problems with having hierarchies like this, as far as I'm aware. The current problem is that nobody wants to use this hierarchy: to get a Monad instance, you have to write four separate instances for your type. What would be nicer is a feature (ConstraintSynonymInstances?) where something like this can be written: instance (Functor Maybe, Apply Maybe, Monad Maybe) where fmap _ Nothing = Nothing fmap f (Just x) = Just (f x) Just f * Just x = Just (f x) _ * _ = Nothing pure = Just f = Just x = f x _ = Nothing = Nothing This would be sugar for instance Functor Maybe where { fmap = ... } instance Apply Maybe where { (*) = ... } instance Monad Maybe where { pure = ... ; (=) = ... } and the last would be sugar for instance Applicative Maybe where { pure = ... } instance Bind Maybe where { (=) = ... } You don't need any new keywords for this, because the above does not conflict with the existing rules for instance declarations. Also, I don't see why it would be a misfeature to have Eq as a superclass of Ord, or Functor as a superclass of Applicative. I see 2 reasons: 1) class functions in reality don't depend of superclass functions 2) Haskell can't check if superclass instance is correspond with class laws Again, I don't see why that makes it a misfeature. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Newclasses

Yes, multi-class instances allow us write type Monad a = (Applicative a, Bind a) But at least 1 issue remains: Applicative : pure; Monad: return Bind : (-); Monad: (=) With MultiClassInstances we could write only instance Monad MyMonad where { pure= ...; (-)= ...} But we don't want to break the existent code. Fortunately, an easy extension FunctionSynonyms could help us: type return = pure-- this allow us to use 'return' instead of 'pure' in instances type (=) = (-)-- this allow us to use '(=)' instead of '(-)' in instances 2) Still remains issue with several default instances, like 'Generic a = ToJSON a' and 'Data a = ToJSON a', which we can't unite to 1 instance 3) If devs of library don't want to change the behavior, (for example divide Monad to Applicative and Bind), but we still want easy connection to that class, newclasses is our choice! Yes, this solution is good! Very nice! I like it! I should name it solution from derivatives. From bottom to top. We have only independent classes and unite them with types. Newclasses solve same problem in integral way. From top to bottom. Instead of having independent little classes, it allow to have big classes with dependences, which are written in newclasses, and they allow to connect easy to any dependent class. newclass Bind a = Monad a = BMonad a where { (=) = (-) } newclass Applicative a = Monad a = ApMonad a where { return = pure } newclass (BMonad a, ApMonad a) = BApMonad a --empty type ApBMonad = BApMonad --then connect these classes: instance Bind MyDataAB where { (-) = ...} instance Applicative MyDataAB where { pure = ... ; (*) = ...} instance ApBMonad MyDataAB --empty --or these instance Monad MyDataM where {return= ... ; (=) = ...} instance MBind MyDataM --empty instance MApply MyDataM --empty instance MApplicative MyDataM--empty instance MFunctor MyDataM--empty If Haskell add MultiClassInstances + FunctionSynonyms, or Newclasses, or both of them, Haskell would be the best language in nearest future!!! About the misfeature. If class is independent of superclass functions and can't check dependence's laws, why does it order to have instances of unnecessary class? Stijn van Drongelen wrote On Thu, Oct 3, 2013 at 8:16 AM, Wvv lt; vitea3v@ gt; wrote: Your first two cases will be fixed in 7.10, as Applicative finally becomes a superclass of Monad. Sure, newclassses not about Applicative and Monads only. This question is more wider. Must Apply be a superclass of Bind? Must Bind be a superclass of Monad? So, must Monad has 2 superclasses at once: Bind and Applicative? Must Semigroupoids be a superclass of Category? Must Category be a superclass of Arrow? There is no theoretical problem here, just a practical one. It would be resolved by solving your 4th problem, for which you don't need newclasses. Consider: {-# LANGUAGE ConstraintKinds #-} class Functor f where { fmap :: (a - b) - f a - f b } class Functor f = Apply f where { (*) :: f (a - b) - f a - f b } class Apply f = Applicative f where { pure :: a - f a } class Apply f = Bind f where { (=) :: (a - f b) - f a - f b } type Monad f = (Applicative f, Bind f) return :: Monad f = a - f a return = pure I might have made some mistakes in the exact hierarchy, but something like this should work. There are no problems with having hierarchies like this, as far as I'm aware. The current problem is that nobody wants to use this hierarchy: to get a Monad instance, you have to write four separate instances for your type. What would be nicer is a feature (ConstraintSynonymInstances?) where something like this can be written: instance (Functor Maybe, Apply Maybe, Monad Maybe) where fmap _ Nothing = Nothing fmap f (Just x) = Just (f x) Just f * Just x = Just (f x) _ * _ = Nothing pure = Just f = Just x = f x _ = Nothing = Nothing This would be sugar for instance Functor Maybe where { fmap = ... } instance Apply Maybe where { (*) = ... } instance Monad Maybe where { pure = ... ; (=) = ... } and the last would be sugar for instance Applicative Maybe where { pure = ... } instance Bind Maybe where { (=) = ... } You don't need any new keywords for this, because the above does not conflict with the existing rules for instance declarations. Also, I don't see why it would be a misfeature to have Eq as a superclass of Ord, or Functor as a superclass of Applicative. I see 2 reasons: 1) class functions in reality don't depend of superclass functions 2) Haskell can't check if superclass instance is correspond with class laws Again, I don't see why that makes it a misfeature. ___ Haskell-Cafe mailing list

### Re: [Haskell-cafe] Newclasses

I don't really understand what a newclass is supposed to be. On Thu, Oct 3, 2013 at 2:15 PM, Wvv vite...@rambler.ru wrote: newclass Bind a = Monad a = BMonad a where { (=) = (-) } I think this means that `BMonad` is supposed to be a new class that has both Bind and Monad in scope, the same as class (Bind a, Monad a) = BMonad a except that the Monad instance's (=) is replaced by (-). If that's what newclass means, it seems absolutely pointless. Does it instead mean that one could write instance Bind MyType where instance BMonad MyType ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Newclasses

Apologies, that wasn't finished. I meant to say, does it mean that by writing a BMonad instance a Monad instance would be automatically generated? If so, that seems like it would cause conflicts in many cases. Regardless, I think newclass needs to be better specified if you want other people to be able to support it. On Thu, Oct 3, 2013 at 7:53 PM, John Lato jwl...@gmail.com wrote: I don't really understand what a newclass is supposed to be. On Thu, Oct 3, 2013 at 2:15 PM, Wvv vite...@rambler.ru wrote: newclass Bind a = Monad a = BMonad a where { (=) = (-) } I think this means that `BMonad` is supposed to be a new class that has both Bind and Monad in scope, the same as class (Bind a, Monad a) = BMonad a except that the Monad instance's (=) is replaced by (-). If that's what newclass means, it seems absolutely pointless. Does it instead mean that one could write instance Bind MyType where instance BMonad MyType ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Newclasses

Hi! Your first two cases will be fixed in 7.10, as Applicative finally becomes a superclass of Monad. I haven't really looked at your third case, so I can't comment on that. Your fourth case is something I'd really like to see solved properly (*together* with a better record system), but as you say, it could be solved separately. Also, I don't see why it would be a misfeature to have Eq as a superclass of Ord, or Functor as a superclass of Applicative. -Stijn ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe