Re: [Haskell-cafe] Factoring into type classes

On Mon, 2009-01-19 at 12:10 -0800, Iavor Diatchki wrote:

Hi,

On Mon, Jan 19, 2009 at 11:06 AM, Jonathan Cast wrote:

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On Mon, 2009-01-19 at 10:59 -0800, Iavor Diatchki wrote:

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Hello, The multitude of newtypes in the Monoid module are a good indication that the Monoid class is not a good fit for the class system

I would say rather that the class system is not a good fit for Monoid. Proposals for local instances, multiple instances, instance import/export control, etc. come up quite frequently on this list; the phenomena in question are not restricted to Monoid.

I disagree with you but that is a moot point because we are discussing Haskell, which does not have any of these features. Also, I find that in many situations where people want to use them, simpler solutions (like some of the ideas I mentioned in my previous post) suffice. That is not to say that we should stop trying to figure out how to improve the class system, but language changes require a lot more work than improving the design of the libraries.

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I usually avoid using the "newtype" trick as I find it inconvenient: usually the newtype does not have the same operations as the underlying type and so it cannot be used directly, and if you are going to wrap thing just when you use the class methods,

OTOH, I think you mean here `when you use class methods and when you use overloaded functions'.

Sure, the point is that you are essentially adding a type annotation, which is like using a non-overloaded function. Compare, for example: "mappend add x y" and "getSum (mappend (Sum x) (Sum y))". I think that the first one is quite a bit more readable but, of course, this is somewhat subjective.

data Iso a b = Iso { to :: a -> b, from :: b -> a } under :: Iso a b -> (b -> b) -> (a -> a) under iso = to iso ~> from iso under2 :: Iso a b -> (b -> b -> b) -> (a -> a -> a) under2 iso = to iso ~> under iso sumIso = Iso Sum getSum (+) = under2 sumIso mappend