Re: Proposal: Functor and friends for the wrappers in Data.Monoid

To clarify the discussion: Let's add the following instances: A: Functor, Foldable, Traversable to Sum, Product, Dual, First, and Last B: Data to everything in Data.Monoid (except Endo): Sum, Product, Dual, First and Last, All, Any, Alt C: Applicative, Monad, MonadFix to Sum, Product, Dual, First and Last D: Data to Identity All seem to agree on A and B, C is "not sure", and D is new item. - Oleg

On 21 Feb 2015, at 17:34, Oleg Grenrus wrote:

I propose to add Functor, Applicative, Monad, Foldable, and Traversable and maybe even MonadFix instances to wrapper newtypes in the Data.Monoid module. The same way as in the semigroups package, e.g. https://hackage.haskell.org/package/semigroups-0.16.1/docs/Data-Semigroup.ht... https://hackage.haskell.org/package/semigroups-0.16.1/docs/Data-Semigroup.ht...

Basically:

instance Functor Sum where fmap f (Sum x) = Sum (f x)

instance Foldable Sum where foldMap f (Sum a) = f a

instance Traversable Sum where traverse f (Sum a) = Sum <$> f a

instance Applicative Sum where pure = Sum a <* _ = a _ *> a = a Sum f <*> Sum x = Sum (f x)

instance Monad Sum where return = Sum _ >> a = a Sum a >>= f = f a

instance MonadFix Sum where mfix f = fix (f . getSum) _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries