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

-1 on C. A container of exactly one element is not a good Applicative/Monad. +1 on A. No opinion about the rest. On 22.02.2015 11:07, Oleg Grenrus wrote:

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

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