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

D is already implemented: https://phabricator.haskell.org/rGHC1f60d635cee1ff3db72e0129f9035b147f52c9c4 https://phabricator.haskell.org/rGHC1f60d635cee1ff3db72e0129f9035b147f52c9c4 https://ghc.haskell.org/trac/ghc/ticket/9664 https://ghc.haskell.org/trac/ghc/ticket/9664 Seems it just (~30 hours ago) landed the 7.10 branch https://git.haskell.org/ghc.git/blob/refs/heads/ghc-7.10:/libraries/base/Dat... https://git.haskell.org/ghc.git/blob/refs/heads/ghc-7.10:/libraries/base/Dat... So, it’s only A, B, C options in discussion anymore. - Oleg

On 22 Feb 2015, at 12:07, 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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