Proposal: Add missing Foldable/Traversable instances for Prelude types

[Conor McBride]

On 21 Jan 2011, at 23:33, Edward Kmett wrote:

> I'd like to propose adding the following missing instances to  
> Data.Foldable and Data.Traversable respectively, since there isn't a  
> canonical location that they could be added outside of those  
> packages without orphans, their definition is unambiguous, and they  
> are quite useful (plus, I happen to need them for a monad  
> transformer in my adjunctions package).
>
> instance Foldable (Either a) where
>   foldMap f (Left a) = mempty
>   foldMap f (Right b) = f b
>
> instance Foldable ((,)e) where
>   foldMap f ea = f (snd ea)
>
> instance Traversable (Either a) where
>   traverse f (Left a) = pure (Left a)
>   traverse f (Right a) = Right <$> f a
>
> instance Traversable ((,)e) where
>   traverse f ~(e,a) = (,) e <$> f a
>
> Discussion period: 2 weeks

+1

Conor