Proposal: Add missing Foldable/Traversable instances for Prelude types

Isaac Dupree ml at
Sat Jan 22 01:27:32 CET 2011

On 01/21/11 18:33, 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

Those look correct.  Is there some reason you chose to make the (,) 
instances less strict in the pair?  For example, traverse on "undefined" 
gives you, modulo the applicative layer, "(undefined, f undefined)".  (I 
haven't thought of a reason (practical or theoretical) for either behavior.)


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