Re: [Haskell-cafe] [announcement] filtrable: class of filtrable containers

On 17/02/2016, Chris Wong wrote:

On Thu, Feb 18, 2016 at 1:57 PM, martin wrote:

...

The only need for the functor requirement is in the default definition of mapMaybe - so why not drop it?

Since fmap can be defined in terms of mapMaybe:

fmap f = mapMaybe (Just . f)

Filtrable should imply Functor in the same way that Traversable implies Foldable.

I think the notion is to make mapMaybe not a class method, and rather define it on its own: mapMaybe f :: (Functor f, Filtrable f) => (a -> Maybe b) -> f a -> f b mapMaybe f = catMaybes . fmap f I like this idea, but worry what the laws would be...