dunai: Add Selective instance

[turion]

I believe that MSFs are an instance of `Selective. It should be roughly implemented as:

instance Monad m => Selective (MSF m a) where
  -- select :: MSF m a (Either e b) -> MSF m a (e -> b) -> MSF m a b
  select msf1 msf2 = proc a -> do
    eb <- msf1 -< a
    case eb of
      Right b -> returnA -< b
      Left e ->
        f <- msf2 -< a
        returnA -< f e

This is not particular to MSFs but works for any instance ArrowChoice, and in fact is implemented for the ArrowMonad newtype. So instead one could also write a deriving via instance.

The advantage of such an instance would be additional control flow like ifS that doesn't need arrow notation.

[ivanperez-keera]

It's interesting.

I think a lot of these examples would deserve at least a blog posts where you explain what it is and how it helps. I'm sure other people would find it interesting too.

[ivanperez-keera]

Have you used this instance a lot in your code, or do you know anyone who is using it?

[turion]

Currently not, but that's only because I haven't written any dunai code lately.