proposal #4095: add Applicative instance for Either

[Henning Thielemann]

On Tue, 25 May 2010, Ross Paterson wrote:

> The proposal is to add this instance to Control.Applicative:
>
>    instance Applicative (Either e) where
>        pure          = Right
>        Left  e <*> _ = Left e
>        Right f <*> r = fmap f r
>
> This is not the only possible instance for Either, but this one is
> compatible with the usual Monad instance.

What other sensible instances are there?

Nevertheless I think, since the Monad instance for Either exists, there is 
no choice other than make the Applicative instance compatible to it and it 
is better than no instance at all, in order to prevent orphan instances in 
user modules.