proposal #4095: add Applicative instance for Either

[Ross Paterson]

On Tue, May 25, 2010 at 03:15:57PM +0200, Henning Thielemann wrote:
> 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?

The "all errors" instance:

instance Monoid e => Applicative (Either e) where
    pure = Right
    Left e1 <*> Left e2 = Left (e1 `mappend` e2)
    Left e1 <*> Right _ = Left e1
    Right _ <*> Left e2 = Left e2
    Right f <*> Right x = Right (f x)