add Applicative instance for Either

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.

Trac metadata

Trac field	Value
Version	6.12.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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