[Haskell-beginners] Applicative instance for Either String

[Stephen Tetley]

Const has an Applicative instance that doesn't touch the "subject"
parameter i.e. the rightmost element of the type.

-- Const - drops /b/
newtype Const a b = Const a  deriving (Eq,Show)


instance Functor (Const a) where
  fmap f (Const a) = Const a

The applicative instance obliges the "data" i.e. the first parameter
to be an instance of Monoid:

instance Monoid a => Applicative (Const a) where
  pure b = Const mzero
  Const f <*> Const v = Const (f `mappend` v)