This module defines a variant of the Alternative and MonadPlus type class. Thanks to quantified constraints, there's really nothing preventing us, in modern Haskell, to define it once and for all at every arity.

Effectively, this is just a clone of the Monoid type class. The reason for preferring to define another class is that applicatives have a natural Monoid instance:

instance (Applicative f, Monoid a) => Monoid (f a) where
  mempty = pure mempty
  (<>) = liftA2 (<>)

In Base, the Monoid instances for ST, IO, and STM are defined like this, and that of Maybe a and Const a b are of a similar nature. See also Ap.

So we typically want a second one. An approach would be to give a monoid instance to a newtype-wrapped version of our functor gadget. But that's very syntactically heave (see Adding, below, though). So instead we propose a dedicated type class, named after the fact that Alternative function have an additive flavour to them (msum, etc…), and situated in the Control hierarchy to represent that it's intended to represent choice between computations.