README

Alternative<F> allows for propagation of 'failure' and 'choice' (in some appropriate sense, depending on the type), as well as provision of a unit/identity value (Empty).

Alternative is a Choice and MonoidK, which means it has a Choose method, a Combine method (which defaults to calling the Choose method), and an Empty method. That creates a semantic meaning for Choose, which is about choice propagation rather than the broader meaning of SemigroupK.Combine. It also allows for Choose and Combine to have separate implementations depending on the type.

The way to think about Choose and the inherited SemigroupK.Combine methods is:

• Choose is the failure/choice propagation operator: |
• Combine is the concatenation/combination/addition operator: +

Any type that supports the Alternative trait should also implement the | operator, to enable easy choice/failure propagation. If there is a different implementation of Combine (rather than accepting the default), then the type should also implement the + operator.

AlternativeLaw can help you test your implementation:

choose(Pure(a), Pure(b)) = Pure(a)
choose(Empty  , Pure(b)) = Pure(b)
choose(Pure(a), Empty  ) = Pure(a)
choose(Empty  , Empty  ) = Empty

It also tests the Applicative and Functor laws.