README

Choice<F> allows for propagation of 'failure' and 'choice' (in some appropriate sense, depending on the type).

Choice is a SemigroupK, but has a Choose method, rather than relying on the SemigroupK.Combine method, (which now has a default implementation of invoking Choose). That creates a new semantic meaning for Choose, which is about choice propagation rather than the broader meaning of 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 Choice 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.

ChoiceLaw can help you test your implementation:

choose(Pure(a),   Pure(b))  = Pure(a)
choose(Fail,      Pure(b))  = Pure(b)
choose(Pure(a),   Fail)     = Pure(a)
choose(Fail [1],  Fail [2]) = Fail [2]

It also tests the Applicative and Functor laws.