What are Alternative's “some” and “many” useful for?

Question

Alternative, an extension of Applicative, declares empty, <|> and these two functions:

One or more:

Answer 1

Will provided a good example motivating the use of those methods, but you seem to still have a misunderstanding about type classes.

A type class definition lists the type signatures for the methods that exist for all instances of the type class. It may also provide default implementations of those methods, which is what is happening with Alternative's some and many methods.

In order to be valid instances, all of the methods have to be defined for the instance. So the ones that you found that did not specifically define instances for some or many used the default implementations, and the code for them is exactly as listed in your question.

So, just to be clear, some and many are indeed defined and can be used with all Alternative instances thanks to the default definitions given with the type class definition.