Comparator combinators

[Zhong Yu]

On Wed, Aug 14, 2013 at 2:49 PM, Brian Goetz <brian.goetz at oracle.com> wrote:
> This may well be our last API loose end...
>
> We currently have a pile of Comparator combinators, all currently called
> comparing() or thenComparing().  Regardless of whether we choose to go
> forward with the simplified overloading plan, these overloads have a
> problem: for implicit lambdas, we can't distinguish between
>
>   comparing(T->U)
> and
>   comparing(T->int)
>
> because we don't type-check the body of the lambda until we do overload
> selection, and don't do overload selection based on whether there are type
> errors in the body (this was roundly rejected as being too brittle).  So for
> lambdas like:
>
>   comparing(x -> x.size())
>
> we've got a circularity -- even under the older, more complex scheme.
>
> We'd thought perhaps that, if we adopted the heuristic that we can eagerly
> give a type to method reference that refers to a non-overloaded method
> (e.g., Person::getAge), then cases like
>
>   comparing(Person::getAge)
>
> can be handled, and this might take some of the pressure off the problem.
>
> For lambdas (with either approach) you can always get what you want with an
> explicit lambda:
>
>   comparing((Person p) -> p.getAge())
>
> since this can be type-checked early.
>
> So the question is, is this good enough, even though it falls afoul of the
> overloading guidelines for implicit lambdas?  Or, should we mangle the names
> of the methods?
>
> This question comes down to whether we think its better to force everyone to
> explicitly pick a method, but then supporting implicit lambdas:
>
>   comparingInt(x -> x.size())

This expression alone should not be compiled; there is no way to know
what x is. The problem here is not overload, therefore de-overload
doesn't change anything.

>
> or forcing users to use non-ambigous method refs or explicit lambdas:
>
>   comparing(Person::getAge)
> or
>   comparing((Person p) -> p.getAge())
>
> Which disambiguation approach is worse?
>