Overload resolution simplification

[Michael Hixson]

I'm sure I sound like the most Comparator-obsessed guy in the world at
this point, but I have to ask...

For a brief time, the Comparator.thenComparing methods had signatures like this:

  <S extends T> Comparator<S> thenComparing(Comparator<? super S> other);

Those "narrowing type" changes were reverted for some reason, removing
the <S> type parameter.  Did it have to do with this topic -- the
ability of the compiler to interpret lambdas/method references in
overloaded methods?  Was that one of the "complex overload
disambiguation scenarios" that was abandoned?

-Michael

On Sat, Aug 10, 2013 at 6:56 AM, Maurizio Cimadamore
<maurizio.cimadamore at oracle.com> wrote:
> On 10/08/13 08:07, Andrey Breslav wrote:
>>
>> The case of overloaded method references worries me as well (lambdas are
>> ok). Note that C# supports overloaded method references (method groups) as
>> arguments and only as arguments. It seems that inference can disambiguate
>> method references rather well if we stick to what Dan proposes about
>> lambdas, because for a method reference there is no body to check. But maybe
>> I'm missing something.
>
> I believe C# is very different w.r.t. Java when it comes to target-typing
> and overload resolution - as such C# is not subject to all the issues we
> have here with 'stuck' expression - i.e. expression such as lambda and/or
> method references that cannot be looked at by the compiler because some type
> information is missing and the compiler cannot safely go ahead and
> instantiate the inference variable that would make it possible for the
> compiler to go ahead.
>
> I think 'comparing' is a good example of what can go wrong; even if we added
> support for overloaded method references (which we had last week), that API
> cannot be compiled by passing in a method reference, as the inference
> variable that is keeping the method reference stuck also appears on the
> 'comparing' return type. Which is, IMHO, a much more subtle explanation than
> 'just don't use an overloaded method reference here'.
>
> If we could have a scheme that worked in all cases, then I'd be totally in
> favor of having a more complex scheme. But, because of Java legacy, I don't
> think such an approach exists here.
>
> The only incremental improvement I see viable here, one that has been
> discussed before, would be to add some logic to detect that all overloaded
> methods force the same choice on the implicit lambda parameter/overloaded
> mref; that would be enough to get past Remi example - but it doesn't scale
> too well to generic methods.
>
> Maurizio
>
>
>>> On Aug 9, 2013, at 2:21 PM, Remi Forax <forax at univ-mlv.fr> wrote:
>>>
>>>> Also I've a nice parsing framework that use type specialised lambda to
>>>> avoid boxing that doesn't compile anymore.
>>>>
>>>> public IntStream parse(BufferedReader reader, ToIntFunction<String> fun)
>>>> {  ... }
>>>> public LongStream parse(BufferedReader reader, ToLongFunction<String>
>>>> fun) { ... }
>>>>
>>>> when called like this: parse(Integer::parseInt).
>>>
>>> Thanks for the use case.
>>>
>>> The 'parse' method is essentially the same shape as the 'map' method that
>>> was discussed by the EG quite a bit, with the eventual conclusion that it
>>> would be clearer to give each method a different name (parseInts,
>>> parseLongs, etc.).
>>>
>>>
>>> http://mail.openjdk.java.net/pipermail/lambda-libs-spec-experts/2013-February/001417.html
>>>
>>> http://mail.openjdk.java.net/pipermail/lambda-libs-spec-experts/2013-March/001441.html
>>>
>>> http://mail.openjdk.java.net/pipermail/lambda-libs-spec-experts/2013-March/001458.html
>>>
>>> Doesn't mean that all other developers must follow our lead, but the fact
>>> that the EG tried it and then concluded that it didn't want overloading here
>>> is a strong argument that this is potentially a bad convention to follow.
>>>
>>> If somebody likes this convention anyway, then we made a special-case
>>> effort to support method references.  Unfortunately, Integer::parseInt is
>>> overloaded and so outside of the set of supported method references.  As I
>>> mentioned in the EG meeting, by drawing the line like this, it's great when
>>> it works, and annoying when it doesn't and you fall off of a cliff.  We
>>> considered using arity (e.g., "is this overloaded with arity 1?"), but that
>>> just moves the line, rather than solving the problem.
>>>
>>> So, I don't love the cliff, but I don't have a good alternative, other
>>> than just not having any special treatment at all.
>>>
>>> —Dan
>
>