Overload resolution simplification

maurizio cimadamore maurizio.cimadamore at oracle.com
Sat Aug 10 10:23:55 PDT 2013

On 10-Aug-13 4:30 PM, Michael Hixson wrote:
> On Sat, Aug 10, 2013 at 8:17 AM, maurizio cimadamore
> <maurizio.cimadamore at oracle.com> wrote:
>> On 10-Aug-13 3:41 PM, Michael Hixson wrote:
>>> 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?
>> I believe so - the signature above has same problems as the 'comparing' one
>> - the compiler can't do much with it, regardless the approach.
>> Maurizio
> That's precisely what I was getting at.  If the current situation is
> no better than the one that led to removing the <S>, then why remove
> the <S>?  I thought it was removed due to the compiler being unable to
> infer <S> in lambdas and method references.  If we'll have to specify
> types in those situations anyway, that explanation no longer makes
> sense.
I believe we are talking about two different methods - thenComparing 
used to be less problematic as it was changed (as you say) not be be a 
generic method, so that problems with stuck lambdas/overloaded method 
references were no longer biting. Since thenComparing is also not 
overloaded, I don't see any problem in using this method with the 
simplified approach too.

> I could be entirely off-base here.  If I'm mixing up issues, just tell me so.
> -Michael
>>> -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

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