Thank you for your informative response.
What you describe sounds very promising. (I hope the extra parentheses will not
be required, though.)
I take it that by “no runtime cost” you mean that something like the following
would do only one heap allocation (for the iterator)?
T[] array;
for (int index, T element : Arrays.indexedElements(array)) {
...
}
Map.entrySet() is not a good test of performance since the Entry objects
already exist and are returned without copying.
Alan
> On Jan 1, 2019, at 11:16 AM, Remi Forax <fo...@univ-mlv.fr> wrote:
>
> ----- Mail original -----
>> De: "Alan Snyder" <javali...@cbfiddle.com>
>> À: "Brian Goetz" <brian.go...@oracle.com>
>> Cc: "core-libs-dev" <core-libs-dev@openjdk.java.net>
>> Envoyé: Vendredi 21 Décembre 2018 03:19:14
>> Objet: Re: enhanced for loop with multiple iteration variables
>
>> RIght, but I don’t see that as a general solution.
>>
>> Its also a bit kludgy, like saying that methods need only one parameter
>> because
>> you can always pass a data object.
>
> yes, that's the point, as part of Valhalla, we want to introduce value types
> in Java.
> A value type is an almost zero cost abstraction that let you group and
> ungroup values.
>
> Also as part of Amber, we want to be able to de-construct record types
> (product types) in order to support pattern matching.
>
> If we have value types and de-construction, we are not far be able to write
> something like:
> for((var key, var value): map.entrySet()) {
> // ...
> }
> with no runtime cost.
>
> So being able to use the enhanced for loop on tuples is clearly something we
> want to have, but to achieve that goal i don't think that introducing a new
> Iterator2 interface is the right way to do that.
>
> regards,
> Rémi
>
>
>>
>>> On Dec 20, 2018, at 2:50 PM, Brian Goetz <brian.go...@oracle.com> wrote:
>>>
>>> For Map, you can do:
>>>
>>> for (Map.Entry<K,V> e : map.entrySet()) { ... }
>>>
>>> and you're already there.
>>>
>>>
>>>
>>> On 12/19/2018 9:54 AM, Alan Snyder wrote:
>>>> Has any consideration been given to supporting iterators that provide more
>>>> than
>>>> one iteration variable in the enhanced for loop?
>>>>
>>>> Obvious uses would be maps (keys and values) and lists (indexes and
>>>> values).
>>>>
>>>> I have in mind keeping the syntactic sugar approach by using one or more
>>>> extensions of the Iterator/Iterable interfaces, such as, for example:
>>>>
>>>> interface Iterator2<E1,E2> extends Iterator<E1> {
>>>> E2 get2();
>>>> }
>>>>
>>>> with the extra methods providing the values for the extra variables
>>>> (associated
>>>> with the previous call to next).
>>>>
>>>> Extending interfaces is not required, but it makes the trailing variables
>>>> optional, which might be useful. For example, the same iterator could
>>>> provide
>>>> values or values and keys.
>>>>
>>>> The fact that this approach only works for a fixed set of numbers of
>>>> variables
>>>> does not bother me unduly.
>>>>
>>>> Alan
>>>>
>
