enhanced for loop with multiple iteration variables

[Alan Snyder]

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 <forax at univ-mlv.fr> wrote:
> 
> ----- Mail original -----
>> De: "Alan Snyder" <javalists at cbfiddle.com>
>> À: "Brian Goetz" <brian.goetz at oracle.com>
>> Cc: "core-libs-dev" <core-libs-dev at 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.goetz at 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
>>>> 
>