Normative: use Numbers instead of reals as counters in Iterator builtins#3776
Normative: use Numbers instead of reals as counters in Iterator builtins#3776michaelficarra wants to merge 3 commits intomainfrom
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michaelficarra
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The rendered spec preview for this PR is available as a single page at https://tc39.es/ecma262/pr/3776 and as multiple pages at https://tc39.es/ecma262/pr/3776/multipage . |
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In #2007 we discussed the issue of counters and IIRC agreed that we should specify them as mathematical values with the understanding that implementations would diverge in the same way they diverge from the requirements to have arbitrarily large strings and callstacks and similar. I think that's better than this. |
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Even JSSE, the entirely AI-generated engine, gets this wrong: https://github.com/pmatos/jsse/blob/main/src/interpreter/builtins/iterators.rs. I guess it didn't read the spec... |
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(When I tried to suggest this on the 'changes' page, GitHub acted weird:) |
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Oh, thank you @jmdyck! |
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| 1. Repeat, while _remaining_ > 0, | ||
| 1. If _remaining_ ≠ +∞, then | ||
| 1. Set _remaining_ to _remaining_ - 1. | ||
| 1. Let _remaining_ be 𝔽(_integerLimit_). |
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Given the check above that _numLimit_ is either Infinity or a safe integer, what is the observable difference between doing all of these steps in the floats space vs mathematical values space?
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I don't think there is one. Just makes it very obvious to implementers that it's okay to do all of this with floats.
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The changes look correct.
I have a slight preference for keep using mathematical values for the take/drop counters, but we already have other two precedents for using floats:
Floats also have the minor advantage of not needing special handling for infinity (but we could also just define what infinite - 1 means for extended mathematical values).
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All known engines are currently non-compliant. If they use Numbers as counters today, they'll stop advancing at 253. So counters passed to user-provided callbacks will always be 253 once they reach that point and take/drop will behave as if Infinity was passed. With this change, internal counters are now Numbers, so the argument passed to user-provided callbacks is expected to repeat at 253 (matching implementations today). In addition, take/drop throw a RangeError on finite inputs ≥ 253.
See tc39/proposal-iterator-includes#12 for context.