Feature #18551: Make `Range#reverse_each` to raise an exception if endless

• Status: Closed
• Assignee: -
• Target version: -

[kyanagi (Kouhei Yanagita)]

https://github.com/ruby/ruby/pull/5498

Currently, Range#reverse_each for an endless range never returns.

% ruby -e '(1..).reverse_each { }'
# never return ...

(This is because Enumerable#reverse_each tries #to_a and #to_a for an endless range comes into an infinite loop.)

I think Range#reverse_each for an endless range should raise an exception, similar to Range#each for a beginless range.

% ruby -e '(..1).each { }' 
-e:1:in `each': can't iterate from NilClass (TypeError)
	from -e:1:in `<main>'

[Eregon (Benoit Daloze)]

There is no Range#reverse_each currently, only Enumerable#reverse_each, and Enumerable can't know about this.
But maybe it makes sense to have Range#reverse_each for this and maybe also for faster reverse_each on Numeric Ranges?
OTOH Range operations that work differently for numerics tend to make Range even more messy when it comes to non-numeric values (more differences in behavior, can't reverse_each efficiently on Date/Time/etc).

[Eregon (Benoit Daloze)]

I checked the PR, seems a useful change to me.

[Dan0042 (Daniel DeLorme)]

Yes, this is a useful change. Although wouldn't it be better to prevent #to_a instead of only #reverse_each ?

Actually there's quite a lot of Enumerable methods that would be better raising an exception for endless ranges.
For example: (1..).select{ true } results in infinite loop and memory consumption.
The only way out is break, but then there's no point using select since it can't return a result.
The same applies to reject, partition, sort, zip, uniq, tally, etc...

[kyanagi (Kouhei Yanagita)]

Eregon (Benoit Daloze) wrote in #note-1:

OTOH Range operations that work differently for numerics tend to make Range even more messy when it comes to non-numeric values (more differences in behavior, can't reverse_each efficiently on Date/Time/etc).

I posted another ticket for performance issue: #18515
I wrote my thoughts there.

[nobu (Nobuyoshi Nakada)]

• Subject changed from Make Range#reverse_each to raise an exception if endless to Make `Range#reverse_each` to raise an exception if endless

[mame (Yusuke Endoh)]

In the previous dev meething, matz accepted this. I will merge the PR

[kyanagi (Kouhei Yanagita)]

• Status changed from Open to Closed

Applied in changeset git|458d079166ce7344c1d44b2403ff963b8c68d7d8.

---

[DOC] Add a mention of [Feature #18551] to NEWS.md