Feature #15747: `(..1).last(2)` should return array but raise TypeError

• Status: Rejected
• Assignee: -
• Target version: -

[wanabe (_ wanabe)]

Range#last with argument for beginless range shouldn't raise exception, should it?

$ ./miniruby -ve 'p (..1).last(2)'
ruby 2.7.0dev (2019-04-04 trunk 67430) [x86_64-linux]
Traceback (most recent call last):
	4: from -e:1:in `<main>'
	3: from -e:1:in `last'
	2: from -e:1:in `to_a'
	1: from -e:1:in `to_a'
-e:1:in `each': can't iterate from NilClass (TypeError)

Range#last without argument works fine.

$ ./miniruby -ve 'p (..1).last'
ruby 2.7.0dev (2019-04-04 trunk 67430) [x86_64-linux]
1

[mame (Yusuke Endoh)]

• Status changed from Open to Feedback

Thank you for playing with a beginless range.

In general, I think that an enumerating operation against a beginless range should raise an exception. It is also the case at Range#last with an argument. Note that the meaning of Range#last with no argument is fairly different from the version with a length argument. See #8739 and #15523.

That said, Range#last might be handled as a special case. I have no strong opinion about this, but an exception is still one of the most reasonable design choices.

1. It is difficult to reversely enumerate the elements of Range from last. AFAIK, there is no precedent.

2. (-Float::INFINITY .. 1).last(2) also raises a TypeError.

(-Float::INFINITY .. 1).last(2) #=> can't iterate from Float (TypeError)

[wanabe (_ wanabe)]

• Tracker changed from Bug to Feature
• ruby -v deleted (ruby 2.7.0dev (2019-04-04 trunk 67430) [x86_64-linux])
• Backport deleted (2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN)

Thank you to response.

mame (Yusuke Endoh) wrote:

In general, I think that an enumerating operation against a beginless range should raise an exception. It is also the case at Range#last with an argument. Note that the meaning of Range#last with no argument is fairly different from the version with a length argument. See #8739 and #15523.

Oh, I've missed #8739 and #15523. Thank you.

That said, Range#last might be handled as a special case. I have no strong opinion about this, but an exception is still one of the most reasonable design choices.

1. It is difficult to reversely enumerate the elements of Range from last. AFAIK, there is no precedent.

2. (-Float::INFINITY .. 1).last(2) also raises a TypeError.

(-Float::INFINITY .. 1).last(2) #=> can't iterate from Float (TypeError)

OK. I have no strong opinion, too.
I have realized this is a not "Bug". I change the tracker "Feature".

[wanabe (_ wanabe)]

• Status changed from Feedback to Rejected

Now, I think this proposal doesn't make sense.
So I reject it. thank you.