Feature #17279: Allow a negative step in Range#step with a block

• Status: Assigned
• Assignee: matz (Yukihiro Matsumoto)
• Target version: -

[mrkn (Kenta Murata)]

Range#step prohibits a negative step when a block is given.

>> (6..3).step(-1) {|i| p i }
Traceback (most recent call last):
        5: from /home/mrkn/.rbenv/versions/2.7/bin/irb:23:in `<main>'
        4: from /home/mrkn/.rbenv/versions/2.7/bin/irb:23:in `load'
        3: from /home/mrkn/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/irb-1.2.4/exe/irb:11:in `<top (required)>'
        2: from (irb):1
        1: from (irb):1:in `step'
ArgumentError (step can't be negative)

But Range#step allows a negative step when it is called without a block. In this case, Range#step creates an ArithmeticSequence, and ArithmeticSequence#each can iterate with a negative step.

>> (6..3).step(-1).each {|i| p i }
6
5
4
3
=> ((6..3).step(-1))

I think the prohibition of a negative step in Range#step has already been meaningless, so it may be better to permit it for consistency.

[hsbt (Hiroshi SHIBATA)]

• Status changed from Open to Assigned

[zverok (Victor Shepelev)]

FWIW, my PR with adjusting Range#step behavior fixes this, too: https://github.com/ruby/ruby/pull/7444 (as part of the unificatioin/simplification process):

From the PR description:

Consistent support for negative step:

p (1..-10).step(-3).to_a 
#=> [1, -2, -5, -8] -- ArithmeticSequence backward iteration, on Ruby 3.3 and my code
(1..-10).step(-3) { p _1 }
# Ruby 3.3: step can't be negative (ArgumentError) -- inconsistent with ArithmeticSequence behavior
# my code: prints 1, -2, -5, -8, consistent with ArithmeticSequence