Bug #21117: Inconsistent behaviour between "_1" and "it" variables

• Status: Closed
• Assignee: prism
• Target version: -
• ruby -v: ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [arm64-darwin24]
• Backport: 3.1: DONTNEED, 3.2: DONTNEED, 3.3: WONTFIX, 3.4: DONE

[radarek (Radosław Bułat)]

I believe variables _1 and it should have consistent behaviour and the same as normal local variables. Here are inconsistencies:

# 1. Assigning new value
[1, 2, 3].each { |v| v = v + 1; p v } # works as expected
[1, 2, 3].each { it = it + 1; p it } # works as expected
[1, 2, 3].each { _1 = _1 + 1; p _1 } # SyntaxError

# 2. Using operators like += on them
[1, 2, 3].each { |v| v += 1; p v } # works as expected
[1, 2, 3].each { it += 1; p it } # SyntaxError but I expected it to work correctly after 1st point
[1, 2, 3].each { _1 += 1; p _1 } # works, which is inconsistent with 1 point
# however, this one does not work
[1, 2, 3].map { _1 += 1 } # runtime error is raised: undefined method '+' for nil (NoMethodError)

If both _1 and it are advertised as block local variables then I would expect that overwriting works correctly, both using expressions like _1 = _1 + 1 and _1 *= 2.

[mame (Yusuke Endoh)]

I believe variables _1 and it should have consistent behaviour

Assignments to it are currently allowed for compatibility considerations, but I believe they may be prohibited in the future.
BTW, _1 and it are different. _1 cannot be written both inside and outside the block, but it is intentionally allowed.

and the same as normal local variables.

No, in my opinion. They are special parameters and should not be considered as just normal local variables. #20965 #21049

[tompng (tomoya ishida)]

Prism and parse.y parses these two code in the example differently.

[1, 2, 3].each { it = it + 1; p it }
[1, 2, 3].each { it += 1; p it }

I think it was designed not to break old code that uses local variable it, so maybe it should be parsed as same as the code below, just like parse.y does.

[1, 2, 3].each { it2 = it2 + 1; p it2 }
[1, 2, 3].each { it2 += 1; p it2 }

[tompng (tomoya ishida)]

• Status changed from Open to Assigned
• Assignee set to prism

These two should be SyntaxError, and is actually SyntaxError with --parser=parse.y

[1, 2, 3].each { _1 += 1; p _1 }
[1, 2, 3].map { _1 += 1 }

Some difference between it and _1 is by design, but the inconsistencies raised in this issue are caused by difference between parse.y and Prism and should be fixed.

[kddnewton (Kevin Newton)]

• Status changed from Assigned to Closed

Applied in changeset git|127325a4bad409ee5da91084fac768934a8fd9e3.

---

[ruby/prism] No writing to numbered parameters

Fixes [Bug #21117]

https://github.com/ruby/prism/commit/19d4bab5a0

[kddnewton (Kevin Newton)]

• Backport changed from 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN to 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: REQUIRED

[k0kubun (Takashi Kokubun)]

• Backport changed from 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: REQUIRED to 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: DONE

ruby_3_4 d3fc56dcfa7b408cc3b6788efad36fd8df3e55da merged revision(s) 127325a4bad409ee5da91084fac768934a8fd9e3.

[nagachika (Tomoyuki Chikanaga)]

• Backport changed from 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: DONE to 3.1: DONTNEED, 3.2: DONTNEED, 3.3: WONTFIX, 3.4: DONE

I decided to mark "WONTFIX" for ruby_3_3. Introducing syntax error might cause unavoidable problem in real world applications.