[ruby-core:120113] [Ruby master Bug#20930] Different semantics for nested `it` and `_1`

Issue #20930 has been updated by k0kubun (Takashi Kokubun).


> I think _1 semantics are slightly better, at least _1 behaves like a normal local variable declared in the outer block then.

Yeah but `_1` isn't declared in the outer block (or anywhere), so `_1` doesn't necessarily need to behave like a local variable declared in the outer block. You could also say "_1 should behave like a normal local variable declared in the inner block", and it seems as plausible as what you said.

> The other problem is how to interpret it. I think Ruby master's it behavior is good. I have experienced mistakes of a name conflict issue of a local variable in a different scope, but not as often. However, just adding a read from it outside of a block only changes the meaning of inner it, which I think would increase the frequency of mistakes very much.

I agree with @mame 's opinion.

When you have nested loops, you would use the iterator of the inner-most block most often. The current behavior of `it` seems to have more use cases than the one of `_1`. Ruby has prioritized solving real-world use cases over just making existing features consistent, so I don't think `it` needs to be consistent with `_1` here.

----------------------------------------
Bug #20930: Different semantics for nested `it` and `_1`
https://bugs.ruby-lang.org/issues/20930#change-110861

* Author: Eregon (Benoit Daloze)
* Status: Open
* ruby -v: ruby 3.4.0dev (2024-12-04T19:29:24Z master 3c91a1e5fd) +PRISM [x86_64-linux]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
```
$ ruby -ve '[1].each { p it; [5].each { p it } }'
ruby 3.4.0dev (2024-12-04T19:29:24Z master 3c91a1e5fd) +PRISM [x86_64-linux]
1
5
$ ruby -ve '[1].each { p _1; [5].each { p _1 } }'
ruby 3.4.0dev (2024-12-04T19:29:24Z master 3c91a1e5fd) +PRISM [x86_64-linux]
1
1
```
Notice the inconsistency, `it` uses the innermost block, `_1` uses the outermost block.
I think `_1` semantics are slightly better, at least `_1` behaves like a normal local variable declared in the outer block then.

Note that on 3.3.5 it was forbidden to nest `_1` which I think might be good for clarity/avoiding ambiguity:
```
$ ruby -ve '[1].each { p _1; [5].each { p _1 } }'
ruby 3.3.5 (2024-09-03 revision ef084cc8f4) [x86_64-linux]
-e:1: numbered parameter is already used in
-e:1: outer block here
[1].each { p _1; [5].each { p _1 } }
ruby: compile error (SyntaxError)
```

---

As an aside, mixing `_1` and `it` is allowed, I think this is [good](https://bsky.app/profile/eregon.bsky.social/post/3lcg4fjcf7225), they are different things so there is not much confusion there:
```
$ ruby -ve '[1].each { p _1; [5].each { p it } }'
ruby 3.4.0dev (2024-12-04T19:29:24Z master 3c91a1e5fd) +PRISM [x86_64-linux]
1
5
$ ruby -ve '[1].each { p it; [5].each { p _1 } }'
ruby 3.4.0dev (2024-12-04T19:29:24Z master 3c91a1e5fd) +PRISM [x86_64-linux]
1
5
```



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