[ruby-core:115629] [Ruby master Feature#18980] Re-reconsider numbered parameters: `it` as a default block parameter

Issue #18980 has been updated by k0kubun (Takashi Kokubun).

Description updated
Status changed from Open to Assigned
Assignee set to k0kubun (Takashi Kokubun)
Target version set to 3.4

In today's Developers Meeting, @matz accepted to warn `it` in Ruby 3.3 and add `it` in Ruby 3.4. Quote from the meeting notes:

> * matz: accept `it` on Ruby 3.4.
>   * ruby 3.3 will warn and ruby 3.4 will use the new semantics

I also copied the discussed specification to the ticket description. In Ruby 3.3, `it` should be warned only when `it` will behave like `_1` in Ruby 3.4.

----------------------------------------
Feature #18980: Re-reconsider numbered parameters: `it` as a default block parameter
https://bugs.ruby-lang.org/issues/18980#change-105563

* Author: k0kubun (Takashi Kokubun)
* Status: Assigned
* Priority: Normal
* Assignee: k0kubun (Takashi Kokubun)
* Target version: 3.4
----------------------------------------
## Problem
Numbered parameters (`_1`, `_2`, ...) look like unused local variables and I don't feel motivated to use them, even though I need this feature very often and always come up with `_1`.

```rb
[1, 2, 3].each { puts _1 }
```

I have barely used it in the last 2~3 years because it looks like a compromised syntax. I even hesitate to use it on IRB.

<details>

<summary>Why I don't use <code>_1</code></summary>

I'm not clever enough to remember the order of parameters. Therefore, when a block has multiple parameters, I'd always want to name those parameters because which is `_1` or `_2` is not immediately obvious. Thus I would use this feature only when a block takes a single argument, which is actually pretty common. 

If I use `_1`, it feels like there might be a second argument, and you might waste time to think about `_2`, even if `_2` doesn't exist, which is a cognitive overhead. If you use `it`, it kinda implies there's only a single argument, so you don't need to spend time remembering whether `_2` exists or not. It is important for me that there's no number in `it`.

</details>

## Proposal

* Ruby 3.3: Warn `it` method calls without a receiver, arguments, or a block.
* Ruby 3.4: Introduce `it` as follows.

### Specification

```rb
[1, 2, 3].each { puts it }
```

`it`s behavior should be as close to `_1` as possible. `it` should treat array arguments in the same way as `_1`. `it` doesn't work in a block when an ordinary parameter is defined. `it` is implemented as a special case of `getlocal` insn, not a method. `it` without an argument is considered `_1` or a normal local variable if defined. `it` is considered a method call only when it has any positional/keyword/block arguments.

<details>

<summary>Full specification</summary>

```ruby
# Ruby 3.4
def foo
  it #=> method call
  _1 #=> method call
  1.times do
    p it #=> 0
    it = "foo"
    p it #=> "foo"

    p _1 #=> 0
    _1 = "foo" # Syntax Error
    p _1 #=> N/A

    p foo #=> method call
    foo = 1
    p foo #=> local var

    it "foo" do # method call (rspec)
    end
  end
  1.times do ||
    p _1 # Syntax Error
    it # method call
  end
  1.times do
    ["Foo"].any? {|| it } # method call
  end
  yield_1_and_2 do # yield 1, 2
    p _1 #=> 1
    p it #=> 1
  end
  yield_ary do # yield [1, 2]
    p _1 #=> [1, 2]
    p it #=> [1, 2]
  end
  1.times do
    p [_1, it] # Syntax Error
    p [_2, it] # Syntax Error
  end
end
```

</details>

## Past discussions
* [Feature #4475] default variable name for parameter: Proposed `it`, and merged as `@1`.
  * 2019/03/13: [DevelopersMeeting20190311Japan](https://docs.google.com/document/d/e/2PACX-1vTUCmj7aUdnMAdunG0AZo0AdWK-9jvfXcB7DWYmzGtmPc0IuIPGn7eLARoR5tBd6XUUB08W-hH74k-T/pub)
  * 2019/04/17: [DevelopersMeeting20190417Japan](https://docs.google.com/document/d/1hw6Xca8arG6b0V63zvWnNEtxIjEjEVzS10KXGhzZpI8/pub)
  * 2019/04/20: [Ruby Committers vs the World](https://youtu.be/5eAXAUTtNYU?t=3118)
* [Feature #15723] Reconsider numbered parameters: Renamed `@1` to `_1`.
  * 2019/08/29: [DevelopersMeeting20190829Japan](https://docs.google.com/document/d/1XypDO1crRV9uNg1_ajxkljVdN8Vdyl5hnz462bDQw34/edit?usp=sharing)
* [Feature #15897] `it` as a default block parameter: Proposed `it`, and got closed because `_1` was merged.
* [Feature #18980] Re-reconsider numbered parameters: (this ticket)
  * 2022/09/08: [Ruby Committers vs the World](https://youtu.be/ajm3lr6Y9yE?si=yyWGiZHoUWqZiTaS&t=2920)

### Compatibility

`it` has not necessarily been rejected by Matz; he just said [it's difficult to keep compatibility](https://bugs.ruby-lang.org/issues/4475#note-6) and [`it` or `this` _could_ break existing code](https://bugs.ruby-lang.org/issues/15723#note-2). It feels like everybody thinks `it` is the most beautiful option but is not sure if `it` breaks compatibility. But, in reality, does `it`?

The following cases have been discussed:

* `it` method, most famously in RSpec: You almost always pass a positional and/or block argument to RSpec's `it`, so the conflict is avoided with my proposal. You virtually never use a completely naked `it` ([comment](https://bugs.ruby-lang.org/issues/15897#note-29)).
* `it` local variable: With the specification in my proposal, the existing code can continue to work if we consider `it` as a local variable when defined.

With the specification in my proposal, existing code seems to break if and only if you call a method `#it` without an argument. But it seems pretty rare (reminder: a block given to an RSpec test case is also an argument). It almost feels like people are too afraid of compatibility problems that barely exist or have not really thought about options to address them.

Also, you could always experiment with just showing warnings, which doesn't break any compatibility. Even if it takes 2~3 years of a warning period, I'd be happy to use that in 3 years.

### Confusion
We should separately discuss incompatible cases and "works but confusing" cases. Potential confusion points:

* RSpec's `it "tests something" do ... end` vs `it` inside the `do ... end`
* `it` could be a local variable or `_1`, depending on the situation

My two cents: You'd rarely need to write `it` directly under RSpec's `it` block, and you would just name a block argument for that case. In a nested block under a test case, I don't think you'd feel `it` is RSpec's. When you use a local variable `it = 1`, you'd use the local variable in a very small scope or few lines because otherwise, it'd be very hard to figure out what the local variable has anyway. So you'd likely see the assignment `it = 1` near the use of the local variable and you could easily notice `it` is not `_1`. If not, such code would be confusing and fragile even without this feature. The same applies when `it` is a method/block argument.

I believe it wouldn't be as confusing as some people think, and you can always choose to not use `it` in places where `it` is confusing.



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