From: "mame (Yusuke Endoh)" <noreply@...>
Date: 2022-07-20T01:58:30+00:00
Subject: [ruby-core:109253] [Ruby master Bug#18837] Not possible to evaluate expression with numbered parameters in it

Issue #18837 has been updated by mame (Yusuke Endoh).


I talked a bit with some of the committers.

@ko1 said that it is difficult to retrieve `_1` afterwards due to the current implementation limitation, since the actual arguments may be dropped by optimization when a block that does not accept formal arguments is called.

Also, @znz pointed out that the meta-programming API to read out only `_1` is somewhat unreasonable, since the meaning of `_1` depends on the presence of _2.

```
{ 1 => "A" }.each do
  p _1 #=> [1, "A"]
end

{ 1 => "A" }.each do
  p _1 #=> 1
  _2
end
```


----------------------------------------
Bug #18837: Not possible to evaluate expression with numbered parameters in it
https://bugs.ruby-lang.org/issues/18837#change-98385

* Author: hurricup (Alexandr Evstigneev)
* Status: Open
* Priority: Normal
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
I presume it's not really a bug, but design flaw, but this is really frustrating. Use case is - debugger.

Here is script with expected behavior:
```
def dumper(bnd)
  puts bnd.local_variable_get 'i'
  puts bnd.eval 'i * 10'
end

[1,2].each { |i| dumper(binding) }
```

And this one attempts to do the same with numbered params:
```
def dumper(bnd)
  puts bnd.local_variable_get('_1')
  puts bnd.eval '_1 * 10'
end

[1,2].each do
  some = _1  # without this line even local_variable_get won't work, still it may be in any place of block
  dumper(binding)
end
```

But `eval` wont ever work and this necessity for using `_1` so binding could see it may be confusing as well. 



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