Bug #18837: Not possible to evaluate expression with numbered parameters in it

• Status: Rejected
• Assignee: -
• Target version: -
• Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN

[hurricup (Alexandr Evstigneev)]

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.

[mame (Yusuke Endoh)]

• Related to Feature #15915: `@1` cannot be achieved in meta-programming added

[mame (Yusuke Endoh)]

I talked a bit with some of the committers.

@ko1 (Koichi Sasada) 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 (Kazuhiro NISHIYAMA) 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

[mame (Yusuke Endoh)]

• Status changed from Open to Rejected

We discussed this issue at the dev meeting. @matz (Yukihiro Matsumoto) agreed that it is difficult to support this for the reasons I said in #note-2.

[alanwu (Alan Wu)]

• Related to Feature #19137: Numbered parameters are not made available to IRB. added

[nobu (Nobuyoshi Nakada)]

• Has duplicate Bug #20308: `it` and numbered parameters do not work in `eval` added