Feat/Bug: Set the value of `_` (underscore) to the value of the last ruby expression

[joe-sharp]

Your proposal

Pry and IRB have a feature in which _ is always set the the value of the last expression. While I realize it may not make sense to do this with debug commands, it would be nice if the feature was there at least for Ruby expressions. The current behavior seems to be setting _ to the last error, but seemingly only for debug commands like ls. This is kind of a bug in the irb_console since IRB has this feature normally but it stops working after entering debug mode.

Additional context

Shell session:

irb(#<Foobar:0x000000011efeb9f0>):001> 3+3
=> 6

irb(#<Foobar:0x000000011efeb9f0>):002> puts _
6
=> nil

irb(#<Foobar:0x000000011efeb9f0>):003> 4+4
=> 8

irb(#<Foobar:0x000000011efeb9f0>):004> info
(rdbg:irb) info
%self = #<Foobar:0x000000011efeb9f0 @foo="bar">
@foo = "bar"

irb:rdbg(#<Foobar:0x000000011efeb9f0>):002> puts _

nil

irb:rdbg(#<Foobar:0x000000011efeb9f0>):003> 4+4
8

irb:rdbg(#<Foobar:0x000000011efeb9f0>):004> puts _

nil
irb:rdbg(#<Foobar:0x000000011efeb9f0>):005> 4+4
8

irb:rdbg(#<Foobar:0x000000011efeb9f0>):006> info
%self = #<Foobar:0x000000011efeb9f0 @foo="bar">
_ = nil
@foo = "bar"

irb:rdbg(#<Foobar:0x000000011efeb9f0>):007> ls Bar
Traceback (most recent call last):
	24: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/session.rb:212:in `block in activate'
	23: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/session.rb:254:in `session_server_main'
	22: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/session.rb:365:in `process_event'
	21: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/session.rb:403:in `wait_command_loop'
	20: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/session.rb:403:in `loop'
	19: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/session.rb:404:in `block in wait_command_loop'
	18: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/session.rb:444:in `wait_command'
	15: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/session.rb:1981:in `intercept_trap_sigint'
	 4: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/irb_integration.rb:9:in `evaluate'
(irb):7:in `set_foo': uninitialized constant Foobar::Bar (NameError)`

irb:rdbg(#<Foobar:0x000000011efeb9f0>):008> puts _
uninitialized constant Foobar::Bar
nil

irb:rdbg(#<Foobar:0x000000011efeb9f0>):009> ls Bar
Traceback (most recent call last):
	24: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/session.rb:212:in `block in activate'
	23: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/session.rb:254:in `session_server_main'
	22: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/session.rb:365:in `process_event'
	21: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/session.rb:403:in `wait_command_loop'
	20: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/session.rb:403:in `loop'
	19: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/session.rb:404:in `block in wait_command_loop'
	18: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/session.rb:444:in `wait_command'
	15: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/session.rb:1981:in `intercept_trap_sigint'
	 4: from /Users/joesharp/.asdf/installs/ruby/2.7.8/lib/ruby/gems/2.7.0/gems/debug-1.9.1/lib/debug/irb_integration.rb:9:in `evaluate'
(irb):9:in `set_foo': uninitialized constant Foobar::Bar (NameError)`

irb:rdbg(#<Foobar:0x000000011efeb9f0>):010> info
%self = #<Foobar:0x000000011efeb9f0 @foo="bar">
_ = #<NameError: uninitialized constant Foobar::Bar>
@foo = "bar"

irb:rdbg(#<Foobar:0x000000011efeb9f0>):011> Bar.methods
eval error: uninitialized constant Foobar::Bar
  (rdbg)/scratch.rb:1:in `set_foo'`
nil

irb:rdbg(#<Foobar:0x000000011efeb9f0>):012> info
%self = #<Foobar:0x000000011efeb9f0 @foo="bar">
_ = nil
@foo = "bar"

irb:rdbg(#<Foobar:0x000000011efeb9f0>):013> Bar.methods
eval error: uninitialized constant Foobar::Bar
  (rdbg)/scratch.rb:1:in `set_foo'`
nil

irb:rdbg(#<Foobar:0x000000011efeb9f0>):014> puts _

nil

[joe-sharp]

Please let me know if the bug portion of this (_ stops working in IRB when entering debug mode) should be filed as a separate issue. Thanks!

[joe-sharp]

Additionally, please note in the "Session Variables" section of https://docs.ruby-lang.org/en/2.2.0/IRB.html#module-IRB-label-Commands that IRB also supports __ (double underscore) and __[line_no] when configured to do so. It always supports the single underscore for the last value though.

[st0012]

Please let me know if the bug portion of this (_ stops working in IRB when entering debug mode) should be filed as a separate issue. Thanks!

Yes please open a separate issue in IRB, thank you!