IRB `__` is undefined about rubyref

[zverok]

@roman-yarovoy It is a funny question. Digging deeep in IRB code have provided me with an answer:

IRB.conf[:EVAL_HISTORY] = 100

in your .irbrc will enable the feature and make __ variable available.
You can check that full IRB docs are completely silent on this little detail (which probably suggests we can do a PR to Ruby ;)), but has some other interesting things. Those things are put in Reference's Mastering IRB section, which also has the link to tranditional IRB docs.

So... Except for "let's fix IRB docs in Ruby repo", I dunno what to say :)

[zverok]

Except for "let's fix IRB docs in Ruby repo", I dunno what to say :)

Well, so I did it for 2.7: https://ruby-doc.org/stdlib-2.7.0/libdoc/irb/rdoc/IRB.html#module-IRB-label-Session+variables
And now the book also contains an explanation: https://rubyreferences.github.io/rubyref/intro/irb.html#session-variables