[ruby-core:71810] [Ruby trunk - Feature #11737] Pass in expression to then block in `case expression`

[s.wanabe@gmail.com]

Issue #11737 has been updated by _ wanabe.


Nobuyoshi Nakada wrote:
> It reminded me a rejected proposal:
> 
> ~~~ruby
> case expr
> when matcher => result
>   ...
> end
> ~~~

Is the proposal [ruby-dev:17615]?
Or other?

----------------------------------------
Feature #11737: Pass in expression to then block in `case expression`
https://bugs.ruby-lang.org/issues/11737#change-55214

* Author: Daniel P. Clark
* Status: Open
* Priority: Normal
* Assignee: 
----------------------------------------
Ruby's `case <expression>` scenario allows the expression to be tested in the `when` test with `:===`, but in the then block the expression is not accessible.  Only if the expression has been assigned to a variable beforehand can it be checked.

~~~ruby
case 4
when ->i{ puts :when; true}
  ->i{ puts i}
else
  :foo
end
# when
#  => #<Proc:0x00000000d91e58@(irb):16 (lambda)> 

case 4
when ->i{ puts :when; true}
  puts _
else
  :foo
end
# when
# #<Proc:0x00000000d91e58@(irb):16 (lambda)>
#  => nil 

case 4
when 4
then _
end
#  => nil

case 4
when 4
then ->i{puts i}
end
#  => #<Proc:0x000000015f9be0@(irb):36 (lambda)> 
~~~

If some one wanted to give an expression after case that wasn't assigned to a variable then there is no access to it in the then block.

I suggest assigning the expression to the `_` variable during a case/when/then scenario.  Here's a rather contrived example use case.

~~~ruby
case Enumerator.new do |y| y << 1; y << 2; y << 3; end
when ->e{ 2.times e.next; true}
then _.peek
end == 3
~~~



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