[ruby-core:101262] [Ruby master Feature#17371] Reintroduce `expr in pat`

Issue #17371 has been reported by ktsj (Kazuki Tsujimoto).

----------------------------------------
Feature #17371: Reintroduce `expr in pat`
https://bugs.ruby-lang.org/issues/17371

* Author: ktsj (Kazuki Tsujimoto)
* Status: Open
* Priority: Normal
----------------------------------------
How about reintroducing `expr in pat`, as akr-san proposed in DevelopersMeeting20201026Japan.

The difference between `expr => pat` and new `expr in pat` is the return value of the expression.

```
# expr => pat
0 => a #=> void (succeeded)
0 => 1 #=> NoMatchingPatternError (failed)

# expr in pat
0 in a #=> true (succeeded)
0 in 1 #=> false (failed)
```

Motivation and use cases are described at [Feature #15865]. 

I pointed out that there is a concern that in this specification a user might overlook the pattern did not match, and changed the return value at [Feature #16355]. 
However, now we already have new "rightward assignment style" pattern matching syntax, so we can say that this problem is solved.

If `expr in pat` is accepted, I also propose that the return value of `expr => pat` on a successful match be the left-hand side value.
Because, in this case, it becomes more clear that the use of `expr => pat` is assignment.




-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:[e-mail]?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>