[ruby-core:107148] [Ruby master Feature#18408] Allow pattern match to set instance variables

Issue #18408 has been updated by palkan (Vladimir Dementyev).


ktsj (Kazuki Tsujimoto) wrote in #note-8:
> If assignment to instance variables (or variables other than local variables) is allowed in pattern matching, there are problems such as:
> 
> 1. The value during pattern matching can be observed by other threads
> 2. The value of the instance variable will be undefined when pattern matching fails (ref: https://github.com/ruby/ruby/blob/v3_1_0/doc/syntax/pattern_matching.rdoc#label-Appendix+B.+Some+undefined+behavior+examples)

In this context, what is the difference between the current approach and the new syntax:

```ruby
case 1
in a if false
in Integer => b
end

# User can do this and result in UB
@a = a
@b = b

# vs.

case 1
in @a if false
in Integer => @b
end
```

?

I understand, that these are too synthetic examples, and new syntax could make it a bit more likely to hit UB; however, we already have such edge cases, even documented, and I think the advantages of adding such syntax overcome the shortcomings.


> However, in this case, the patterns available for normal pattern matching and right assignment will be different so I am negative about this change.

Totally agree.




----------------------------------------
Feature #18408: Allow pattern match to set instance variables
https://bugs.ruby-lang.org/issues/18408#change-95998

* Author: Dan0042 (Daniel DeLorme)
* Status: Assigned
* Priority: Normal
* Assignee: ktsj (Kazuki Tsujimoto)
----------------------------------------
I expected this to work:

```ruby
42 => @v
```

But instead it raises "syntax error, unexpected instance variable"

Is this intentional?



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