[ruby-core:107294] [Ruby master Feature#18408] Allow pattern match to set instance variables

Issue #18408 has been updated by Dan0042 (Daniel DeLorme).


Eregon (Benoit Daloze) wrote in #note-17:
> In practice it should rarely matter because if a clause isn't matched the code should obviously not look at variables not set by that clause.

I'm not sure I agree with that. When I have `if condition; x=42; end` I expect `x` to be nil if the branch was not taken. Not some undefined semi-random value. And sometimes I depend on that behavior in my code. I understand pattern matching doesn't work that way, and I understand why and how pattern matching works the way it does, but only because I read this thread. Otherwise I would assume the variable is only set when the pattern matches successfully.

So I had expected this to work and was fairly surprised that it doesn't:
```ruby
case [1,2,3,4]
in [*,a,1,*]
  puts "before one: #{a}"
in [*,b,2,*]
  puts "before two: #{b}"
in [*,c,3,*]
  puts "before three: #{c}"
else
  puts "no match"
end
#I expected a==nil since the first pattern didn't match, but instead it's a==3
if !a
  puts "nothing before one"
end
```

I don't know if the current behavior fits Matz' POLS, but it doesn't fit mine. (Not saying it necessarily should, but it's at least a data point for one person who finds this behavior surprising).


----------------------------------------
Feature #18408: Allow pattern match to set instance variables
https://bugs.ruby-lang.org/issues/18408#change-96191

* 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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