From: "Eregon (Benoit Daloze)" <noreply@...>
Date: 2021-10-21T10:24:33+00:00
Subject: [ruby-core:105727] [Ruby master Feature#18083] Capture error in ensure block.

Issue #18083 has been updated by Eregon (Benoit Daloze).


ko1 (Koichi Sasada) wrote in #note-20:
> Sorry, my proposal is introducing dynamic scope, not syntactical scope like:

Ah, I misunderstood.

Then I think deprecating `$!` is the right way, since that seems the only way to be able to analyze decently if an exception's backtrace is needed and do the important optimizations I mentioned in earlier comments.
This would help a lot for making Ruby exceptions faster.
I'm pretty sure @headius would agree since he did a similar optimization, but currently quite limited because of `$!` being accessible inside calls.

And of course it would avoid the problem of `$!` having confusing semantics, by telling users to use `rescue => exc` instead which is clear, simple and clean.

If needed maybe we can avoid warning for `rescue $!` since that's fairly harmless, but probably that's very rarely used in library/app code (i.e., only used in REPL and for trying things).

---

We could also warn whenever `$!` is not used syntactically in the `rescue` clause, but IMHO better deprecate it entirely.

----------------------------------------
Feature #18083: Capture error in ensure block.
https://bugs.ruby-lang.org/issues/18083#change-94226

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
----------------------------------------
As discussed in https://bugs.ruby-lang.org/issues/15567 there are some tricky edge cases.

As a general model, something like the following would be incredibly useful:

``` ruby
begin
 ...
ensure => error
  pp "error occurred" if error
end
```

Currently you can get similar behaviour like this:

``` ruby
begin
  ...
rescue Exception => error
  raise
ensure
  pp "error occurred" if error
end
```

The limitation of this approach is it only works if you don't need any other `rescue` clause. Otherwise, it may not work as expected or require extra care. Also, Rubocop will complain about it.

Using `$!` can be buggy if you call some method from `rescue` or `ensure` clause, since it would be set already. It was discussed extensively in https://bugs.ruby-lang.org/issues/15567 if you want more details.



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