Issue #15567 has been updated by ioquatix (Samuel Williams).


Thanks for the discussion. I'll try to summarise my position briefly.

> Maybe we should open a new issue so that Timeout.timeout uses an Exception and not throw?

I agree Timeout should raise an exception internally and we should change this. Happy to work on a PR with you all. I am very strongly in favour of this. If we decide to do this, we should let Rails team know about our intended change so they don't end up working themselves into a corner.

> Yes, it's used anytime you want to return a response immediately. I just used redirect as an example.

Yes, I also do this and agree with Jeremy's position here. While I do respect everyone's input, to me, exception = abort transaction, throw/break/next = commit.

Therefore, I do feel strongly that we need some precise way to capture the exception as we go through the ensure block. The previous suggestion makes sense to me, as in:

```
begin
  raise "Boom"
ensure => exception
end
```

This actually makes it very easy to do the correct thing. The alternatives are tricky to get right:

```
failed = false
begin
rescue Exception # "Exception" usually missing... so buggy.
  # error
  failed = true
  raise
ensure
  if failed
    abort
  else
    commit
  end
end
```

I'm pretty sure I've got bugs in my code due to this, due to mis-understanding the flow control. So, I think we should make this easier for developers to get right.

----------------------------------------
Feature #15567: Allow ensure to match specific situations
https://bugs.ruby-lang.org/issues/15567#change-92324

* Author: ioquatix (Samuel Williams)
* Status: Rejected
* Priority: Normal
* Assignee: ioquatix (Samuel Williams)
----------------------------------------
There are some situations where `rescue Exception` or `ensure` are not sufficient to correctly, efficiently and easily handle abnormal flow control.

Take the following program for example:

```
def doot
	yield
ensure
	# Did the function run to completion?
	return "abnormal" if $!
end

puts doot{throw :foo}
puts doot{raise "Boom"}
puts doot{"Hello World"}

catch(:foo) do
	puts doot{throw :foo}
end
```

Using `rescue Exception` is not sufficient as it is not invoked by `throw`.

Using `ensure` is inefficient because it's triggered every time, even though exceptional case might never happen or happen very infrequently.

I propose some way to limit the scope of the ensure block:

```
def doot
	yield
ensure when raise, throw
	return "abnormal"
end
```

The scope should be one (or more) of `raise`, `throw`, `return`, `next`, `break`, `redo`, `retry` (everything in `enum ruby_tag_type` except all except for `RUBY_TAG_FATAL`).

Additionally, it might be nice to support the inverted pattern, i.e.

```
def doot
	yield
ensure when not return
	return "abnormal"
end
```

Inverted patterns allow user to specify the behaviour without having problems if future scopes are introduced.

`return` in this case matches both explicit and implicit.




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