[ruby-core:104149] [Ruby master Feature#15567] Allow ensure to match specific situations

Issue #15567 has been updated by Eregon (Benoit Daloze).


Thanks for the link.

Is `throw` used for other things than `redirect`?

If `redirect` would use a regular exception, and for instance pass `[]` as the backtrace, it would probably be as efficient, but then gives the possibility to differentiate for example with `rescue => e; e&.should_commit?` or so.
`throw`/`catch` has AFAIK no way to communicate any information, so there is no way to know if the `throw` is meant as a convenience return like for `redirect` or as some kind of bailout which should not commit the transaction.
It can't even be differentiated from return/break so it seems a bad idea to use `throw` in the first place.

It feels weird that Timeout uses `throw` and not `raise`, and it seems counter-intuitive (everyone knows `Timeout.timeout {}` raises Timeout::Error, and exception, `throw` is unexpected)
It also adds quite some complexity: https://github.com/ruby/timeout/blob/4893cde0eda321448a1a86487ac9b571f6c35727/lib/timeout.rb#L29-L50
Does anyone know what's the point of that?

What I could find is https://github.com/ruby/timeout/commit/238c003c921e6e555760f8e96968562a622a99c4
and https://bugs.ruby-lang.org/issues/8730

IMHO `rescue Exception` without re-raise is always the bug.

----------------------------------------
Feature #15567: Allow ensure to match specific situations
https://bugs.ruby-lang.org/issues/15567#change-92321

* 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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