[ruby-core:104131] [Ruby master Feature#15567] Allow ensure to match specific situations

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


I suspect that there is existing buggy code which has ensure blocks and checks `$!` which fails in the case I mentioned.

I think the ensure block should be able to capture the current exception without using a global.

I like the proposed syntax:

```
begin
  ...
ensure => exception
  if exception
    abort
  else
    commit
  end
end
```

It allows existing code that uses `if $!` to be retrofitted to do the correct thing.

----------------------------------------
Feature #15567: Allow ensure to match specific situations
https://bugs.ruby-lang.org/issues/15567#change-92303

* 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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