[ruby-core:94739] [Ruby master Feature#12543] explicit tail call syntax: foo() then return

Issue #12543 has been updated by duerst (Martin Dst).


mame (Yusuke Endoh) wrote:
> I'm strongly against "ON by default".  It makes the backtrace difficult to understand.  Consider the following program:


> If tail-call optimization is used by default, it will print:
> 
> ```
> Traceback (most recent call last):
> 	1: from test.rb:9:in `<main>'
> test.rb:2:in `foo': unhandled exception
> ```

This should be changed to something like:
```
Traceback (most recent call last):
      1: from test.rb:9:in `<main>'
      [some frames omitted due to tail call optimization, use --tail-call-optimization-off for more details]
test.rb:2:in `foo': unhandled exception
```
Of course, the exact name of the exception and the wording of the message can still be improved. Implementation should be easy, just set a flag on the stack frame above the one that is eliminated by the tail call optimization.

> The frame of `bar` is removed due to tail-call optimization, so the debugger must guess how it reached at Line 2 from Line 9.

Guessing is of course not prohibited, but better use the option to get the full trace.

> This issue would be incredibly difficult when multiple frames are omitted.  It would be not so rare on practical programs.  I believe that "easy to debug" is one of the most important properties in Ruby.

I agree that "easy to debug" is important for Ruby. But I don't think my proposal makes debugging very difficult.

----------------------------------------
Feature #12543: explicit tail call syntax: foo() then return
https://bugs.ruby-lang.org/issues/12543#change-81357

* Author: mame (Yusuke Endoh)
* Status: Assigned
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 
----------------------------------------
How about introducing a new syntax for tail call?

```
def foo()
  foo()
end
foo() #=> stack level too deep
```

```
def bar()
  bar() then return
end
bar() #=> infinite loop
```

* no new keyword (cf. `goto foo()`)
* no conflict with any existing syntax
* an experimental patch is available (attached)
* no shift/reduce nor reduce/reduce conflict in parse.y

-- 
Yusuke Endoh <[e-mail]>

---Files--------------------------------
then_return.patch (9.18 KB)


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