bug#74870: cl-labels and cl-flet don't create named blocks

bug#74870: cl-labels and cl-flet don't create named blocks

[Jan Jouleodov via Bug reports for GNU Emacs, the Swiss army knife of text editors]

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In ANSI CL, these should create a named CL:BLOCK, allowing one to do a local return via CL:RETURN-FROM. In Emacs, this is not the case, resulting in an uncaught exception.

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bug#74870: cl-labels and cl-flet don't create named blocks

[Eli Zaretskii]

> Date: Fri, 13 Dec 2024 23:30:41 +0000
> From:  Jan Jouleodov via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> In ANSI CL, these should create a named CL:BLOCK, allowing one to do a local return via
> CL:RETURN-FROM. In Emacs, this is not the case, resulting in an uncaught exception.

I'm guessing this is a documentation bug, in that this particular
aspect of CL is not emulated by cl-labels.

Stefan, am I right?

bug#74870: cl-labels and cl-flet don't create named blocks

[Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors]

>> In ANSI CL, these should create a named CL:BLOCK, allowing one to do a local return via
>> CL:RETURN-FROM. In Emacs, this is not the case, resulting in an uncaught exception.
> I'm guessing this is a documentation bug, in that this particular
> aspect of CL is not emulated by cl-labels.
> Stefan, am I right?

Could be.  Or it could be an oversight, maybe dating back to many many
years ago, and that nobody noticed until now.

Whichever was the original reason, now we get to decide whether we keep
the behavior or not.  Following Common Lisp's lead should not introduce any
backward compatibility issue, all it would cost us is a slightly more
costly macroexpansion for those macros.


        Stefan

bug#74870: cl-labels and cl-flet don't create named blocks

[Jan Jouleodov via Bug reports for GNU Emacs, the Swiss army knife of text editors]

> Whichever was the original reason, now we get to decide whether we keep
> the behavior or not. Following Common Lisp's lead should not introduce any
> backward compatibility issue, all it would cost us is a slightly more
> costly macroexpansion for those macros.
>
>
> Stefan

Is there any reason why one would not want to *always* emulate the CL
behavior in cl-lib? I could only think of a backward compatibility
problem before CL was standardized, but I am not familiar with the time
frame of cl-lib to know if that's really the case.

bug#74870: cl-labels and cl-flet don't create named blocks

[Michael Heerdegen via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Eli Zaretskii <eliz@gnu.org> writes:

> I'm guessing this is a documentation bug, in that this particular
> aspect of CL is not emulated by cl-labels.

I dug a bit into the history.

`flet's implementation indeed lost its implicit `cl-block' in

  de7e2b36875 Get rid of cl-lexical-let, keeping only lexical-let for
  compatibility.
  Stefan Monnier <monnier@iro.umontreal.ca>, Thu Jun 7 2012

probably by accident.

OTOH it looks like `cl-labels' never created implicit blocks.  As the
manual describes it as especially useful for recursive local function
bindings, this case is maybe also a bit different.


Michael.