Re: Is multi-level function return possible?

On Mon, 10 Mar 2014 15:46:31 -0400, noitalmost <noitalmost@cox.net>
wrote:


>Is this a doable thing for a Pascal-like language that is meant to be
>compiled? I have a multi-level break which works within a procedure, but the
>return across procedure boundaries seems to add a lot of complications.
>
>I don't recall this being discussed in the dragon book, perhaps because it's
>just a silly idea. But implementing the multi-level break got me curious
>whether it's possible.
>[Sure, you can do it. PL/I let you jump out of a routine and I think Algol60
>may have, too. You just need to keep your stack frames sufficiently linked
>so that you can find the one you want and unwind everything below it. Look
>at the way Algol and Pascal reference variables in blocks outside the current
>routine and you'll get most of what you need. -John]


What you're looking for sounds similar to the setjmp/longjmp feature
in C and exception handling in C++. If you're going to attract C++
users to your language, the try/catch block would be a good thing to
imitate.


Unisys ALGOL has a TRY statement; see
http://public.support.unisys.com/aseries/docs/clearpath-mcp-15.0/pdf/86000098-511.pdf
for details.


I believe that exception handling as it's usually implemented is
driven by the calling routine, while it sounds like what you want is
for a nested procedure to return from an outer procedure. Unisys
ALGOL does at least some of this with a "bad" GO TO statement, which
branches out of one or more blocks. If an outer procedure END
statement has a label, and if you branch to it from within a contained
procedure, you've accomplished your multi-level return.


Louis

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