# [racket] Multi-return Function Call

 From: David Fisher (dave at casadefisher.com) Date: Tue Feb 21 09:50:06 EST 2012 Previous message: [racket] Multi-return Function Call Next message: [racket] Multi-return Function Call Messages sorted by: [date] [thread] [subject] [author]

The implementation looks good!  It is indeed possible to implement ,\_MR
via exceptions, local continuations, or prompts as Anthony has here.  While
I can't speak for Olin, I expect that his point is that implementing the
transformations we discuss in the paper practically would require a
(possibly byte-code) compiler that knew about multi-returns internally.  I
don't know Racket internals well enough to discuss prompts, but I imagine
they don't allow the kinds of tail-call optimizations that such a compiler
would.

Dave

On Tue, Feb 21, 2012 at 9:32 AM, J. Ian Johnson <ianj at ccs.neu.edu> wrote:

> Perhaps I remembered wrong about interpretation, but I do know he said
> that he didn't have a good implementation story - he must not have been
> very satisfied with that journal article.
>
> -Ian
> ----- Original Message -----
> From: "Sam Tobin-Hochstadt" <samth at ccs.neu.edu>
> To: "J. Ian Johnson" <ianj at ccs.neu.edu>
> Cc: "Anthony Carrico" <acarrico at memebeam.org>, users at racket-lang.org
> Sent: Tuesday, February 21, 2012 8:45:06 AM GMT -05:00 US/Canada Eastern
> Subject: Re: [racket] Multi-return Function Call
>
> That seems very surprising, since the journal paper (JFP 2006) about
> MRLC talks a lot about implementation strategies, and includes
> performance measurements for a compiler that implements MRLC.
>
> I think Anthony's implementation resembles the implementation in terms
> of exceptions discussed in section 11.1 of that paper.
>
> On Tue, Feb 21, 2012 at 8:15 AM, J. Ian Johnson <ianj at ccs.neu.edu> wrote:
> > I don't have the time right now to go through your implementation, but I
> will say that I've talked to Olin directly about implementing \lambda_{MR}
> and he answered that it he could not think of any good implementation story
> beyond interpretation.
> >
> > -Ian
> > ----- Original Message -----
> > From: "Anthony Carrico" <acarrico at memebeam.org>
> > To: users at racket-lang.org
> > Sent: Monday, February 20, 2012 8:21:06 PM GMT -05:00 US/Canada Eastern
> > Subject: [racket] Multi-return Function Call
> >
> > It is only 60 LOC, so go ahead and take a look:
> >  https://github.com/acarrico/multi
> >
> >
> > AUTHOR: Anthony Carrico <acarrico at memebeam.org>
> >
> > This is an implementation of Olin Shivers'/David Fisher's
> > "Multi-return Function Call". I'll quote from the abstract of their
> > paper:
> >
> >  "It is possible to extend the basic notion of 'function call' to
> >  allow functions to have multiple return points. This turns out to be
> >  wide-ranging tour of such a feature..."
> >
> > The paper is at:
> >  http://www.ccs.neu.edu/home/shivers/papers/mrlc.pdf
> >
> > I'm not aware of any published implementation of this feature. I wrote
> > this version after looking at the language creation features in
> > Racket. I was wondering how Racket might host languages with features
> > that weren't supported by its primitives and multi came to mind, but
> > after reading through the Racket documentation on continuations, I
> > began to think that it was possible to implement multi as a plain old
> > macro with continuation marks and prompts.
> >
> > My syntax has minor differences from the paper. Return points are
> > indexed from ZERO, not one, since that is how vectors (etc.) are
> > indexed in Scheme. Also, the indexed return point syntax isn't
> > prefixed with #, and the lambda return point syntax doesn't have a
> > lambda keyword.
> >
> > Look through test.rkt for a bunch of examples.
> >
> > ISSUES:
> >
> > I'd like to hear from someone who knows Racket really well to tell me
> > if I'm barking up the wrong tree. Is there a better way to provide
> > multi? Is what I have done reasonable?
> >
> > The macros could be better, and give better messages, etc. Please send
> > a patch if you are good at that sort of thing.
> >
> > The multi macro would need to be improved to capture the super tail
> > call semantics:
> >  * A prompt should only be used when introducing lambda return points.
> >  * Unused prompts could be popped off the "stack" before setting
> >    up new return points.
> >
> > --
> > Anthony Carrico
> >
> >
> > ____________________
> >  Racket Users list:
> >  http://lists.racket-lang.org/users
> > ____________________
> >  Racket Users list:
> >  http://lists.racket-lang.org/users
>
>
>
> --
> sam th
> samth at ccs.neu.edu
>
> ____________________
>  Racket Users list:
>  http://lists.racket-lang.org/users
>
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