# [racket] Multi-return Function Call

 From: J. Ian Johnson (ianj at ccs.neu.edu) Date: Tue Feb 21 08:15:14 EST 2012 Previous message: [racket] Multi-return Function Call Next message: [racket] Multi-return Function Call Messages sorted by: [date] [thread] [subject] [author]

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
a surprisingly useful mechanism. This article conducts a fairly
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

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