Re: Multiple Values in Kawa

[Per Bothner <per at bothner dot com>]

Nigel Dolby wrote:
> Given that Kawa is pretty effective for prototyping and that the Common 
> Lisp handling of multiple values provides a significant increment of 
> prototyping capability, it seems to me that it would be worth trying to 
> make the Common Lisp functionality available in Kawa Scheme. This 
> doesn't necessarily mean that it should be a default. The crucial 
> consideration is that, if I modify an existing function to return 
> multiple values when it previously returned a single one, all existing 
> calls on the modified function should continue to work as before without 
> any code changes (thus, let-binding to capture the first value is not a 
> preferred solution).

Is it really any easier than changing:

(define (existing-foo)
  (calculate-value1))

to:

(define(foo-2)
  (list (calculate-value1) (calculate-value2))
(define (existing-foo)
  (car (foo-2)))

The difference between the two approaches is that you "overload"
based on the return context.  Is i convenient?  Sure.  Does it
make anything noticable easier?  I'm not convinced.  Is it worth
it?  Perhaps.

> Without any knowledge of the implementation, it 
> seems that it might be possible to add a form similar to "values", 
> perhaps named "optional-values" or "ignorable-values", that would 
> explicitly call for the Common Lisp functionality in the code of the 
> function that is returning the multiple values, which is already 
> necessarily undergoing change in the prototyping scenario. This would 
> allow people who prefer to use strict-checking functionality to use 
> "values", and those who prefer flexibility to use the new form. If this 
> is feasible, it seems that it might satisfy all the requirements and do 
> so without foreclosing sequence functionality in XQuery.

It's not trivial to do it without penalizing the normal single-value
solution.  It is possible, but it probably requires changing mechanism
by which Scheme function calls are implemented in the Java VM.  I do
want to do that anyway to support continuations.

So support for this feature (or the Common Lisp mechanism) may be
considered as part of adding support to full continuatons - but
not until then.
--
	--Per Bothner
per@bothner.com   http://per.bothner.com/