[racket] Multiple return values

From: Tomasz Rola (rtomek at ceti.com.pl)
Date: Thu Dec 15 12:17:42 EST 2011

On Thu, 15 Dec 2011, Zayr Okale wrote:

> I even do understand what multiple return values are useful for in CL: "okay,
> the function calculates other potentially useful values anyway, so no reason
> not to make them available".
> Unfortunately, this scenario doesn't apply to Racket. And this is exactly what
> prompted my question. Since one of the reasons behind multiple return values
> is, as David Van Horn pointed out, symmetry with multiple input values
> (function arguments), then why optional input values are allowed, but optional
> output values aren't?

This question is probably better asked to people behind RnRS. I was unable 
to give you any interesting examples with Scheme, because I didn't know 
them. Actually, I was a bit unsure about this multiple values stuff 
myself, as I had been reading R5RS some time ago (I am yet to find time 
for R6RS, so maybe there is more about values, but I don't know right 

After I exchanged punches with CL, I've got a bit better understanding of 
the issue. Or so I hope.

Your question sounded like one of more general nature, which is why I 
allowed myself to do this CL/C intrusion here.

> The situation when all the return values are of equal importance, yet
> returning a struct or a list is not convenient is, IMHO, quite rare.

I don't want to bet on this :-) .

I think that in some cases you may emulate values by operating on list. 
Like, return a list of values and later use apply.

However, once you have to produce list of specified size, it becomes 
inconvenient, IMHO. Because no matter what you want, you have to make this 
one specific list and later go through it to access the elements (this can 
be optimized, but one shouldn't count on such happy end).

So I think using list/struct forces an overhead when later you want to 
make use of the values. On the other hand, with values (again, sorry for 
CL), one can have:

[2]> (multiple-value-bind (f r) (floor 130 11) (list f r))

(11 9)

This is from CL HyperSpec. Here, your data is "inserted" into your 
namespace, and this can be paired with (declare (ignore ...)) to make it 
perform better.

Situations like this can be rare in other languages, simply because for 
mul-vals to work and make sense it requires a bit bigger ecosystem of 
language constructs and, like above, assumption about how and where things 
are going to be optimized. Above, I think the minimal approach would be to 
analyze entire multiple-value-bind expression.

I keep an eye on Racket but I don't know it too well (just one 
non-trivial program bettered with profiler and few smaller ad hoc pieces 
over few years period), so I am unsure how much sense is there in using 
values in it.

Tomasz Rola

** A C programmer asked whether computer had Buddha's nature.      **
** As the answer, master did "rm -rif" on the programmer's home    **
** directory. And then the C programmer became enlightened...      **
**                                                                 **
** Tomasz Rola          mailto:tomasz_rola at bigfoot.com             **

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