[plt-scheme] How do I convert (values ...) to something I can do something with?

From: Jay McCarthy (jay.mccarthy at gmail.com)
Date: Wed Mar 10 00:27:03 EST 2010

I use multiple value returns _a lot_.

I try to never overload lists or pairs and instead always make
structs. I try to limit myself to structs that "make sense" in the
wild as opposed to ones that will only be produced/consumed by a
single function; I find that in those cases I really want multiple

However, multiple values have a problem that when you get more than a
few it is complicated to remember which is which because all you have
is positions. At least with function calls you can have keyword args.
In those cases, I let myself use a struct, but I feel bad about it.

My two gil,


On Tue, Mar 9, 2010 at 9:38 PM, Todd O'Bryan <toddobryan at gmail.com> wrote:
> Stylistically, is one better/more idiomatic than the other? I thought
> about returning a list, but I have this aversion to complex nested
> structures that aren't self-descriptive. (Yeah, I know--if
> s-expressions freak me out, I'm looking for trouble coding in Scheme.)
> I could define a struct so that the parts have names; maybe that's the
> best thing to do.
> I guess I decided to return multiple-values because it forces me to
> check myself. I can't just assign the return value from the function
> to a single variable, so if I screw up and forget that it returns
> multiple values, I should get an error fairly early. Within my code,
> the multiple-value thing isn't a problem at all. It's just when I was
> trying to write test cases that it became annoying.
> Does anyone else sometimes feel like you're learning a foreign
> language by listening to tapes? I have this constant worry that, while
> I think I'm speaking Scheme, when I finally start talking to native
> speakers, they're going to find my accent either incomprehensible or
> really funny.
> Todd
> On Tue, Mar 9, 2010 at 11:26 PM, Matthias Felleisen
> <matthias at ccs.neu.edu> wrote:
>> Sure, return a list instead of multiple values. -- Matthias
>> On Mar 9, 2010, at 11:14 PM, Todd O'Bryan wrote:
>>> Thanks. I saw those, but they didn't register...
>>> If (foo 'x) returns multiple values, is there any easier way than
>>> (call-with-values (lambda () (foo 'x)) list)
>>> to convert it to a list?
>>> On Tue, Mar 9, 2010 at 11:05 PM, Carl Eastlund <cce at ccs.neu.edu> wrote:
>>>> On Tue, Mar 9, 2010 at 11:02 PM, Todd O'Bryan <toddobryan at gmail.com> wrote:
>>>>> I've written a function that returns multiple values. But now I can't
>>>>> figure out how to check the silly thing.
>>>>> Sorry if this is a stupid question, but how do I convert a (values
>>>>> ...) expression into something I can wrap my parentheses around?
>>>>> Todd
>>>> If you know exactly how many values you'll be getting, use let-values
>>>> or define-values.  If you don't, use call-with-values.  They're all in
>>>> the Help Desk.
>>>> --Carl
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Jay McCarthy <jay at cs.byu.edu>
Assistant Professor / Brigham Young University

"The glory of God is Intelligence" - D&C 93

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