I don't fully understand the use case... but have you looked at 'apply'?
> (define (sum4 a b c d)
(+ a b c d))
> (apply sum4 1 2 (list 3 4))
10
On Fri, Jul 22, 2016 at 7:58 PM, David Storrs <david.sto...@gmail.com>
wrote:
> Thanks Jon, I appreciate the clear explanation.
>
> I'm using call-with-values in database code in order to turn a list into
> an acceptable set of bind parameters. Here's an example:
>
> (query-exec conn "insert into foo (bar, baz) values ($1, $2)" (some-func))
>
> some-func returns a list, '("bob", "george"). query-exec throws an
> exception because it wants to see two elements for the bind parameters, not
> a list. The solution I found was this:
>
> (call-with-values
> (lambda () (apply values the-list))
> (curry query-exec conn stmt))
>
> I know that I could have dug the parameters out with (car) and (cadr), but
> this seemed like it would be a common enough pattern that it was worth
> coming up with a standard answer that would work for any number of params.
>
> This is what got me thinking about how exactly (values) worked.
>
>
> On Fri, Jul 22, 2016 at 7:28 PM, Jon Zeppieri <zeppi...@gmail.com> wrote:
>
>>
>>
>> On Fri, Jul 22, 2016 at 6:30 PM, David Storrs <david.sto...@gmail.com>
>> wrote:
>>
>>>
>>> The best mental model I've been able to come up with is that (values)
>>> returns its results vertically but most functions expect to get them
>>> horizontally and will die if there are parallel lines of uncollected
>>> values. Under this model (call-with-values) collects values from the Y
>>> axis and pivots them back onto the X axis for consumption by regular
>>> functions. This feels like a fragile analogy though. Would someone
>>> please fill me in on how it actually works?
>>>
>>>
>>
>> When you do:
>>
>> (values ...)
>>
>> in a DrRacket window, it does list the values vertically, but that's
>> probably not a very useful way to think about what `values` is. You can
>> think of `values` as a constructor of a second-class tuple. The fact that
>> the resultant tuple is "second-class" is very important here. You can't
>> give a name to this tuple. For example,
>>
>> (define foo (values 1 2 3))
>>
>> is illegal. You can't pass it as an argument to a procedure:
>>
>> (list (values 1 2 3))
>>
>> is likewise illegal. There is, in fact, very little you can do with this
>> tuple other than let `call-with-values` unpack it.
>>
>> `call-with-values` is special (like call-with-continuation is special).
>> Its first argument, a zero-arity procedure, produces a values which are
>> then passed *as separate arguments* -- not as a second-class tuple -- to
>> its second argument. That's what `call-with-values` does: it translates
>> this second-class tuple into separate procedure arguments.
>>
>> `call-with-values` is rarely used directly. It's far more common to use
>> `define-values` or `let-values` (or one of its variants). For example,
>> although you can't do:
>>
>> (define foo (values 1 2 3))
>>
>> You can do:
>>
>> (define-values (a b c) (values 1 2 3))
>>
>> There is one special case to keep in mind: a unary use of `values`, e.g.:
>>
>> (values 1)
>>
>> is exactly equivalent to its argument. That is, (values 1) *is exactly*
>> 1. That's not true for any other arity of `values`. (This is also how
>> *first*-class tuples in ML behave.)
>>
>> For my own part, I'm not a big fan of `values` and `call-with-values`.
>> Their use is for returning multiple values from a procedure -- which you
>> can also do by returning a first-class, composite piece of data, like a
>> vector or a list. The one advantage of second-class tuples is that, because
>> there's so little you can do
>> with them, they're easier to give optimized representations.
>>
>> - Jon
>>
>>
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