On the old SRFI 102 list, Shiro Kawai gave an example of where it
could be used ([1]).

Let me quote:

    I rather think that the point of the arity query procedure is for
some kind of crude optimization, rather than strict error checking,
given the nature of the ambiguity Thomas mentioned.

    For example, higher order procedure can branch out the typical
    cases to avoid folding arguments (it can be abstracted to a macro).

    (define (complement fn)
      (case (arity fn) ;; some optimization
        ((0) (lambda () (not (fn))))
        ((1) (lambda (x) (not (fn x))))
        ((2) (lambda (x y) (not (fn x y))))
        (else (lambda args (not (apply fn args))))))

(Note that this example has been written for the old API, but this is
irrelevant.)

I am not sure about the usefulness of the example, though. We have
already case-lambda for an optimized version (given that the
underlying Scheme has case-lambda as a core form):

(define (complement fn)
  (case-lambda
    (() (not (fn)))
    ((x) (not (fn x)))
    ((x y) (not (fn x y)))
    (x* (not (apply fn x*)))))

Marc

--

[1] https://srfi-email.schemers.org/srfi-102/msg/2772015/

Am Do., 18. Juni 2020 um 00:23 Uhr schrieb Arthur A. Gleckler
<xxxxxx@speechcode.com>:
>
> On Wed, Jun 17, 2020 at 3:18 PM Alex Shinn <xxxxxx@gmail.com> wrote:
>
>>
>> Note this wasn't rhetorical - I don't think a SRFI should be accepted without any examples.
>
>
> That's a good point.  I've put checking for examples in my finalization and new-draft checklists.