[racket] How to get arity of make-object? Found answer

From: Asumu Takikawa (asumu at ccs.neu.edu)
Date: Fri Jun 6 14:18:21 EDT 2014

On 2014-06-06 13:31:17 -0400, Matthias Felleisen wrote:
> I think we can get them in this sense:
> > > (arity-includes? (procedure-arity (case-lambda [(x) x][(x y) x][(x y z) x])) 1)
> > #t
> > > (arity-includes? (procedure-arity (case-lambda [(x) x][(x y) x][(x y z) x])) 3)
> > #t
> > > (arity-includes? (procedure-arity (case-lambda [(x) x][(x y) x][(x y z) x])) 4)
> > #f
> but I agree that it is of questionable value. -- Matthias

I'm not sure this predicate, if implemented, can return useful results.
To make this more concrete, consider the following interaction:

  -> (require racket/date)
  -> (define base-class%
       (class object%
         (init x y z)))
  -> (define weird-class%
       (class base-class%
         (if (= (date-week-day (current-date)) 5)
             (super-new [x 1]))))

Today (since it's Friday), the `weird-class%` class will take three init
arguments. If you try this tomorrow, it should take two init arguments.

Even worse, the class could change the number of init arguments it takes
based on the parity of the millisecond field, a random coin-flip, or by
doing face recognition with your webcam.

It might be that the arity actually will have changed from the time of
the predicate call to the actual use of the constructor.


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