[plt-scheme] keyword arguments (in v4.0, maybe)

At Tue, 12 Jun 2007 17:56:00 -0500, "Robby Findler" wrote:
> On 6/12/07, Matthew Flatt <mflatt at cs.utah.edu> wrote:
> > That is, `apply' still works for
> > procedures that have optional keyword arguments. But it doesn't
> > work for procedures that require some keyword arguments.
> 
> What is the difference between an optional keyword argument whose
> default is to signal an error and a required keyword argument?

There's no difference in "kw.ss". But they would indeed be different in
what I'm suggesting, due to things like `keyword-procedure-arity'.

It's analogous to the difference between these two:

  (lambda args (unless (= (length args) 1) (error "no")) ....)
  (lambda (a) ....)

The difference between these is visible via `procedure-arity'.


In the keyword case, here's how the two different things would be
written:

 (lambda (#:y y) ....)                    ; required
 (lambda (#:y [y (error "no y!")]) ....)  ; "optional"

With the former, applying it to no arguments triggers an exception
about a missing `#:y' argument, or at least "no matching case". With
the latter, applying it to no arguments triggers an exception with "no
y!". That's not much of a difference.

But `keyword-procedure-arity' would say that the former requires a #:y
argument, while it would say that the latter optionally accepts a #:y
argument.

In fact, maybe `procedure?' reports #f for the first and #t for the
second. (This design point is still open, I think.)


Matthew