Re: void? throws an exception

[Per Bothner <per at bothner dot com>]

On 01/04/2011 07:26 PM, alex mitchell wrote:
> I just updated to the latest version in svn after quite some time (previous
> version that I was running was 6632), and found that where I had previously
> been using the procedure void? in my code, now I'm getting an exception (see
> below).

I checked in a fix for this.  Your expression should now work.

However, note that I don't promise to support this.  void is not a value
- it is *zero* values.  I've considered having having multiple values
in an argument expand to multiple arguments:
    (fun a (values b c) d) === (fun a b c d) ;; possible future
The experimental Q2 languages does this:
http://per.bothner.com/blog/2010/Q2-extensible-syntax/
I'm not doing this for Kawa-Scheme partly because I'm not sure of the
best semantics and partly because of performance concerns.

I agree the "API" for multiple values in general and void in particular
needs to be clarified/fixed.  This ties in with sequences and list
comprehensions, which I'm working on.

> (define (void? anything)
>   (equal? #!void anything))

This may turn into a compile-time error ...  In general, depending on
the value of an expression that is guaranteed to return void (i.e. #!void)
is likely to be an error, so I think it may be more helpful to prohibit
such expressions rather than allow the.

> but its strange that this previously worked and now doesn't. I'm guessing
> this is a problem with the automatic generation of the type-testing
> predicate.

I'm surprised.  I don't think I declared void? before.  It's certainly
not there in the Kawa 1.9.1 shipped with Fedora 14.

(Sigh - any volunteer to take over Kawa packing for Fedora - or Debian 
or Ubuntu?)
--
	--Per Bothner
per@bothner.com   http://per.bothner.com/