Re: Confusing error message

[Charles Turner <chturne at gmail dot com>]

On 17/04/2011 08:04, Per Bothner wrote:
> > Type checking seems to be a mixed bag in Kawa, most of the time the type
> > errors seem to emanate from a ClassCastException, which seems quite
> > elegant.
> > I'll have to look into why cdr doesn't do it like this, but
> > when I find out why, would it be okay to try and make it use the
> > ClassCastException as well?
>
> I think so.

I had a look at this today, but the Kawa source got the better of me. 
For some stupid reason, I figured that the ClassCastException would be 
thrown with a custom detail string, how wrong I was.
For the contrived example of evaluating (cddr '()) [one that throws 
ClassCastExceptions currently] the exception is thrown from 
ModuleMethod.apply1 in the expression,

return module.apply1(this, arg1);

I was looking at this in a debugger and noticed that module is of type 
"lists" which from a quick inspection doesn't have an apply1 method 
(it's a scheme file), I'm struggling to understand how this is working 
at all! The stack trace suggests the error really came from the 
following in lists.scm,

(define-procedure cdr
  setter: set-cdr!
  (lambda (x :: <pair>) name: 'cdr
      x:cdr))

I suppose from the x :: <pair>, since at this point, x is a list. I 
can't figure out where to catch this exception before it gets parcelled 
up. I could regex out the types in from Throwable.getMessage() and then 
look them up, but this seems excessively kludgey.

I'm sorry I'm making such a pigs ear of this seemingly simple change!

Thanks for reading,

Charles.