Polynomial & Vector: Debugging Update

I've tracked down the next bug mentioned at the end of my last post, and again it comes as a side effect of the hack to make it easier to code Polynomial addition. In that hack, we extend the Polynomial coefficients array with 0. But if the coefficients are Vectors, the actual extension value should be Vector.new(0, 0, 0). Just plain 0 means we eventually try to add 0 to a Vector, and since Vector doesn't support addition with a scalar, it falls back to a normal addition operator, which tries to convert Vector to a Num.

Thoughts: I don't see any obvious way to figure out the correct "zero" to extend the coefficients with. Nor do I see any obvious way to allow you to add 0 to a Vector. (At least without opening up general vector plus scalar math, which is a bad idea IMO.)

It also suggests that it may be a very good idea to go beyond implementing the operators that work to implement dying versions of the operators that shouldn't be allowed, because allowing Perl 6 to fall back to its own operators will produce less useful error messages. (Or worse, they might even "work".)

I can see one more potential error like this lurking in the Polynomial.prefix<-> code...

Polynomial & Vector: Debugging

So, I was just going to post on working around the bug mentioned in my last post, when I noticed Moritz++ had commented with a suggested debugging approach. And we're off and running!

multi method Num()
{
die "Cannot call Num on Vector!";
}

It's not the most elegant error message, but it will do. Adding that switches from the fairly useless Method 'Num' not found for invocant of class 'Vector'
in Main (file src/gen_setting.pm, line 206) message to the extremely interesting

Cannot call Num on Vector!
in method Vector::Num (file lib/Vector.pm, line 24)
called from method Polynomial::new (file , line )
called from sub infix:* (file lib/Polynomial.pm, line 106)
called from Main (file , line )

Aha! Not a Rakudo bug at all, this is a legit issue. When I added the fix to remove trailing zero coefficients, I wrote @x[*-1] == 0 to check for them. Odds seem very good that == calls .Num. So... a change to .abs here is probably more realistic, anyway. (Maybe? I need to ponder that.)

Hmmm... that moves the error elsewhere, to (according to the backtrace) infix:+ (file , line ). I need to go to bed now, so the final debugging will have to wait. But at a minimum, Moritz's suggestion of adding a .Num that dies appears to be a valuable Perl 6 debugging trick.