Monday, October 12, 2009

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...

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