Sunday, October 11, 2009

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/, line 206)
message to the extremely interesting

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

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