Thursday, November 26, 2009

Aha!

A post-Thanksgiving dinner $work debugging session left me with some spare time to poke at my SVG.pm bug: that is, the "too many positional arguments: 3 passed, 1 expected" error I got when I tried to crank up the number of samples I was taking of the curve. I finally added enough says to track the error down to the simplest of operations: @N[$i] += $temp;, where $temp was 0.00686851014887172. This wasn't one of my fancy overloaded operators, it was a basic Perl 6 numeric operator. What could go wrong?

Well, I added a bunch more stuff to the say. And it turns out we are adding two Rats, 424/61731 and 832/61731. What could go wrong?

> say 424/61731 + 832/61731
too many positional arguments: 3 passed, 1 expected

If you look into infix:<+>(Rat, Rat), it just does a naive fractional add, relying on Rat.new to reduce the fraction. The problem here is that means the denominator of our new fraction starts its life as 61731 * 61731.

> say (61731 * 61731) div 246924
No applicable candidates found to dispatch to for 'infix:div'

I'm not sure what the best approach to fixing this is. Obviously I can change my code to do Nums. But I think the first step is to file a Rakudo bug.

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