Accepting Equality

Recently I realized that Numeric and Real need to implement ACCEPTS. As far as I know, the Spec has nothing to say about what this ACCEPTS should do, so I looked at the current implementation. There are two versions of Num.ACCEPTS, one which takes anything and uses == to compare, and one which takes a Complex and checks that the imaginary part is zero, and the real part is equal to the Num. Both ACCEPTS have special cases for NaN, needed because NaN != NaN.

Talking about this with Jonathan, I was initially stumped. If every class had to have an ACCEPTS for its own type, for Real, and for Numeric, things would be a mess. Then it occurred to me: why not implement it by simply calling to ==? As long as == is doing the right thing, ACCEPTS will do the right things as well. (Errr, modulo the NaN issue, which might get tricky.)

Of course, that depended on a notion of “right thing” for ==. Quick check: does a Real number equal the Complex version of the same number?

> 1 == 1 + 0i

Huh. Well, let’s get a second opinion:

colomon: alpha: say 1 == 1 + 0i
p6eval: alpha 30e0ed: OUTPUT«1␤»

Whoops. The False answer in current Rakudo is because of my reworking of Numeric comparison. At the time, I was thinking of sorting, and figured it would do no harm if 1 < 1+0i. But of course that looks silly in the context of ==! Apparently the test suite doesn’t have any tests for this case.

As frequently happens with these posts, I started this one without any clear sense of where I should go, but now I’ve got a notion:

1) Fix == so that Reals and equivalent Complexes are equal again.

2) Make a generic ACCEPTS for Numeric that works based on ==.

3) Somehow fold NaN handling into the mix.

Hmmm. Okay, my actual first step is going to be to completely muck up Numeric.ACCEPTS (intentionally), and see what, if any, tests fail, so I can find out the state of testing ACCEPTS.

Thanks to the Perl Foundation and Ian Hague for supporting this work.


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