fix(parser): make if statements with 'not' work

[lkipke]

Change Summary

This allows if statements with a logical not operator, and closes #329.

[Piera]

Trying to get my head around this -- had a couple of questions. 🤔

[Piera]

Thanks for the updates!

[lkipke]

Considering the order of operations goes relational > bitshift > additive > multiplicative > exponential > prefixUnary, I was going to add some unit tests that did things like not 2 * 3. However, that elicits strange behavior on RBI (not 2 * 3 = -7 for some reason...) and brs prevents comparing boolean NOT to non-boolean 2 * 3.

[Piera]

🤔 interesting!

[sjbarag]

Hmm, seems like there's still some bugs in brs that we'll need to shake out then! From RBI:

print not  0 ' => -1
print not  1 ' => -2
print not -1 ' =>  0
print not  2 ' => -3
print not -2 ' =>  1

which looks like RBI is using a two's complement representation for numbers and does something slightly unexpected with it. not someInteger in RBI seems to take someInteger, convert to a binary representation, perform a bitwise negation on it (all 0s become 1s, all 1s become 0s), then convert back to a decimal representation in the Two's complement understanding. So for not 0 (using just 4-bits for now because I'm lazy), we have:

not 0 (decimal) == not 0b0000 == 0b1111 == -1 (decimal in two's complement)

How strange! In the not 2 * 3 example, we get:

not (2 * 3) == not 6 (decimal) == not 0b0110 == 0b1001 == -7 (decimal in two's complement)

All-up, it looks like RBI supports negating decimals, but brs doesn't! I'll file a bug to fix that a bit later. At least the parser is still correct 🎉

[sjbarag]

Very exhaustive tests! Thanks for including them here 😃