[RFC 58] Add partial arithmetic for integer types

[mfelsche]

This PR adds partial arithmetic functions and operators to all integer types.

Normal classes can also provide partial arithmetic operators, like +?, -?, *?, etc. by implementing add_partial, sub_partial, mul_partial etc. the same way the normal operators work on classes.

I limited this PR to +?, -?, *?, /? and %?.

Two question arised during implementation:

• Should we extend these methods to all numeric types, especially to floating point operations, erroring whenever NaN or Inf or -Inf would be returned?
• Should we add shl_partial, shr_partial and neg_partial too? As part of this PR?

Closes #2814

[mfelsche]

I added a Division test to make sure division by zero is zero and that Int.min_value()/-1 is also zero. The latter does not seem to be the case on arm and armhf:

detailed logs of failing job

/home/pony/project/packages/builtin_test/_test.pony:1801: Assert eq failed.  Expected (0) == (-170141183460469231731687303715884105728)

I think this should be handled as a separate issue.

[jemc]

Two question arised during implementation

I'd suggest to stick to the scope of the original RFC for the purposes of this PR, and leave out both of these extensions. These be handled in a separate RFC.

I think this should be handled as a separate issue.

Agreed. In a separate issue, we can make sure this result is deterministic across platforms, and if someone wants the faster, nondeterministic behaviour, they can use /~ (div_unsafe).

[mfelsche]

Thanks @jemc yeah, i will stick the failing division test into the new issue and remove it here. This RFCs feature is working fine it seems.

[mfelsche]

I removed the division test and will stick this PR to the RFC scope and at some point create a new RFC to extend the partial arithmetic both in terms of functions and in terms of scope (to include floating point).

[jemc]

There's a few style issues in this file - everywhere where you have two blank lines, it should be one blank line. See https://github.com/ponylang/ponyc/blob/master/STYLE_GUIDE.md#blank-lines

[mfelsche]

One minor drawback that this implementation contains is that previously, when implementing custom operator functions, say add, one could chose between a partial add(other: T)? or a total add(other: T), now, for the operators touched here, there is no partial add, just add_partial. This is not super-intuitive and needs explaining. But i cannot come up with a better way to offer both total and partial add for integers. But imho this is the only case where having both versions makes sense.

[SeanTAllen]

Was the "how we teach this" from the RFC merged? I don't see anything re:

"The tutorial should introduce the different ways of doing arithmetic in greater detail, checked (addc etc.), unchecked (add etc.). unsafe ( add_unsafe etc.) and partial (add_partial). So that users know what options they have and what the drawbacks might be (e.g. performance against correctness)."

We need to get that in for the RFC to be considered closed. As it is, the RFC isn't finished.

[SeanTAllen]

Also, this wasn't noted in Last Week in Pony.

[SeanTAllen]

@mfelsche can you add release notes for this?

[mfelsche]

The tutorial still needs to be updated. I am working on this in the little sparetime i have. ETA is t + 3-5 days.

This wasnt noted on lwip as it is not done yet, as you correctly stated.

[SeanTAllen]

@mfelsche can you add release notes for this?

[mfelsche]

Release Notes Draft

With this RFC Pony finally adds the means to do partial arithmetic on integers. It introduces a new set of operators: +?, -?, *?, /? and %? that all raise an error when they detect overflow/underflow or division by zero. The non-partial versions all wrapped around on overflow/underflow or defined division by zero to be 0, if this is not acceptable for you, you now can make use these operators. And if erroring is not your way, you can go with addc, subc, mulc, divc or remc which return the result together with a flag denoting overflow/underflow or division by zero. Check out the full story on the tutorial: https://tutorial.ponylang.io/expressions/arithmetic.html