Perl 6 CaR TPF Grant: Monthly Report (May, 2018)

This document is the May, 2018 progress report for The Perl Foundation's Perl 6 Constant and Rationals Grant.

Tangibles Produced

This month I was working in docs and roast repos, in separate car-grant-midrat branches. So far, they contain 29 documentation commits and 11 spec commits adding about 5,824 words of documentation and 152 tests.

The tests specced the MidRat type. The documentation, along with documenting MidRat/MidRatStr types, centered largely around new Language/Numerics page that describes all of the available Perl 6 numeric types—including native and atomic numerics—their purpose, properties, and hierarchy. This guide was not on the list of deliverables of the Grant and has been produced as a bonus item.

General Info

I started the grant by working on adding the proposed MidRat/MidRatStr type pair. This was the most contentious part of the Rationals Work Proposal that before the grant underwent three revisions, settling on solving the problem by adding the MidRat type.

I went full-on with Test/Documentation Driven Development and I think the result is a fantastic example of the power of this approach. After adding 5,824 words of docs and 152 tests, I still haven't written a single line of code of the implementation in the compiler itself.

The process highlighted several issues with MidRat type and I've tentatively decided not to implement it at this time. Its functionality will be subsumed with the plain Rat type. The plan to change Rat type to use uint64 native type for the denominator is tentatively canceled and instead an Int denominator will accept denominators over 64 bits in size in all the cases where a MidRat type were going to be produced. At the same time, the Rat will degrade to a Num in all the cases where a MidRat were to do so.

Doing so robs us of potential performance improvements due to using native denominator, however we still have the option of implementing a MidRat-type solution open in the future. Were we to go with implementing MidRat right now, we'd be locking ourselves into supporting it basically forever, and exposing ourselves to yet unforeseen design issues.

Issues/Clarifications of MidRat

What is a .Numeric/.Real of a MidRat?

Writing documentation first showed an ambiguity with what .Numeric/.Real methods should return for a MidRat. The work proposal had MidRat as an allomorph of Rat and FatRat, and while .Numeric on IntStr allomorph returns its numeric component, a MidRat would have two such components.

This issue is complicated by the fact that prefix:<+> uses .Numeric, so that means 0 +,2) would produce a Rat objet, yet,2) would produce a potentially different object. To maintain consistency there, at the time I settled on simply having .Numeric/.Real coerce the MidRat to a Rat, which means that had the potential of degradation of that Rat to a Num. Bring the MidRatStr into play and you get three different types by chaining .Numeric calls. That was already slightly weird.

Convergence to Plain Rat

While documenting infectiousness of numerics, the question of infectiousness of MidRat came about. The original work proposal was vague in this area but after documenting the MidRat type, it became obvious that its infectiousness must be the same as if Rat type was used instead. That's the first point of convergence to Rat.

The second became apparent when writing degradation tests. The work proposal argued for MidRat to be an allomorph of Rat and FatRat types. A MidRat has infectiousness of a Rat. But that means this code…

my FatRat $a = get-rational;
my FatRat $b = get-rational;
say $a + $b;

…has the potential to produce a Num of all things (MidRats degrade to Rat and that can degrade to a Num). That's certainly unacceptable, so by this point, I decided to toss FatRat from the ancestors of MidRat, leaving it be just a subclass of Rat. This largely resolved the issue of what to return from .Numeric/.Real methods.


By now you can see that the MidRat got reduced to just being a Rat with non-native denominator. Adding two extra types (MidRat/MidRatStr) to an already populous zoo of Perl 6 numerics for such a tiny detail was hard for me to justify. So, after flipping a coin to confirm my gut feeling, I decided to abandon the idea of MidRat and to repurpose the Rat type to resolve all of the issues MidRat were meant to resolve.

The documentation and tests I wrote won't go to waste: they're simply going to be modified to document/test the MidRat's functionally from the point of view of the Rat type that subsumed it.

Going Forward

I still plan on doing the Bonus Work of the grant, fixing bugs in native uint64 attributes, despite no longer planning to use them directly in the denominator of the Rat type. I'll also try to make a perf improvement by selecting native/non-native NQP ops for dividing Rats, to make up for not using native attributes in them.

In lieu of implementing the MidRat type originally promised, I'll work on resolving some additional bugs. Likely trying to resolve value drift in MoarVM Numification of Rationals that currently does not select the closest representable Num.

For the next month, I plan to work more on Rationals, fixing the data race and normalization of zero-denominator Rationals.

Related Work Done By Others

  • AlexDaniel++ fixed object-identity bug in Rationals. The fix was done using the .REDUCE-ME method, which has the data race and will go away in about a month, but in the meantime, object identity will be correct.
  • thundergnat++ did some work on improving performance of Rat.Str and FatRat.Str and fixed an issue with unwanted trailing zeros.

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