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Sun, 13 Sep 2009

When we reach 100% we did something wrong


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<meta> This is not a marketing blurb as I originally intended to write. It is quite a lengthy and a bit philosophical brain dump. If my regular readers skip it that's fine; I promise that my next post will be shorter, more exciting and easier to digest.</meta>

A few times per week we (the Rakudo development team) update our spectest graph which shows how many tests Rakudo passed in the past and now, and how many tests there are in the official test suite.

This is a very useful information, but still its use is limited. Among other reactions we also received two kinds of responses that I want to talk about. One is "And when Rakudo passes all tests, Perl 6 is ready, right?", the other is "I don't care how many tests you pass until it's 100% (the latter seen in a reddit or Y-combinator comment, for example).

Implications

The first reaction points to a larger problem - how can we know if our tests actually cover the whole specification? The answer is we can't. Ever. We can identify parts of the spec that are very well covered by tests (for example multi dispatch), and parts that are covered badly or not at all (IO, and way too many other areas). But there is a huge number of ways the various parts of the specification interact (growing exponential in the number of specified facts), which makes it impossible to reach complete covering.

Still we can read each paragraph in the specs and see if we have an appropriate test, achieving some kind of shallow coverage.

But worse still there are many things implicit but not explicit in the spec. I write "worse" because for the test suite writer it's a pain, but on the other hand if everything was stated explicitly, we'd waste too much time on trivialities.

One example is a bug which Rakudo had in the beginning: recursion would screw up some lexical variables rather badly. When Patrick and Jonathan looked into fixing it, I wanted to contribute my part and provided some tests. And then I didn't know where to put them. The specs talk about subroutines, and about lexical variables. But it does not talk about the interaction between recursion and lexicals - because it's obvious that each instance of a function gets its own lexical pad on recursion. Perl 5 does this just fine, we carry that semantics over to Perl 6. Perfectly sane, but it's an implication nonetheless.

And, after much talking, I finally come to my first main point: We can't measure test coverage of such implications. We have no automatic way to turn implications into explicit statements either. We can't know if spec and the implications have a decent test coverage.

We won't ever reach 100% passing tests

Most people consider Perl 5 "done". Not in the sense that there's nothing left to do, but that there are stable releases, wide acceptance, on generally technical and social maturity. And yet Perl 5 does not pass all of its test.

It might come as a shock to you if you haven't looked at Perl 5's source tree, and never compiled it from source and run the tests (and actually looked at the results), but it's true.

$ ~/tmp/perl> ack -w 'skip(?:_all)?' t|wc -l
381
$ ~/tmp/perl> ack -w 'todo' t|wc -l
48

There are many valid reasons why some tests should not be run - for example the tested feature might not be available on the current platform, or maybe a previous failure would make their outcome inconclusive.

There are also valid reasons why some tests are marked as TODO (which means they are run, but don't pass) - for example to test a bug that is not yet fixed, or for behavior that is subject to change but has not yet been adapted.

So even with huge efforts our chances of ever unconditionally passing each and every test in the Perl 6 test suite are practically 0. When a run over the whole test suite shows that all tests passed, the first thing I'll do is to check our test module and test harness for errors - maybe some bug prevented them from successfully identifying errors.

If you compare how the tests for Perl 5 and Perl 6 were originally written, you'll see that in the case of Perl 5 a huge amount was written against a working implementation, while many Perl 6 tests were whipped up by reading the spec and transforming it into code, or even as feature requests for pugs. So it won't surprise you that in the Perl 6 test suite there is a larger amount of contradictory tests, written either against different versions of the specification, or by various people who understood the spec differently. We'll work hard to weed out such contradictions, but it would be an illusion to assume that we will fully succeed.

Are you moody?

When you read the previous paragraphs you might think I'm moody, and quite negative about Perl 6. I'm not. I just faced the reality that each software project of at least modest complexity faces, and brain-dumped it here.

In fact I'm quite optimistic. Many areas of Perl 6 improve at an impressive rate these days: the specification, Rakudo, smop+mildew, the websites, available libraries and applications, and last but not least: the community.

Having written quite some Perl 6 code (see my SVG related posts on this blog) over the previous weekend I think that developing Perl 6 has never been more fun - at least not to me ;-)

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