Raku testing and conditional compilation

I've been really falling in love with the Raku programming language – it's powerful, expressive, has great support for introspection, strong pattern matching, and extremely good support for metaprogramming. I'm pretty much sold on using it for any project where I don't need raw performance. (When I do need raw performance, I still reach for Rust).

That said, there are a few niceties and patterns I miss from Rust. Fortunately, Raku is powerful enough to make nearly all of them possible, usually with just a few lines of code.

Today, I'd like to talk about one example: Writing unit tests in the same file as the code under test, without any runtime cost thanks to conditional compilation.

Organizing tests

Putting code and unit tests together in the same file is generally not standard procedure in Raku. Instead, as explained in the Test docs, you typically put all tests in a /t directory, and name each file with the same name as the code under test – except with a .t extension instead of a .rakumod or .pm6 extension. So, if you're testing the code in a fibonacci.pm6 file, you'd put the tests in /t/fibonacci.t.

This works, and if you prefer it, you should certainly feel free to stick with that method. It can be a clean way to organize your code, especially if you tend to write longer modules (which can make the module+test combination unmanageably large).

On the other hand, I really like Rust's approach to organizing unit tests: put each test in the same file as the code being tested. This has a few pretty large advantages:

The typical downsides of tests in the same file

Given these advantages, why do so many languages default to organizing their tests outside of the file/module/package/namespace containing the code under test? In short, performance.

In most interpreted languages (and more than a few compiled ones), adding test code to the main file would have a significant run-time cost. Even if the test code isn't executed at run time, it's still present in the file; it still needs to be parsed before the program can be run (or, for a compiled program, it still bloats the binary).

Rust's solution

How does Rust get around this? As performance-focused as Rust is, there's no way it would pay a runtime cost for its tests. And, indeed, it doesn't: it uses conditional compilation to compile test code only when running tests. The standard way to write tests in Rust looks like this:

mod test {
    use super::*;
    fn first_test() {}

In case you don't speak Rust, the #[cfg(test)] on the first line is an attribute that tells the Rust compiler to only compile the following block when it's running a test. If you compile that project without passing the test argument, you'll get exactly the same binary you would have gotten without writing a single line of test code. Rust achieves what it's always looking for: the zero-cost abstraction.

Bringing this solution to Raku

Raku doesn't have conditional compilation in the same way Rust does (at least not yet, anyway!). But it does have a sophisticated notion of compile-time programming that's powerful enough to give us the equivalent – though, as we'll see, it's so powerful that we'll have to be careful to get exactly what we want at the truly zero cost that Rust provides.

At the most basic level, how can we get started with compile-time programming in Raku? Well, we can begin with a BEGIN block (or a CHECK block). Both of these blocks execute code only at compile time, with BEGIN executing at the beginning of compile time, while CHECK executes at the end. So, imagine you have code like this:

CHECK { say 'compiling' }
say 'running';

If you invoke this file with raku -c, you'll compile it without running it, and you'll get "compiling" as your output. If you run this file with raku, you'll both compile and run it, and you'll get both "compiling" and "running" as output.

CHECK blocks aren't good enough

Using CHECK blocks, however, won't quite get us where we want to be. If you split this out into its own module, and compile it, you'll notice that it does not produce the same bytecode as the file without the CHECK block. That is, this code:

unit module Example;
say 'running';

and this code

unit module Example;
CHECK { say 'compiling' }
say 'running';

don't produce the same bytecode. Specifically, the first version produces only 24 KiB of bytecode, while the second produces 52 KiB. For a long time, I couldn't figure out what caused this discrepancy – but Johnathan Worthington was kind enough to explain it to me: Raku has the concept of nested compile times and runtimes, which means that the bytecode needs to include some output for the inner compile time (even though it doesn't get executed in the run time we really care about).

Now, this isn't really a big deal at all–as Johnathan also explained, Rakudo is very good at minimizing the cost of bytecode that's never run. But we're going for zero cost, so we'll need to do better.

Switching to DOC

The key to doing so lies in the DOC block – the block Raku provides to execute code when generating documentation. Like the CHECK and BEGIN phasers, DOC blocks aren't invoked during runtime; unlike those blocks, DOC blocks avoid creating a nested runtime inside the compile time. With that in mind, let's update our code:

unit module Example;
DOC CHECK { say 'compiling' }
say 'running';

Great, now we're back to the 24 KiB we'd have had without any compile time code.

Using tests without use test

This almost gets us to a perfect solution – but not quite. If we add a use Test; line to our code, we're right back to seeing our bytecode size shoot up (this time, all the way to 88 KiB). What's going on?

Once again, Johnathan had the answer:

So far as use goes, its action is performed as soon as it is parsed. Being inside a DOC CHECK block does not suppress that - and in general can not, because the use might bring in things that need to be known in order to finish parsing the contents of that block.

Ok, so use statements are special and will add to our bytecode even when they live inside a DOC CHECK block. How can we fix that? Easy, just don't use use. Ok, next question: how can we test anything without a use Test line?

We can take advantage of another Rakudo option, -M. This flag loads a module immediately before running the program – if we invoke our module with raku -MTest, then we can call all test functions we want without ever needing to use Test.

Compiler fight

So, we have our test code working perfectly, but Rakudo isn't at all happy with our non-test code. When we run our code without -MTest, Rakudo complains that whatever Test functions we're using are Undeclared routines – even though they're inside DOC CHECK blocks and won't get executed. At this point in figuring this all out, I was just about ready to swear at Rakudo: yes, the routine is undefined now, but it won't be when you need to call it! Grrrrr.

Fortunately for this blog post and my sanity, Raku offers us an easy out: to use the is method from Test, we add a single line: multi is(|) { callsame }. This satisfies Rakudo with a placeholder &is symbol for the times that we're not loading Test, while still letting us invoke the actual is function when we have loaded Test. (If you haven't come across callsame or the cool things you can do with it before, then the relevant docs are well worth a read.)

Putting it all together

Despite the length of this post, all of this results in just a few lines of code. Here's what testing a simple Fibonacci function would look like:

# ./bin/fibonacci
use v6d;
use Fibonacci;

#| Print the first N Fibonacci numbers
sub MAIN(Int $N) { say fibonacci($N).join("\n") }
# ./lib/Fibonacci.pm6
use v6d;
unit module Fibonacci;

#| Return a list of the first $n Fibonacci numbers
sub fibonacci(Int $n --> List) is export { 
    state @fib = 1, 1, * + * … ∞;

DOC CHECK { multi is-deeply(|) { callsame }
   fibonacci(1).&is-deeply((1,), 'Fibonacci of 1');
   fibonacci(5).&is-deeply((1,1,2,3,5), 'Fibonacci of 5');

With that code, you can test with raku --doc -c -MTest lib/Fibonacci, which produces

ok 1 - Fibonacci of 1
ok 2 - Fibonacci of 2
Syntax OK

And you can run it with raku -Ilib bin/fibonacci which produces

  bin/fibonacci <N> -- Print the first N Fibonacci numbers

(I still can't get over how easy Raku makes producing nice usage output!). Or, for actual output, with raku -Ilib bin/fibonacci 5:


And – perhaps best of all! – our byte code is only 40 KiB. And that's exactly the same size as if we'd omitted the tests entirely. A true zero cost abstraction, in a single line of code.

Summing up

This may not be %100 seamless. In particular, if your tests use a lot of functions from Test, declaring the multis could get annoying. But, in a single line of code, we were able to build a powerful feature and one I've been missing from Rust. And that's a small taste of what I love so much about Raku.

If you have any thoughts about this post, especially including any ways to make this even better, I'd love to hear them. You can email me, find me on the #raku IRC channel, or post to this post's thread on r/rakulang.