Day 5 – How to use the docs

…or why (nearly) everything is an object.

Spread out over all the pages of the official documentation of Perl 6, the class Signature is linked to more then 40 times. That shows not only that signatures are everywhere but also demonstrates a peculiarity of Perl 6 that will become more important as the language matures. No really, we are not done yet.

But first let’s have a look at some code.

multi sub trait_mod:<is> (Sub $s, :$foo!) is foo {
    say 'is foo called'
}
sub bar {}
&trait_mod:(&bar, :foo);

Here we define a trait that happens to be of type Sub which Rakudo (re)defines in Sub.pm. So there is actually a class for this build-in type. In fact, there are classes for anything besides classes and grammars. Having a Class for class would be a bit circular and prevent a slang to define its own object system. That you can do with Metamodel::ClassHOW, which is the Class behind class. But back to the trait.

A trait is somewhat macroish in nature as it is applied at compile time to some object its signature refers to. That allows the MMD to pick the candidate defined above when we use it with is foo. (There are a whopping 91 candidates of is defined in Rakudo.) Since it’s called at compile time it has to be called with something that exists at that time. All those classes for Sub, Label and friends provide a nice target for such a call. And yes, you can apply a trait to a label if you call it the right way in a BEGIN phaser. Combined with the power of compile time mixins and the the mighty .wrap you can go wild and improve the tool that is most dear to you — the Perl 6 compiler. If you observed the example carefully you likely spotted that the trait is applied to itself. If you really want to you could have infinite recursion at compile time.

Perl 6 is a dynamic dynamic language. The programs you write are dynamic and you can mend them and bend them to your will, both at compile- and runtime. And so you can do with the compiler. And that leads us to how to use the docs. Since language features are represented as a class, we link heavily to those classes to provide detailed discussions and you are encouraged to follow those links before you continue with the main paragraph the link refers from.

Macros will make good use of all those objects. With any luck next year will show us how.