Day 19 – Introspection

Perl 6 is an Object-Oriented Programming Language. Well, really, it’s multi-paradigm; but one of the options (that’s used throughout its core) is definitily Object Orientation.

However, as the (current) perl6.org tells us, Perl 6 supports “generics, roles and multiple dispatch”, which are some very nice features, and already covered in other advent calendar posts.

But the one we’re going to take a look at today is the MOP. “MOP” stands for “Meta-Object Protocol”. It means that, instead of objects, classes, etc defining the language; they’re actually a part you can change (or just look at) from the user’s side of things.

Indeed, in Perl 6, you can add methods to a type, remove some, wrap methods, augment classes with more capabilities (OO::Actors and OO::Monitors are two such examples), or you could totally redefine it (and, say, use a Ruby-like object system. Such an example here).

But today, instead, we’re gonna look at the first part: Introspection. Looking at a type after it’s been built, getting to know it, and use these informations.

The module we’re going to build together is based on a need for the Sixcheck module (a QuickCheck-like module): generate some random data for a type, then feed that data to the function we’re testing, and check some post-condition.

So, we write our first version:

my %special-cases{Mu} =
  (Int) => -> { (1..50).pick },
  (Str) => -> { ('a'..'z').pick(50).join('') },
;

sub generate-data(Mu:U \t) {
  %special-cases{t} ?? %special-cases{t}() !! t.new;
}

generate-data(Int);

okay, that’s the first version we wrote. We note a few things:

• We specify the key type for %special-cases. That’s because, by default, the type is Str. Obviously, we don’t want our types to be stringified. What we actually do is specify they’re going to be subtypes of “Mu” (which is at the top of the types “food chain”)
• We put parentheses around Int and Str, to avoid stringification.
• We use the :U in the function’s argument’s type. That means the value has to be undefined. Type objects (as in Int, Str, etc) are undefined, so it serves us well (a different unknown value you’ve probably seen is Nil).
• Type objects really are… Objects, like any other object (more on that later).
  That’s why we can call .new on them, like here. It’s the same as directly calling Int.new, for example (that’s useful for consistency, and autovivification).
• We provide a fallback for Int and Str, because calling Int.new and Str.new (0 and “”) would not give us any randomness in the data we create
• Perl 6 automatically returns the last expression in a function, so we didn’t have to put a return there.

That’s our code to generate the data, fair and square. But we’re gonna need to generate much more than those simple examples.

The least we need to support is classes with properties: we want to look at the list of attributes, generate data for their type, and feed them to the constructor (the only classes we support right now are those with a parameterless constructor).

We need to be able to look inside a class. What we’re going to reach for, in Perl 6 terms, is the Meta-Object Protocol (or MOP for short). First off, let’s define a sample data class for our, huh, blog (the author is sorry for such a lack of imagination).

class Article {
  has Str $.title;
  has Str $.content;
  has Int $.view-count;
}
# we can manually create instances this way:
Article.new(title => "Perl 6 Advent, Day 19",
            content => "Magic!",
            view-count => 0);

But we don’t want to create the article by hand. We want to pass the class Article to our generate-data function, and get an Article back (with random data inside). Let’s go back to our REPL…

> say Article.^attributes;
(Str $!title Str $!content Int $!view-count)
> say Article.^attributes[0].WHAT;
(Attribute)

If you’ve clicked on the MOP link, you shouldn’t be surprised that we get a 3-elements array. If you’re still surprised by the syntax, .^ is the meta-method call. What it means is that a.^b translates to a.HOW.b(a).

If we want to know what’s available to us, we could just ask (removing the anonymous ones):

> Attribute.^methods.grep(*.name ne '<anon>')
(compose apply_handles get_value set_value container readonly package inlined WHY set_why Str gist)
> Attribute.^attributes
Method 'gist' not found for invocant of class 'BOOTSTRAPATTR'

Whoops… Seems like this is a bit too meta. Thankfully, we can use a very nice property of Rakudo: a lot of it is written in Perl 6! To know what’s available to us, we can just look up the source:

#     has str $!name;
...
#     has Mu $!type;

We got the name for the key, and the type to generate the value. Let’s see…

> say Article.^attributes.map(*.name)
($!title $!content $!view-count)
> say Article.^attributes.map(*.type)
((Str) (Str) (Int))

Yep! Seems correct! (If you’re wondering why we get $! (private) twigils back, it’s because $. only means a getter method will be generated. The attribute itself is still private, and accessible in the class).

Now, the only thing we need to build is a loop…

my %args;
for Article.^attributes -> $attr {
 %args{$attr.name.substr(2)} = generate-data($attr.type);
}
say %args.perl;

This is an example of what could be printed:

{:content("muenglhaxrvykfdjzopqbtwisc"), :title("rfpjndgohmasuwkyzebixqtvcl"), :view-count(45)}

You get different results each time you run your code (I don’t think it’ll produce an article worth reading, however…).

Only thing left to do is pass them to Article‘s constructor:

say Article.new(|%args);

(the prefix pipe | allows us to pass %args as named arguments, instead of a single positional argument). Again, you should get something like this printed:

Article.new(title => "kyvphxqmejtuicrbsnfoldgzaw", content => "jqbtcyovxlngpwikdszfmeuahr", view-count => 26)

Yay! We managed to create an Article instance “blindly”, without knowing nothing special about Article at all. Our code can be used to generate data for any constructor that expects its class attributes to be passed. Done!

Before I go, I need to remind you of one things: with great power comes a lot of “WTF”. If you’re only taking a peek like we do here, and gathering information, it should be all fine. But don’t go around and change the internals of Rakudo’s classes, because no one knows what’d happen then :-). As always, have a moderate amount to fun introspecting.

PS: Left as an exercise to the reader is the recursive implementation, move the loop to generate-data, so that we could add a User $.author attribute to Article, and get this one constructed as well. Good luck!