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Sun, 22 Jan 2017

Perl 6 By Example: Perl 6 Review


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This blog post is part of my ongoing project to write a book about Perl 6.

If you're interested, please sign up for the mailing list at the bottom of the article, or here. It will be low volume (less than an email per month, on average).


In the previous "Perl 6 by Example" blog posts we've discussed some examples interleaved with the Perl 6 mechanics that make them work. Here I want to summarize and deepen the Perl 6 knowledge that we've touched on so far, removed from the original examples.

Variables and Scoping

In Perl 6, variable names are made of a sigil, $, @, % or &, followed by an identifier. The sigil implies a type constraint, where $ is the most general one (no restriction by default), @ is for arrays, % for hashes (associative arrays/maps), and & for code objects.

Identifiers can contain - and ' characters, as long as the character after it is a letter. Identifiers must start with a letter or underscore.

Subroutines and variables declared with my are lexically scoped. They are visible from the point of the declaration to the end of the current {}-enclosed block (or the current file, in case the declaration is outside a block). Subroutine parameters are visible in the signature and block of the subroutine.

An optional twigil between the sigil and identifier can influence the scoping. The * twigil marks a dynamically scoped variable, thus lookup is performed in the current call stack. ! marks attributes, that is, a per-instance variable that's attached to an object.

Subroutines

A subroutine, or short sub, is a piece of code with its own scope and usually also a name. It has a signature which specifies what kind of values you have to pass in when you call it:

sub chunks(Str $s, Int $chars) {
#         ^^^^^^^^^^^^^^^^^^^^ signature
#   ^^^^^^ name
    gather for 0 .. $s.chars / $chars - 1 -> $idx {
        take substr($s, $idx * $chars, $chars);
    }
}

The variables used in the signature are called parameters, whereas we call the values that you pass in arguments.

To refer to a subroutine without calling it, put an ampersand character in front of it, for example

say &chunks.^name;      # Sub

to call it, simply use its name, followed by the list of arguments, which can optionally be in parentheses:

say chunks 'abcd', 2;   # (ab cd)
say chunks('abcd', 2);  # (ab cd)

You only need the parentheses if some other construct would otherwise interfere with the subroutine call. For example if you intend to write

say chunks(join('x', 'ab', 'c'), 2);

and you leave out the inner pair of parentheses:

say chunks(join 'x', 'ab', 'c', 2);

then all the arguments go to the join function, leaving only one argument to the chunks function. On the other hand it is fine to leave out the outer pair of parentheses and write

say chunks join('x', 'ab', 'c'), 2;

because there's no ambiguity here.

One case worth noting is that if you call a subroutine without arguments as the block of an if condition or a for loop (or similar constructs), you have to include the parentheses, because otherwise the block is parsed as an argument to the function.

sub random-choice() {
    Bool.pick;
}

# right way:
if random-choice() {
    say 'You were lucky.';
}

# wrong way:
if random-choice {
    say 'You were lucky.';
}

If you do happen to make this mistake, the Perl 6 compiler tries very hard to detect it. In the example above, it says

Function 'random-choice' needs parens to avoid gobbling block

and when it tries to parse the block for the if-statement, it doesn't find one:

Missing block (apparently claimed by 'random-choice')

When you have a sub called MAIN, Perl 6 uses its signature to parse the command line arguments and pass those command line arguments to MAIN.

multi subs are several subroutines with the same name but different signatures. The compiler decides at run time which of the candidates it calls based on the best match between arguments and parameters.

Classes and Objects

Class declarations follow the same syntactic schema as subroutine declarations: the keyword class, followed by the name, followed by the body in curly braces:

class OutputCapture {
    has @!lines;
    method print(\s) {
        @!lines.push(s);
    }
    method captured() {
        @!lines.join;
    }
}

By default, type names are scoped to the current namespace, however you can make it lexically scoped by adding a my in front of class:

my class OutputCapture { ... }

Creating a new instance generally works by calling the new method on the type object. The new method is inherited from the implicit parent class Any that all types get:

my $c = OutputCapture.new;

Per-instance state is stored in attributes, which are declared with the has keyword, as seen above in has @!lines. Attributes are always private, as indicated by the ! twigil. If you use the dot . twigil in the declaration instead, you have both the private attribute @!lines and a public, read-only accessor method:

my class OutputCapture {
    has @.lines;
    method print(\s) {
        # the private name with ! still works
        @!lines.push(s);
    }
    method captured() {
        @!lines.join;
    }
}
my $c = OutputCapture.new;
$c.print('42');
# use the `lines` accessor method:
say $c.lines;       # [42]

When you declare attributes with the dot twigil, you can also initialize the attributes from the constructor through named arguments, as in OutputCapture.new( lines => [42] ).

Private methods start with a ! and can only be called from inside the class body as self!private-method.

Methods are basically just subroutines, with two differences. The first is that they get an implicit parameter called self, which contains the object the method is called on (which we call the invocant). The second is that if you call a subroutine, the compiler searches for this subroutine in the current lexical scope, and outer scopes. On the other hand, the methods for a method calls are looked up in the class of the object and its superclasses.

Concurrency

Perl 6 provides high-level primitives for concurrency and parallel execution. Instead of explicitly spawning new threads, you are encouraged to run a computation with start, which returns a Promise. This is an object that promises that in the future the computation will yield a result. The status can thus be Planned, Kept or Broken. You can chain promises, combine them, and wait for them.

In the background, a scheduler distributes such computations to operating system level threads. The default scheduler is a thread pool scheduler with an upper limit to the number of threads to use.

Communication between parallel computations should happen through thread-safe data structures. Foremost among them are the Channel, a thread-safe queue, and Supply, Perl 6's implementation of the Observer Pattern. Supplies are very powerful, because you can transform them with methods such as map, grep, throttle or delayed, and use their actor semantic to ensure that a consumer is run in only one thread at a time.

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