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NAME

Synopsis_11 - Modules

AUTHOR

Larry Wall <larry@wall.org>

VERSION

Maintainer: Larry Wall <larry@wall.org>
Date: 27 Oct 2004
Last Modified: 16 Nov 2005
Number: 11
Version: 6

Overview

This synopsis discusses those portions of Apocalypse 12 that ought to have been in Apocalypse 11.

Modules

As in Perl 5, a module is just a kind of package. Unlike in Perl 5, modules and classes are declared with separate keywords, but they're still just packages with extra behaviors.

A module is declared with the module keyword. There are two basic declaration syntaxes:

module Foo;	# rest of scope is in module Foo
...

module Bar {...}	# block is in module Bar

The first form is allowed only as the first statement in the file.

Since there are no barewords in Perl 6, module names must be predeclared, or use the sigil-like ::ModuleName syntax. The :: prefix does not imply top-levelness as it does in Perl 5. (Use ::* or GLOBAL:: for that.)

A bare module declarator declares an our module name in the current package. At the start of the file, the current package is *, so the first declaration in the file is automatically global.

You can use our module to explicitly declare a module in the current package (or module, or class). To declare a lexically scoped module, use my module. Module names are always searched for from innermost scopes to outermost. As with an initial ::, the presence of a :: within the name does not imply globalness (unlike in Perl 5).

The ::* namespace is not "main". The default namespace for the main program is ::*Main, which it switches to from * as soon as it sees the first declaration, if that declaration doesn't set the package name. (Putting module Main; at the top of your program is redundant, except insofar as it tells Perl that the code is Perl 6 code and not Perl 5 code. But it's better to say "use v6" for that.)

But note that if you say

use v6;
module Foo {...}

you've just created Main::Foo, not *Foo.

Module traits are set using is:

module Foo is bar {...}

Exportation

Exportation is now done by trait declaration on the exportable item:

                                           # Tagset...
sub foo is export(:DEFAULT)         {...}  #  :DEFAULT, :ALL
sub bar is export(:DEFAULT :others) {...}  #  :DEFAULT, :ALL, :others
sub baz is export(:MANDATORY)       {...}  #  (always exported)
sub bop is export                   {...}  #  :ALL
sub qux is export(:others)          {...}  #  :ALL, :others

Importation

All imports are done by default into the current lexical scope (rather than the current package, as in Perl 5).

use Sense <common @horse>;

You can be explicit about the desired scoping:

use Sense :my<common> :our<@horse> :state<$haslearnedsome>;

That's pretty much equivalent to:

use Sense;
my &common ::= &Sense::common;
our @horse ::= @Sense::horse;
state $haslearnedsome ::= $Sense::haslearnedsome;

In the absence of a specific scoping specified by the caller, the module may also specify a different scoping default by use of :our or :state tags as arguments to is export. (Of course, mixing incompatible scoping in different scopes is likely to lead to confusion.)

Versioning

When at the top of a file you say something like

module Cat;

or

class Dog;

you're really only giving one part of the name of the module. The full name of the module or class includes other metadata, in particular, the version, and the author.

Modules posted to CPAN or entered into any standard Perl 6 library are required to declare their full name so that installations can know where to keep them, such that multiple versions by different authors can coexist, all of them available to any installed version of Perl.

The syntax of a versioned module or class declaration has three parts separated by hyphens. The three parts are the short name of the class/module, its version number, and a URI identifying the author (or authorizing authority). For example:

class Dog-1.2.1-cpan:JRANDOM;
class Dog-1.2.1-http://www.some.com/~jrandom;
class Dog-1.2.1-mailto:jrandom@some.com;

Such a declaration automatically aliases the full name of the class (or module) to the short name. So for the rest of the lexical scope, Dog refers to the longer name.

If there are extra classes or modules or packages declared within the same file, they implicitly have a long name including the file's version and author, but you needn't declare them again.

Since these long names are the actual names of the classes, when you say:

use Dog;

you're really wildcarding the unspecified bits:

use Dog-(Any)-(Any);

And when you say:

use Dog-1.2.1;

you're really asking for:

use Dog-1.2.1-(Any);

Saying 1.2.1 specifies an exact match on the version number, not a minimum match. To match more than one version, put a range operator in parens:

use Dog-(1.2.1..1.2.3);
use Dog-(1.2.1..^1.3);
use Dog-(1.2.1...);

Subversions are wildcarded, so 1.2 really means 1.2.0.... If you say:

use v6;

which is short for:

use Perl-6;

you're asking for any version of Perl 6. Say:

use Perl-6.0;
use Perl-6.0.0;
use Perl-6.2.7.1;

if you want to lock in a particular set of semantics at some greater degree of specificity. And some large company ever forks Perl, you can say

use Perl-6-cpan:TPF

to guarantee that you get the unembraced Perl. :-)

For wildcards any valid smartmatch selector works:

use Dog-(1.2.1 | 1.3.4)-(/:i jrandom/);
use Dog-(Any)-(/^cpan\:/)

Parens are optional on a closure smartmatcher. The preceding may also be written:

use Dog-{$^ver ~~ 1.2.1 | 1.3.4}-{$^auth ~~ /:i jrandom/};
use Dog-{$^ver ~~ Any}-{$^auth ~~ /^cpan\:/}

In any event, however you select the module, its full name is automatically aliased to the short name for the rest of your lexical scope. So you can just say

my Dog $spot .= new("woof");

and it knows (even if you don't) that you mean

my Dog-1.3.4-cpan:JRANDOM $spot .= new("woof");

The use statement actually allows a language on the front of a module name, so that you can use modules from other languages. The language is separated by a colon. For instance:

use perl5:ACME::Bleach-1.0-DCONWAY;
use ruby:Rails <PR_MACHINE>;

Forcing Perl 6

To get Perl 6 parsing rather than the default Perl 5 parsing, we said you could force Perl 6 mode in your main program with:

use Perl-6;

Actually, you can just start your main program with any of:

use v6;
module;
class;

Those all specify the latest Perl 6 semantics, and are equivalent to

use Perl-(v6.0...)-(Any);

To lock the semantics to 6.0.0, say:

use v6.0.0;

In any of those cases, strictures and warnings are the default in your main program. But if you start your program with a bare version number or other literal:

v6.0.0;
v6;
6;
"Coolness, dude!";

it runs Perl 6 in "lax" mode, without strictures or warnings, since obvously a bare literal in a void context ought to have produced a warning. (Invoking perl with -e6 has the same effect.)