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Perl6::Bible::S10

NAME

Synopsis_10 − Packages

AUTHOR

Larry Wall <larry AT wall DOT org>

VERSION

  Maintainer: Larry Wall <larry AT wall DOT org>
  Date: 27 Oct 2004
  Last Modified: 22 Feb 2006
  Number: 10
  Version: 4

Overview

This synopsis summarizes Apocalypse 10, which discusses packages despite never having been written.

Packages

As in Perl 5, packages are the basis of modules and classes. Unlike in Perl 5, modules and classes are declared with separate keywords, but they’re still just packages with extra behaviors.

An ordinary package is declared with the "package" keyword. It can only be used with a block:

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

A named package declaration can occur as part of an expression, just like named subroutine declarations.

As a special exception, if a braceless "package" declaration occurs as the first thing in a file, then it’s taken to mean that the rest of the file is Perl 5 code.

    package Foo;        # the entire file is Perl 5
    ...

This form is illegal in the middle of a Perl 6 file.

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

A bare "package" declarator declares an "our" package within the current package (or module, or class, or role, or...). Use "*" or "GLOBAL::" to declare a global package name.

To declare a lexically scoped package, use "my package". Package 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). True globals are always in the "GLOBAL::" namespace, which has the shortcut "*" where that is not ambiguous with "real" operators.

The "*" namespace is not "main". The default namespace for the main program is *Main in Perl 6. All files start out being parsed in the "*" package, but switch to some other package scope depending on the first declaration. If that first declaration is not a package variant, then the parsing switches to the "*main" package for Perl 5 code and the "*Main" package for Perl 6 code.

Package traits are set using "is":

    package Foo is bar {...}

All symbolic links are done with the "::($expr)" syntax, which is legal in any variable, package, module, or class name anywhere a "::Ident" is legal. The string returned by the expression will be parsed for "::" indicating subpackage names. Do not confuse this with the

    Foo::{$key}

syntax that lets you do a lookup in a particular symbol table. In this case, the key is not parsed for "::". It’s just a hash lookup.

Autoloading

The package is the namespace that controls autoloading. There is still an "AUTOLOAD" hook that behaves as in Perl 5. However, that is being replaced by various autoload hooks that distinguish declaration from definition, and various types from one another. In particular:

    AUTOSCALAR
    AUTOARRAY
    AUTOHASH
    AUTOSUB
    AUTOMETH

stand in for the declaration of objects; they are called when anyone is searching for a name in the package (or module, or class), and the name doesn’t already exist in the package. (In particular, ".can" calls "AUTOMETH" when trying to determine if a class supports a particular method.) The routines are expected to return a reference to an object of the proper sort (i.e. a variable, subroutine, or method reference), or undef if that name is not to be considered declared. That object need not be defined yet, though the routine is allowed to define it, and even install it into the symbol table if it likes.

When someone tries to actually call or access an undefined object (which may have come from one of the routines above, or might have just been declared with a body of "{...}"), a different set of hooks is used to define actual behavior at the last moment:

    AUTOSCALARDEF
    AUTOARRAYDEF
    AUTOHASHDEF
    AUTOSUBDEF
    AUTOMETHDEF

These routines are expected to define the object, but not to call it, since the call is already "scheduled" from somewhere else. (The "goto &$AUTOLOAD" is implicit, in other words. But you can hijack the call via the "call" builtin, in which case the autoloader behaves just like a wrapper--see S06.)

In any case, there is no longer any magical $AUTOLOAD variable. The name being declared or defined can be found in $_ instead. The name does not include the package name. You can always get your own package name with "$?PACKAGENAME".

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