/ 
Perl6-Bible-0.32
River stage zero No dependents
/ Perl6::Bible::S10

TITLE

Synopsis 10: Packages

AUTHOR

Larry Wall <larry@wall.org>

VERSION

Maintainer: Larry Wall <larry@wall.org>
Date: 27 Oct 2004
Last Modified: 24 Apr 2007
Number: 10
Version: 7

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 executable statement 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

A package (or any other similar namespace) can control autoloading. However, Perl 5's AUTOLOAD is being superseded by MMD autoloaders that distinguish declaration from definition, but are not restricted to declaring subs. A run-time declarator multisub is declared as:

multi CANDO ( MyPackage, $type, $name, *%args --> Container)

which stands in for the declaration of a container object within another container object; it is 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 CANDO when trying to determine if a class supports a particular method.) The arguments to CANDO include type information on what kind of object is expected in context, or this may be intuited from the name requested. In any case, there may be multiple CANDO routines that are dispatched via MMD:

multi CANDO ( MyPackage, Item, $name, *%args --> Container)
multi CANDO ( MyPackage, Array, $name, *%args --> Container)
multi CANDO ( MyPackage, Hash, $name, *%args --> Container)
multi CANDO ( MyPackage, Code, $name, *%args --> Container)

The package itself is just passed as the first argument, since it's the container object. Subsequent arguments identify the desired type of the inner container and the "name" or "key" by which the object is to be looked up in the outer container. Such a name does not include its container name, unlike Perl 5's magical $AUTOLOAD variable. Nor does it include the type information of a Code object's "long name"; this information comes in via the type parameter, and may be matched against using ordinary subsignature matching:

multi CANDO ( MyPackage, &:($), $name, *%args --> Container)     # 1 arg
multi CANDO ( MyPackage, &:($,$), $name, *%args --> Container)   # 2 args

The slurpy %args hash is likely to be empty in standard Perl 6 usage, but it's possible that some dialects of Perl will desire a mechanism to pass in additional contextual information, so this parameter is reserved for such purposes.

The CANDO is expected to return an inner container object of the proper sort (i.e. a variable, subroutine, or method object), or a proxy object that can "autovivify" lazily, or undef if that name is not to be considered declared in the namespace in question. (Only bare undef is interpreted as "not there", since typed undefs may function as autovivifiable proxy objects. See S12.)

The declaration merely defines the interface to the new object. That object need not be completely defined yet, though the CANDO routine is certainly allowed to define it eagerly, and even install the inner object into the outer container (the symbol table) if it wants to cache the declaration.

At declaration time it might not yet be known whether the inner container object will be used in lvalue or rvalue context; the use of a proxy object can supply either readonly or rw semantics later.

When the package in question is a class, it is also possible to declare real methods or submethods:

multi method CANDO ($self: Code, $name, *%args --> Container)

multi submethod CANDO ($self: Item, $name, *%args --> Container)

The method form is inherited by subclasses. Submethods are never inherited but may still do MMD within the class. (Ordinary multisubs are inherited only to the extent allowed by the MMD mechanism.)