S12-objects.pod

=encoding utf8

=head1 TITLE

Synopsis 12: Objects

=head1 AUTHOR

Larry Wall <larry@wall.org>

=head1 VERSION

  Maintainer: Larry Wall <larry@wall.org>
  Date: 27 Oct 2004
  Last Modified: 21 Jan 2009
  Number: 12
  Version: 68

=head1 Overview

This synopsis summarizes Apocalypse 12, which discusses object-oriented
programming.

=head1 Classes

A class is a module declared with the C<class> keyword.  As with
modules, the public storage, interface, and name of the class is
represented by a package and its name, which is usually (but not
necessarily) a global name.

Taken as an object, a class represents all of the possible values of
its type, and the class object can thus be used as a proxy for any
"real" object of that type in calculating what a generic object of
that type can do.  The class object is an Object, but it is not a
Class, because there is no mandatory Class class in Perl 6.  We wish
to support both class-based and prototype-based OO programming.
So all metaprogramming is done through the current object's C<HOW>
object, which can delegate metaprogramming to any metamodel it likes.
However, by default, objects derived from Object support a fairly
standard class-based model.

There are two basic class declaration syntaxes:

    class Foo;          # rest of file is class definition
    ...

    class Bar {...}     # block is class definition

The first form is allowed only as the first declaration in a compilation
unit (that is, file or eval string).

In either case, the code represented by C<...> executes at compile
time as the body of a method of the metaclass, which is responsible
for interpreting the keywords of the class definition.  (And since a
class is also a module, it also handles any module-oriented keywords.
You can export subs from a class at "use" time, for instance.)

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

Classes are primarily for instance management, not code reuse.
Consider using C<roles> when you simply want to factor out
common code.

Perl 6 supports multiple inheritance, anonymous classes, and autoboxing.

All public method calls are "virtual" in the C++ sense.  More
surprisingly, any class name mentioned in a method is also considered
virtual, that is, polymorphic on the actual type of the object.

You may derive from any built-in type, but the derivation of a low-level
type like C<int> may only add behaviors, not change the representation.
Use composition and/or delegation to change the representation.

Since there are no barewords in Perl 6, bare class names must be
predeclared.  You can predeclare a stub class and fill it in later
just as you would a subroutine.  Alternately, you can define a local
class or type variable using the C<::> type sigil.  In an rvalue
context the C<::> prefix is a no-op, but in a declarational context,
it binds a new type name within its declared scope.

Without a C<my> or other scoping declarator, a bare C<class>
declarator declares an C<our> declarator, that is, a name within
the current package.  Since class files begin parsing in the
C<GLOBAL> package, the first class declaration in the file installs
itself as a global name, and subsequent declarations then install
themselves into the current class rather than the global package.

Hence, to declare an inner class in the current package (or module, or
class), use C<our class> or just C<class>.  To declare a lexically
scoped class, use C<my class>.  Class names are always searched
for from innermost scopes to outermost.  As with an initial C<::>,
the presence of a C<::> within the name does not imply globalness
(unlike in Perl 5).  So the outward search can look in children
of the searched namespaces.

Class traits are set using C<is>:

    class MyStruct is rw {...}

An "isa" is just a trait that happens to be another class:

    class Dog is Mammal {...}

MI is specified with multiple C<is> modifiers:

    class Dog is Mammal is Pet {...}

Roles use C<does> instead of C<is>:

    class Dog is Mammal does Pet {...}

You may put these inside as well:

    class Dog {
        is Mammal;
        does Pet;
        ...
    }

Every object (including any class object) delegates to an instance of
its metaclass.  You can get at the metaclass of any object via the
C<HOW> method, which returns an instance of the metaclass.  A "class" object is just considered an "empty"
instance in Perl 6, more properly called a "prototype" object, or just
"protoobject".
The actual class object is the metaclass object pointed to by the
C<HOW> syntax.  So when you say "C<Dog>", you're referring to both a
package and a protoobject, that latter of which points to the
actual object representing the class via C<HOW>.  The protoobject
differs from an instance object not by having a different
type but rather in the extent to which it is defined.  Some objects
may tell you that they are defined, while others may tell you that
they are undefined.  That's up to the object, and depends on how the
metaclass chooses to dispatch the C<.defined> method.

The notation C<^Dog> is syntactic sugar for C<Dog.HOW()>, so C<^> can be
considered the "class" sigil when you want to talk about the current
metaclass instance.

Classes are open and non-final by default, but may easily be closed
or finalized not by themselves but by the entire application, provided
nobody issued an explicit compile-time request that the class stay open
or non-final.  (Or a site policy could close any applications that use
the policy.)  Platforms that do dynamic loading of sub-applications
probably don't want to close or finalize classes wholesale, however.

Roles take on some of the compile-time function of closed classes,
so you should probably use those instead anyway.

A private class can be declared using C<my>; most privacy issues are
handled with lexical scoping in Perl 6.  The fact that importation
is lexical by default also means that any names your class imports
are also private by default.

=head1 Methods

Methods are routines declared in a class with the C<method> keyword:

    method doit ($a, $b, $c) { ... }
    method doit ($self: $a, $b, $c) { ... }
    method doit (MyName $self: $a, $b, $c) { ... }
    method doit (::?CLASS $self: $a, $b, $c) { ... }

Declaration of the invocant is optional.  You may always access the
current invocant using the keyword C<self>.  You need not declare the
invocant's type, since the lexical class of the invocant is known in any
event because methods must be declared in the class of the invocant,
though of course the actual (virtual) type may be a derived type of
the lexical type.  You could declare a more restrictive type, but
that would probably be a bad thing for proper polymorphism.  You may
explicitly type the invocant with the lexical type, but any check for
that will be optimized away.  (The current lexically-determined class
may always be named as C<::?CLASS> even in anonymous classes or roles.)

To mark an explicit invocant, just put a colon after it:

    method doit ($x: $a, $b, $c) { ... }

This is true also for multi methods:

    multi method doit ($x: $a; $b; $c) { ... }

If you declare an explicit invocant for an Array type using an array variable,
you may use that directly in list context to produce its elements

    method push3 (@x: $a, $b, $c) { ... any(@x) ... }

Note that the C<self> function is not context sensitive and thus always
returns the current object as a single item even in list context.
Hence if your current object happens to be an array but you did not
declare it with an explicit array variable, you need to explicitly
access the elements of the array somehow:

    any(self)     # WRONG
    any(self[])   # okay
    any(@(self))  # okay
    any(@self)    # WRONG unless you declared @self yourself

Private methods are declared using C<my>:

    my method think (Brain $self: $thought)

(Such methods are completely invisible to ordinary method calls, and are
in fact called with a different syntax that uses C<!> in place of the C<.>
character.  See below.)

To call an ordinary method with ordinary method-dispatch semantics,
use either the dot notation or indirect object notation:

    $obj.doit(1,2,3)
    doit $obj: 1,2,3

Indirect object notation now requires a colon after the invocant,
even if there are no arguments after the colon:

    $handle.close;
    close $handle:;

To reject method call and only consider subs, simply omit the colon
from the invocation line:

    close($handle);
    close $handle;

However, here the built-in B<IO> class defines C<method close () is export>,
which puts a C<multi sub close (IO)> in scope by default.  Thus if the
C<$handle> evaluates to an IO object, then the two subroutine calls above
are still translated into method calls.

Dot notation can omit the invocant if it's in C<$_>:

    .doit(1,2,3)

Note that there is no corresponding notation for private methods.

    !doit(1,2,3)        # WRONG, would be parsed as not(doit(1,2,3))
    self!doit(1,2,3)    # okay

There are several forms of indirection for the method name.  You can
replace the identifier with a quoted string, and it will be evaluated
as a quote and then the result of that is used as the method name.

    $obj."$methodname"(1,2,3)   # use contents of $methodname as method name
    $obj.'$methodname'(1,2,3)   # no interpolation; call method with $ in name!

    $obj!"$methodname"          # indirect call to private method name

[Note: to help catch the mistaken use of C<< infix:<.> >> as a string
concatenation operator, Perl 6 will warn you about "useless use of
quotes" at compile time if the string inside quotes is an identifier.
(It does not warn about non-identifier strings, but such strings are
likely to produce missing method errors at run time in any case.)
Also, if there is whitespace around an intended C<.> concatenation,
it cannot be parsed as a method call at all; instead it fails at
compile time because standard Perl 6 has a pseudo C<< infix:<.> >> operator
that always fails at compile time.]

For situations where you already have a method located, you
can use a simple scalar variable in place of method name:

    $methodobj = $foo ?? &bar !! &baz;
    $obj.$methodobj(1,2,3)

or more succinctly but less readably:

    $obj.$($foo ?? &bar !! &baz)(1,2,3)

The variable must contain a C<Callable> object (usually of type C<Code>), that is, a closure of some
sort.  Regardless of whether the closure was defined as a method or
a sub or a block, the closure is called directly without any class
dispatch; from the closure's point of view, however, it is always
called as a method, with the object as its first argument, and the
rest of the arguments second, third, and so on.   For instance, such
a closure may be used to abstract a "navigational" path through a
data structure without specifying the root of the path till later:

    $locator = -> $root, $x, $y { $root.<foo>[$x]<bar>{$y}[3] }
    $obj.$locator(42,"baz")  # $obj<foo>[42]<bar><baz>[3]

    $locator = { .<here> }
    $obj.$locator            # $obj<here>

As a convenient form of documentation, such a closure may also be written
in the form of an anonymous method:

    $locator = method ($root: $x, $y) { $root.<foo>[$x]<bar>{$y}[3] }
    $obj.$locator(42,"baz")  # $obj<foo>[42]<bar><baz>[3]

    $locator = method { self.<here> }
    $obj.$locator            # $obj<here>

Note however that, like any anonymous closure, an anonymous method
can only be dispatched to directly, like a sub.  You may, of course,
bind an anonymous method to the name of a method in a class's public
interface, in which case it is no longer anonymous, and may be
dispatched to normally via the class.  (And in fact, when the normal
method dispatcher is calling individual candidates in its candidate
list, it calls each candidate as a sub, not as a method, or you'd
end up with recursive dispatchers.)  But fundamentally, there's
no such thing as a method closure.  The C<method> declarator on an
anonymous method has the primary effect of making the declaration
of the invocant optional.  (It also makes it an official C<Routine>
that can be returned from, just as if you'd used C<sub> to declare it.)

Instead of a scalar variable, an array variable may also be used:

    $obj.@candidates(1,2,3)

As with the scalar variant, string method names are not allowed, only
C<Callable> objects, The list is treated as a list of candidates to
call.  After the first successful call the rest of the candidates are
discarded.  Failure of the current candidate is indicated by calling
C<nextwith> or C<nextsame> (see L</Calling sets of methods> below).

Note also that the 

    $obj.$candidates(1,2,3)

form may dispatch to a list of candidates if C<$candidates> is either
a list or a special C<Code> object representing a partial dispatch to a
list of candidates.  If C<$candidates> (or any element of C<@candidates>)
is a List or Array object it is expanded out recursively until C<Callable>
candidates are found.  The call fails if it hits a candidate that is
neither C<Callable> nor expandable.

Another form of indirection relies on the fact that operators are named
using a variant on hash subscript notation, which gives you these forms:

    $x.infix:{$op}($y)
    $x.prefix:{$op}
    $x.postfix:{$op}

Generally you see these with the literal angle bracket form of subscript:

    $a.infix:<*>($b)      # equivalent to $a * $b
    $a.prefix:<++>        # equivalent to ++$a
    $a.postfix:<++>       # equivalent to $a++

If you omit the syntactic category, the call will be dispatched according
to the number of arguments either as "prefix" or as "infix":

    $a.:<+>($b)           # equivalent to $a + $b
    $a.:<++>              # equivalent to ++$a
    $a.:<!>               # equivalent to !$a
    @a.:<[*]>             # equivalent to [*] @a

But it's probably better to spell out the syntactic category when
the actual operator is not obvious:

    $x.infix:{$op}($y)
    $x.prefix:{$op}

You must use a special syntax to call a private method:

    $mybrain!think($pinky)
    self!think($pinky)

For a call on your own private method, you may also use the attribute-ish form:

    $!think($pinky)     # short for $(self!think($pinky))

Parentheses (or a colon) are required on the dot/bang notations if there
are any arguments (not counting adverbial arguments).  There may be
no space between the method name and the left parenthesis unless you
use the dot form of parentheses or otherwise make use of "unspace":

    .doit       # okay, no arguments
    .doit()     # okay, no arguments
    .doit ()    # ILLEGAL (two terms in a row)
    .doit.()    # okay, no arguments, same as .doit()
    .doit\ ()   # okay, no arguments, same as .doit() (unspace form)
    .doit\ .()  # okay, no arguments, same as .doit.() (unspace form)

However, you can turn any of the legal forms above into a list
operator by appending a colon:

    .doit: 1,2,3        # okay, three arguments
    .doit(1): 2,3       # okay, one argument plus list
    .doit (): 1,2,3     # ILLEGAL (two terms in a row)
    .doit.(1): 2,3      # okay, same as .doit(1,2,3)
    .doit\ .(1,2): 3    # okay, same as .doit(1,2,3)

In particular, this allows us to pass a closure in addition to the
"normal" arguments:

    .doit: { $^a <=> $^b }              # okay
    .doit(): { $^a <=> $^b }            # okay
    .doit(1,2,3): { $^a <=> $^b }       # okay

In case of ambiguity between indirect object notation and dot form,
the nearest thing wins:

    dothis $obj.dothat: 1,2,3

means

    dothis ($obj.dothat(1,2,3))

and you must say

    dothis ($obj.dothat): 1,2,3

or

    $obj.dothat.dothis: 1,2,3

if you mean the other thing.

Also note that if any term in a list is a bare closure or pointy
sub, it will be considered to be the final argument of its list
unless the closure's right curly is followed immediately by comma
or colon.  In particular, a method call does *not* extend
the list, so you can say:

    @list.grep: { $_ % 2 }.map: { $_ - 1 }.say

and that will be taken as equivalent to

    @list.grep({ $_ % 2 }).map({ $_ - 1 }).say

Methods (and subs) may be declared as lvalues with C<is rw>.  You can
use an argumentless C<rw> method anywhere you can use a variable,
including in C<temp> and C<let> statements.  (In fact, you can use an
C<rw> method with arguments as a variable as long as the arguments are
used only to identify the actual value to change, and don't otherwise
have strange side effects that differ between rvalue and lvalue usage.
Setter methods that expect the new value as an argument do not fall
into the well-behaved category, however.)

Method calls on mutable scalars always go to the object contained in
the scalar (autoboxing value types as necessary):

    $result = $object.doit();
    $length = "mystring".codes;

Method calls on non-scalar variables just calls the C<Array>, C<Hash>
or C<Code> object bound to the variable:

    $elems = @array.elems;
    @keys  = %hash.keys;
    $sig   = &sub.signature;

Use the prefix C<VAR> macro on a scalar variable to get at its
underlying C<Scalar> object:

    if VAR($scalar).readonly {...}

C<VAR> is a no-op on a non-scalar variables and values:

    VAR(1);     # 1
    VAR(@x);    # @x

There's also a corresponding C<< postfix:<.VAR> >> macro that can be used
as if it were a method:

    if $scalar.VAR.readonly {...}

(But since it's a macro, C<VAR> is not dispatched as a real method.
To dispatch to a real C<.VAR> method, use the indirect C<$obj."VAR">
form.)

You can also get at the container through the appropriate symbol table:

    if MY::<$scalar>.readonly {...}

=head1 Class methods

Other OO languages give you the ability to declare "class" methods that either don't
need or actively prohibit calls on instances.  Perl 6 gives you a choice.
If you declare an ordinary method, it can function as a "class" method when you
pass it a protoobject such as "C<Dog>" regardless of how defined the prototype
object is, as long as the method body doesn't try to access any information that
is undefined in the current instance.

Alternately, you can associate a class method with the current metaclass instance,
which as a singleton object knows your package, and can function as a more traditional
"class" method:

    our $count;
    method ^count { return $count }

Such a I<metaclass method> is always delegated to the C<HOW> object just as methods like
C<.does> are, so it's possible to call this as C<Dog.count> or C<$dog.count>.
However, best practice is probably to call such a class method as C<Dog.^count>
or C<$dog.^count> to make it clear that it's in its own namespace separate
from ordinary methods, and so that your class method cannot be accidentally
overridden by an ordinary method in a subclass--presuming you don't want to
allow for that possibility.

=head1 Submethods

Submethods are for declaring infrastructural methods that shouldn't
be inherited by subclasses, such as initializers:

    submethod BUILD ($arg) {
        $.attr = $arg;
    }

Apart from the keyword, submethod declaration and call syntax is
identical to method syntax.  You may mix methods and submethods of
the same name within the class hierarchy, but only the methods are
visible to derived classes via inheritance.  A submethod is called
only when a method call is dispatched directly to the current class.

Conjecture: in order to catch spelling errors it is a compile-time
warning to define a submethod in any class that does not inherit the
corresponding method name from some base class.  More importantly, this
would help safeguard Liskov substitutability. (But note that the
standard C<Object> class already supplies a default C<BUILD> and C<new>.)

=head1 Attributes

Attributes are stored in an opaque datatype, not in a hash.  Not even
the class has to care how they're stored, since they're declared much like
ordinary variables.  Instead of C<my>, use C<has>:

    class Dog is Mammal {
        has $.name = "fido";
        has $.tail is rw;
        has @.legs;
        has $!brain;
        ...
    }

Public attributes have a secondary sigil of "dot", indicating
the automatic generation of an accessor method of the same name.
Private attributes use an exclamation to indicate that no public accessor is
generated.

        has $!brain;

The "true name" of the private variable always has the exclamation, but
much like with C<our> variables, you may declare a lexically scoped alias
to the private variable by saying:

        has $brain;     # also declares $!brain;

As with the C<!> declaration, no accessor is generated.

And any later references to the private variable within the same block
may either use or omit the exclamation, as you wish to emphasize or
ignore the privacy of the variable.  Outside the block, you must use
the C<!> form.  If you declare with the C<!> form, you must use that
form consistently everywhere.  If you declare with the C<.> form, you
also get the private C<!> form as a non-virtual name for the actual
storage location, and you may use either C<!> or C<.> form anywhere
within the class, even if the class is reopened.  Outside the class
you must use the public C<.> form, or rely on a method call (which
can be a private method call, but only for trusted classes).

For public attributes, some traits are copied to the accessor method.
The C<rw> trait causes the generated accessor to be declared C<rw>,
making it an lvalue method.  The default is a read-only accessor.

If you declare the class as C<rw>, then all the class's attributes
default to C<rw>, much like a C struct.

You may write your own accessors to override any or all of the
autogenerated ones.

The attribute variables may be used within instance methods to refer
directly to the attribute values.  Outside the instance methods, the
only access to attributes is through the accessors since an object has
to be specified.  The dot form of attribute variables may be used in
derived classes because the dot form always implies a virtual accessor
call.  Every I<dot> declaration also declares a corresponding private
I<exclamation> storage location, and the exclamation form may be used
only in the actual class, not in derived classes.  Reference to the
internal storage location via C<$!foo> should generally be restricted
to submethods.  Ordinary methods should stick to the C<$.foo> form.

In fact, within submethods, use of the C<$.foo> form on attributes
that are available as C<$!foo> (that is, that are declared directly
by this class) is illegal and produces a dire compile-time warning
(which may be suppressed).  Within a submethod the C<$.foo> form may
only be used on attributes from parent classes, because only the parent
classes' part of the object is guaranteed to be in a consistent state
(because C<BUILDALL> call's the parent classes' C<BUILD> routines
first).  If you attempt to get around this by declaring C<BUILD> as
a method rather than a submethod, that will also be flagged as a dire
(but suppressible) compile-time warning.  (It is I<possible> to define
an inheritable C<BUILD> routine if you have access to all the metadata
for the current class, but it's not easy, and it certainly doesn't
happen by accident just because you change C<submethod> to C<method>.)

Because C<$.foo>, C<@.foo>, C<%.foo>, C<&.foo> are just shorthands of
C<self.foo> with different contexts, the class does not need to declare
any of those as an attribute -- a C<method foo> declaration can work
just as well.

The dot form can take an argument list as well.  These are all equivalent:

    self.foo(1,2,3);    # a regular method call
    self.foo.(1,2,3);   # ditto
    $.foo(1,2,3);       # calls self.foo under $ context
    $.foo.(1,2,3);      # ditto
    @.foo(1,2,3);       # calls self.foo under @ context
    @.foo.(1,2,3);      # ditto

Pseudo-assignment to an attribute declaration specifies the default
value.  The value on the right is treated as an implicit closure and
evaluated at object build time, that is, when the object is being
constructed, not when class is being composed.  To refer to a value
computed at compilation or composition time, you can either use a
temporary or a temporal block of some sort:

    has $.r = rand;     # each object gets different random value

    constant $random = rand;
    has $.r = $random;  # every object gets same value

    has $.r = BEGIN { rand };
    has $.r = INIT { rand };
    has $.r = ENTER { rand };
    has $.r = FIRST { rand };
    has $.r = constant $myrand = rand;

When it is called at C<BUILD> time, the topic of the implicit closure
will be the attribute being initialized, while "self" refers to the
entire object being initialized.  The closure will be called at the
end of the C<BUILD> only if the attribute is not otherwise initialized
in either the signature or the body of the C<BUILD>.

Class attributes are declared with either C<my> or C<our>.  The only
difference from ordinary C<my> or C<our> variables is that an accessor
is generated according to the secondary sigil:

    our $.count;        # generates a public read-only .count accessor
    our %!cache is rw;  # generates no public accessor
    my  $.count;        # generates a public read-only .count accessor
    my  %!cache is rw;  # generates no public accessor

=head1 Construction and Initialization

All classes inherit a default C<new> constructor from C<Object>.  It
expects all arguments to be named parameters initializing attributes of
the same name.  You may write your own C<new> to override the default,
or write constructors with any other name you like.  As in Perl 5,
a constructor is any routine that calls C<bless>.  Unlike in Perl 5,
you call it as a method on the class object (though any object may be
used as a class object), passing the candidate as the first argument.
To bless a hash as in Perl 5, say:

    $object = $class.bless({k1 => $v1, k2 => $v2, ...});

If the candidate is omitted, a candidate object is implicitly created in
the current class by calling C<CREATE>:

    $object = $class.bless(k1 => $v1, k2 => $v2, ...)
    $object = $class.bless(:k1($v1), :k2($v2), ...)  # same

(The default C<CREATE> makes an opaque object.)

In addition to the candidate position argument, C<bless> also
allows one or more positional arguments representing autovivifying
protoobjects.  Such an object looks like a prototype followed by a
hash subscript (see "Autovivifying objects" below).  These are used
to initialize superclasses.

Other than the candidate object and any autovivifying protoobjects,
all arguments to C<bless> must be named arguments, not positional.
Hence, the main purpose of custom constructors is to turn positional
arguments into named arguments for C<bless>.  The C<bless> method
allows an object to be used for its class invocant.  (Your constructor
need not allow this).  In any case, the object is not used as a prototype.
Use C<.clone> instead of C<.bless> if that's what you mean.

Any named arguments to C<bless> are automatically passed to the
C<CREATE> and C<BUILD> routines.  If you wish to pass special options
to the C<CREATE> routine (such as an alternate representation),
call C<CREATE> yourself and then pass the resulting candidate to C<.bless>:

    my $candidate = $class.CREATE(:repr<P6opaque>);
    $object = $class.bless($candidate, :k1($v1), :k2($v2))

For the built-in default C<CREATE> method, C<P6opaque> is the default
representation.  Other possiblilities are C<P6hash>, C<P5hash>,
C<P5array>, C<PyDict>, C<Cstruct>, etc.

The C<bless> function automatically calls all appropriate C<BUILD>
routines by calling the C<BUILDALL> routine for the current class,
which initializes the object in least-derived to most-derived order.
C<DESTROY> and C<DESTROYALL> work the same way, only in reverse.

The default C<BUILD> and C<BUILDALL> are inherited from C<Object>,
so you need to write initialization routines only if you wish to
modify the default behavior.  The C<bless> function automatically
passes the appropriate argument list to the C<BUILD> of its various
parent classes.  If the type of the parent class corresponds to one
of the protoobjects passed to bless, that protoobject's argument
list is used.  Otherwise all the arguments to bless are passed to
the parent class's C<BUILD>.  For the final C<BUILD> of the current
object, all the arguments to C<bless> are passed to the C<BUILD>, so
it can deal with any protoobjects that need special handling.  (It is
allowed to pass protoobjects that don't correspond to any parent class.)

    class Dog is Animal {...}
    my $pet = Dog.new( :name<Fido>, Animal{ :blood<warm>, :legs(4) } );

Here we are using an autovivifying C<Animal> protoobject to specify what
the arguments to C<Animal>'s C<BUILD> routine should look like.  (It does
not actually autovivify an C<Animal> apart from the one being created.)

You can clone an object, changing some of the attributes:

    $newdog = $olddog.clone(:trick<RollOver>);

You can write your own C<BUILD> submethod to control initialization.
If you name an attribute as a parameter, that attribute is initialized
directly, so

    submethod BUILD ($!tail, $!legs) {}

is equivalent to

    submethod BUILD ($tail, $legs) {
        $!tail = $tail;
        $!legs = $legs;
    }

Whether you write your own C<BUILD> or not, at the end of the C<BUILD>,
any default attribute values are implicitly copied into any attributes
that haven't otherwise been initialized.

=head1 Mutating methods

You can call an in-place mutator method like this:

    @array .= sort;

If there is a C<self:sort> operator defined, that will be used.  Otherwise
one will be autogenerated from the ordinary C<sort> operator, on the
order of:

    @array = @array.sort;

One handy place for an in-place mutator is to call a constructor on a
variable of a known type:

    my Dog $spot .= new(:tail<LONG> :legs<SHORT>);

=head1 Calling sets of methods

For any method name, there may be some number of candidate methods
that could handle the request: typically, inherited methods or
multi variants.  The ordinary "dot" operator dispatches
to a method in the standard fashion.  There are also "dot" variants
that call some number of methods with the same name:

    $object.meth(@args)   # calls one method or dies
    $object.?meth(@args)  # calls method if there is one, otherwise undef
    $object.*meth(@args)  # calls all methods (0 or more)
    $object.+meth(@args)  # calls all methods (1 or more)

The method name may be quoted when disambiguation is needed:

    $object."+meth"(@args)
    $object.'VAR'(@args)

As with ordinary calls, the identifier supplying the literal method
name may be replaced with an interpolated quote to specify the method
name indirectly.  It may also be replaced with an array to specify
the exact list of candidates to be considered:

    my @candidates := $object.WALK(:name<foo>, :breadth, :omit($?CLASS));
    $object.*@candidates(@args);

The C<WALK> method takes these arguments:

    :canonical      # canonical dispatch order
    :ascendant      # most-derived first, like destruction order
    :descendant     # least-derived first, like construction order
    :preorder       # like Perl 5 dispatch
    :breadth        # like multi dispatch

    :super              # only immediate parent classes
    :name<name>         # only classes containing named method declaration
    :omit(Selector)     # only classes that don't match selector
    :include(Selector)  # only classes that match selector

Any method can defer to the next candidate method in the list by
the special functions C<callsame>, C<callwith>, C<nextsame>, and
C<nextwith>.  The "same" variants reuse the original argument list
passed to the current method, whereas the "with" variants allow a
new argument list to be substituted for the rest of the candidates.
The "call" variants dispatch to the rest of the candidates and return
their values to the current method for subsequent processing, whereas
while the "next" variants don't return, but merely defer to the rest
of the candidate list:

    callsame;           # call with the original arguments (return here)
    callwith();         # call with no arguments (return here)
    callwith(1,2,3);    # call with a new set of arguments (return here)
    nextsame;           # redispatch with the original arguments (no return)
    nextwith();         # redispatch with no arguments (no return)
    nextwith(1,2,3);    # redispatch with a new set of arguments (no return)

For dispatches using C<.> and C<.?>, the return value is the
C<Capture> returned by the first method completed without deferring.
(Such a return value may in fact be failure, but it still counts as a
successful call from the standpoint of the dispatcher.)  Likewise the
return value of C<.*> and C<.+> is a list of C<Captures> returned by
those methods that ran to completion without deferring to next method.

It is also possible to trim the candidate list so that the current
call is considered the final candidate.  (This is implicitly the case
already for the dispatch variants that want a single successful call.)
For the multiple call variants, C<lastcall> will cause the dispatcher
to throw away the rest of the candidate list, and the subsequent
return from the current method will produce the final C<Capture>
in the returned list.

=head1 Parallel dispatch

Any of the method call forms may be turned into a hyperoperator by
treating the method call as a postfix:

    @object».meth(@args)   # calls one method on each
    @object».?meth(@args)  # calls method if there is one on each
    @object».*meth(@args)  # calls all methods (0 or more) on each
    @object».+meth(@args)  # calls all methods (1 or more) on each
    @object».=meth(@args)  # calls mutator method on each
    @object»!meth(@args)   # calls private method on each

The return value is a list with exactly the same number of elements
as C<@object>.  Each such return value is a Capture or List of Captures
as specified above for the non-hyper "dot" variants.

Hyperoperators treat a junction as a scalar value, so saying:

    $junction».meth(@args);

is just like:

    $junction.meth(@args);

To hyperoperate over the values of a junction you have to explicitly
pull out the values:

    $junction.values».meth(@args);

As with other forms of method call, the "meth" above may be replaced
with a quoted string or variable to do various forms of indirection.

Note that, as with any hyper operator, the methods may be evaluated
in any order (although the method results are always returned in the
same order as the list of invocants).  Use an explicit loop if you
want to do something with ordered side effects, such as I/O.

=head1 Multisubs and Multimethods

The "long name" of a subroutine or method includes the type signature
of its invocant arguments.  The "short name" doesn't.  If you put
C<multi> in front of any sub (or method) declaration, it allows
multiple long names to share a short name, provided all of them are
declared C<multi>.  (Putting C<proto> on the first such declaration has
the same effect, but usually you want to put the C<multi> explicitly
anyway for documentation.)  If a sub (or method) is not marked
with C<multi> and it is not within the package or lexical scope of
a C<proto> of the same short name, it is considered unique, an I<only>
sub.  You may mark a sub explicitly as C<only> if you're worried it
might be within the scope of a C<proto>, and you want to suppress
any other declarations within this scope.  An C<only> sub (or method)
doesn't share with anything outside of it or declared prior to it.
Only one such sub (or method) can inhabit a given namespace, and it
hides any outer subs (or less-derived methods) of the same short name.

The default C<proto> declarations provided by Perl from the global
scope are I<not> automatically propagated to the user's scope
unless explicitly imported, so a C<sub> declaration there that
happens to be the same as a global multi is considered C<only> unless
explicitly marked C<multi>.  In the absence of such an explicit C<sub>
declaration, however, the global proto is used by the compiler in
the analysis of any calls to that short name.  (Since only list
operators may be post-declared, as soon as the compiler sees a
non-listop operator it is free to apply the global C<proto> since
any user-defined C<only> version of it must of necessity be declared
earlier in the user's lexical scope or not at all.)

A C<proto> may share dispatch with multis declared after it in the same scope,
but in that case it functions only as the final tie-breaker if the
inner multis can't decide among themselves what to do.  (It may
then, of course, decide to redispatch outside of the current scope.)

Within its scope,
the signature of a C<proto> also nails down the presumed order
and naming of positional parameters, so that any multi call with named
arguments in that scope can presume to rearrange those arguments into
positional parameters based on that information.  (Unrecognized names
remain named arguments.)  Any other type information or traits attached
to the C<proto> are also shared by the routines within its scope,
so a C<proto> definition can be used to factor out common traits.
This is particularly useful for establishing grammatical categories
in a grammar by declaring a C<proto> C<token> or C<proto> C<rule>.  (Perl 6's
grammar does this, for instance.)

You can have multiple C<multi> variables of the same name in the
same scope, and they all share the same storage location and type.
Usually these are declared by one C<proto> declaration at the top,
and leaving the C<multi> implicit on the rest of the declarations.
You might do this when you suspect you'll have multiple declarations
of the same variable name (such code might be produced by a macro
or by a code generator, for instance) and you wish to suppress any
possible warnings about redefinition.

In contrast, C<multi> routines can have only one instance of the long
name in any namespace, and that instance hides any outer (or less-derived)
routines with the same long name.  It does not hide any routines with
the same short name but a different long name.  In other words, multis
with the same short name can come from several different namespaces
provided their long names differ and their short names aren't hidden
by an C<only> declaration in some intermediate scope.

When you call a routine with a particular short name, if there are
multiple visible long names, they are all considered candidates.
They are sorted into an order according to how close the run-time types
of the arguments match up with the declared types of the parameters of
each candidate.  The best candidate is called, unless there's a tie,
in which case the tied candidates are redispatched using any additional
tiebreaker long names (see below).

If a tie still results, only candidates marked with the C<default> trait are
considered, and the best matching default routine is used.  If there
are no default routines, or if the available defaults are also tied,
a final tie-breaking proto sub is called, if there is one (see above).
Otherwise the dispatch fails.

The order in which candidates are considered is defined by a
topological sort based on the "type narrowness" of each candidate's
long name, where that in turn depends on the narrowness of each
parameter that is participating.  Identical types are considered tied.
Parameters whose types are not comparable are also considered tied.
A candidate is considered narrower than another candidate if at least
one of its parameters is narrower and all the rest of its parameters
are either narrower or tied.  This defines the partial ordering of
all the candidates.  If the topological sort detects a circularity in
the partial ordering, all candidates in the circle are considered tied.
A warning will be issued at C<CHECK> time if this is detected and there is
no default candidate to fall back to.

Ordinarily all the parameters of a multi sub are considered for dispatch.
Here's a declaration for an integer range operator with two parameters
in its long name:

    multi sub infix:<..>(Int $min, Int $max) {...}

Sometimes you want to have parameters that aren't counted as part of the
long name.  For instance, if you want to allow an optional "step" parameter
to your range operator, but not consider it for multi dispatch, then put a
double semicolon instead of a comma before it:

    multi sub infix:<..>(Int $min, Int $max;; Int $by = 1) {...}

The double semicolon, if any, determines the complete long name of
a multi.  (In the absence of that, a double semicolon is assumed
after the last declared argument, but before any return signature.)
Note that a call to the routine must still be compatible with
subsequent arguments.

[Conjecture: However, a given multi may advertise multiple long names,
some of which are shorter than the complete long name.  This is done
by putting a semicolon after each advertised long name (replacing
the comma, if present).  A semicolon has the effect of inserting two
candidates into the list.  One of them is inserted with exactly the
same types, as if the semicolon were a comma.  The other is inserted
as if all the types after the semicolon were of type Any, which puts
it later in the list than the narrower actual candidate.  This merely
determines its sort order; the candidate uses its real type signature
if the dispatcher gets to it after rejecting all earlier entries on the
candidate list.  If that set of delayed candidates also contains ties,
then additional semicolons have the same effect within that sublist
of ties.  Note, however, that semicolon is a no-op if the types after
it are all C<Any>.  (As a limiting case, putting a semicolon after
every parameter produces dispatch semantics much like Common Lisp.
And putting a semicolon after only the first argument is much like
ordinary single-dispatch methods.)  Note: This single-semicolon syntax
is merely to be considered reserved until we understand the semantics
of it, and more importantly, the pragamatics of it (that is, whether
it has any valid use case).  Until then only the double-semicolon
form will be implemented in the standard language.]

Within a class, C<multi submethod> is visible to both method-dispatch
and subroutine-dispatch.  A C<multi method> never participates in the
subroutine-dispatch process.  It is dispatched just like a normal
method, then the tie-breaking rules of the previous paragraph are applied.
That is, the shortest long name of a multi method includes I<only> the
single invocant, and any additional semicolons may only indicate long names
to be used as tiebreakers.

Conjecture: In order to specify dispatch that includes the return
type context, it is necessary to place the return type before the double
semicolon:

    multi infix:<..>(Int $min, Int $max --> Iterator;; Int $by = 1) {...}
    multi infix:<..>(Int $min, Int $max --> Selector;; Int $by = 1) {...}

Note that such a declaration might have to delay dispatch until the
actual desired type is known!  (Generally, you might just consider
returning a flexible C<Range> object instead of an anonymous partial
dispatch that may or may not be resolved at compile time via type
inferencing.  Therefore return-type tiebreaking need not be supported
in 6.0.0 unless some enterprising soul decides to make it work.)

=head2 Method call vs. Subroutine call

The caller indicates whether to make a method call or subroutine
call by the call syntax.  The "dot" form and the indirect object form
default to method calls.  All other prefix calls default to subroutine calls.
This applies to prefix unary operators as well:

    !$obj;  # same as $obj.prefix:<!>

A method call considers only methods (including multi-methods and submethods)
from the class hierarchy of its invocant, and fails if none is found.  The
object in question is in charge of interpreting the meaning of the method
name, so if the object is a foreign object, the name will be interpreted
by that foreign runtime.

A subroutine call considers only visible subroutines (including
submethods) of that name.  The object itself has no say in the
dispatch; the subroutine dispatcher considers only the types the
arguments involved, along with the name.  Hence foreign objects passed
to subroutines are forced to follow Perl semantics (to the extent
foreign types can be coerced into Perl types, otherwise they fail).

There is no fail-over either from subroutine to method dispatch or
vice versa.  However, you may use C<is export> on a method
definition to make it available also as a multi sub.  As with indirect
object syntax, the first argument is still always the invocant,
but the export allows you to use a comma after the invocant instead of
a colon, or to omit the colon entirely in the case of a method with
no arguments other than the invocant.  Many standard methods (such
as C<IO::close> and C<Array::push>) are automatically exported to the
global namespace by default.  For other exported methods, you will not
see the multi sub definition unless you C<use> the class in your scope,
which will import the multi sub lexically, after which you can call it
using normal subroutine call syntax.

Note that explicit use of a syntactic category as a method name
overrides the choice of dispatcher, so

    $x.infix:<*>($y)

and

    infix:<*>($x,$y)

are exactly equivalent.  That is, both calls use the subroutine/multi
dispatcher, not the method/single dispatcher.  Likewise

    foo($bar)

can be written

    $bar.prefix:<foo>()

with the same meaning.  To get single dispatch of that method
name to a foreign function, you must say:

    $bar.'prefix:<foo>'()

Most foreign languages are not going to understand such a method name,
however.

=head1 Multi dispatch

Multi submethods work just like multi methods except they are constrained
to an exact type match on the invocant, just as ordinary submethods are.

Perl 6.0.0 is not required to support multiple dispatch on named parameters,
only on positional parameters.  Note that most builtins will map known
named parameters to positional via a C<proto> declaration.

Within a multiple dispatch, C<nextsame> means to try the next best
match, or next best default in case of tie, or the proto sub if there
is one.

Attributes are tied to a particular class definition, so a multi method
can only directly access the attributes of a class it's defined within
when the invocant is the "self" of that attribute.
However, it may call the private attribute accessors from a different
class if that other class has indicated that it trusts the class the
multi method is defined in:

    class MyClass {
        trusts YourClass;
        ...
    }

The syntax for calling back to C<MyClass> is C<$obj!MyClass::meth()>.

The C<sub> keyword is optional immediately after a C<proto>, C<multi>,
or C<only> keyword.

A C<proto> declaration may not occur after a C<multi> declaration in the
same scope.

=head1 Roles

Classes are primarily in charge of object management, and only
secondarily in charge of software reuse.  In Perl 6, roles take over
the job of managing software reuse.  Depending on how you care to look
at it, a role is like a partial class, or an interface with default
implementation, or a set of generic methods and their associated data,
or a class closed at compile time.

Roles may be composed into a class at compile time, in which case
you get automatic detection of conflicting methods.  A role may also
be mixed into a class or object at run time to produce an anonymous
derived class with extra capabilities, but in this case conflicting
methods are overridden by the new role silently.  In either case,
a class is necessary for instantiation--a role may not be directly
instantiated.

A role is declared like a class, but with a C<role> keyword:

    role Pet {
        method feed ($food) {
            $food.open_can;
            $food.put_in_bowl;
            self.eat($food);
        }
    }

A role may not inherit from a class, but may be composed of other
roles.  However, this "crony" composition is not evaluated until
class composition time.  This means that if two roles bring in the
same crony, there's no conflict--it's just as if the class pulled in
the crony role itself and the respective roles didn't.  A role may
never conflict with itself regardless of its method of incorporation.
A role that brings in two conflicting crony roles *may* resolve them
as if it were a class.  This solution is accepted by the class unless
the class supplies its own solution.  If two different roles resolve
the same crony conflict two different ways, those roles are themselves
in conflict and must be resolved by a "more derived" role or the class.

A role doesn't know its own type until it is composed into a class.
Any mention of its main type (such as C<::?CLASS>) is generic, as is
any reference to C<self> or the type of the invocant.  You can use
a role name as a type, but only for constraints, not for declaring
actual objects.  (However, if you use a role as if it were a class,
an anonymous class is generated that composes the role, which provides
a way to force a role to test its crony composition for infelicities.)

A role's main type is generic by default, but you can also parameterize
other types explicitly using type parameters:

    role Pet[::Petfood = TableScraps] {
        method feed (Petfood $food) {...}
    }

(Note that in this case you must not use ::Petfood in the inner declaration,
or it would rebind the type to type of the actual food parameter.)

If a role merely declares methods without defining them, it degenerates
to an interface:

    role Pet {
        method feed ($food) {...}
        method groom () {...}
        method scratch (:$where) {...}
    }

Note that, while these methods must become available at class
composition time, they might be supplied by any of: another role,
the class itself, or some superclass.  We know the methods that are
coming from the other roles or the class, but we don't necessarily know
the complete set of methods supplied by our super classes if they are
open or rely on wildcard delegation.  However, the class composer is
allowed to assume that only currently declared superclass methods or
non-wildcard methods are going to be available.  A stub can always
be installed somewhere to "supply" a missing method's declaration.

Roles may have attributes:

    role Pet {
        has $.collar = Collar.new(Tag.new);
        method id () { return $.collar.tag }
        method lose_collar () { undefine $.collar }
    }

If you want to parameterize the initial value of a role attribute,
be sure to put a double semicolon if you don't want the parameter to
be considered part of the long name:

    role Pet[::ID;; $tag] {
        has ID $.collar .= new($tag);
    }

Within a role the C<has> declarator always indicates the declaration
from the viewpoint of the class.  Therefore a private attribute declared
using C<has> is private to the class, not to the role.  You may wish to declare an attribute
that is hidden even from the class; a completely private role
attribute may be declared like this:

    my $!spleen;

The name of such a private attribute is always considered lexically scoped.
If a role declares private lexical items, those items are private to
the role due to the nature of lexical scoping.  Accessors to such
items may be exported to the class, but this is not the default.
In particular, a role may say

    trusts ::?Class;

to allow C<self!attr()> access to the role's C<$!attr> variables with the
class or from other roles composed into the class.  Conflicts between
private accessors are also caught at composition time, but of course
need not consider super classes, since no-one outside the current
class (or a trusted class) can call a private accessor at all.
(Private accessors are never virtual, and must be package qualified
if called from a trusted scope other than our own.  That is, it's
either C<self!attr()> or C<$obj!TrustsMe::attr().>)

A role may also distinguish a shared method

    has method foo ...
    method foo ...      # same

from a nonshared private method:

    my method !foo ...
    my method foo ...   # same, but &foo is aliased to &!foo

Generally you'd just use a lexically scoped sub, though.

    my sub foo ...

[Conjectural: To put a private sub into the class, say

    our sub !foo ...

]

A role can abstract the decision to delegate:

    role Pet {
        has $groomer handles <bathe groom trim> = hire_groomer();
    }

Note that this puts the three methods into the class as well as
C<$groomer>.  In contrast, "C<my $!groomer>" would only put the
three methods; the attribute itself is private to the role.

A role is allowed to declare an additional inheritance for its class when
that is considered an implementation detail:

    role Pet {
        is Friend;
    }

A class incorporates a role with the verb "does", like this:

    class Dog is Mammal does Pet does Sentry {...}

or equivalently, within the body of the class closure:

    class Dog {
        is Mammal;
        does Pet;
        does Sentry;
        ...
    }

There is no ordering dependency among the roles.

A class's explicit method definition hides any role definition of
the same name.  A role method in turn hides any methods inherited
from other classes.

If there are no method name conflicts between roles (or with the
class), then each role's methods can be installed in the class.  If,
however, two roles try to introduce a method of the same name the
composition of the class fails.  (Two C<has> attributes of the same
name, whether public or private, are simply merged into one slot,
provided the types are the same; otherwise, the composition fails.
Role-private attributes are not merged, and from the viewpoint of
the composition, don't even exist, except to allocate a slot for each
such attribute.)

There are several ways to solve method conflicts.  The first is simply to
write a class method that overrides the conflicting role methods, perhaps
figuring out which role method to call.

Alternately, if the role's methods are declared C<multi>, they can be
disambiguated based on their long name.  If the roles forget to declare
them as multi, you can force a multi on the roles' methods by installing
a proto stub in the class being constructed:

    proto method shake {...}

(This declaration need not precede the C<does> clause textually, since
roles are not actually composed until the end of the class definition,
at which point we know which roles are to be composed together
in a single logical operation, as well as how the class intends to
override the roles.)

The proto method will be called if the multi fails:

    proto method shake { warn "They couldn't decide" }

Run-time mixins are done with C<does> and C<but>.  The C<does> binary
operator is a mutator that derives a new anonymous class (if necessary)
and binds the object to it:

    $fido does Sentry

The C<does> infix operator is non-associative, so this is a syntax error:

    $fido does Sentry does Tricks does TailChasing does Scratch;

You can, however, say

    $fido does Sentry;
    $fido does Tricks;
    $fido does TailChasing;
    $fido does Scratch;

And since it returns the left side, you can also say:

    ((($fido does Sentry) does Tricks) does TailChasing) does Scratch;

Unlike the compile-time role composition, each of these layers on a new
mixin with a new level of inheritance, creating a new anonymous class
for dear old Fido, so that a C<.chase> method from C<TailChasing> hides a
C<.chase> method from C<Sentry>.

You can also mixin a precomposed set of roles:

    $fido does (Sentry, Tricks, TailChasing, Scratch);

This will level the playing field for collisions among the new
set of roles, and guarantees the creation of no more than one more
anonymous class.  Such a role still can't conflict with itself, but it
can hide its previous methods in the parent class, and the calculation
of what conflicts is done again for the set of roles being mixed in.
If you can't do compile-time composition, we strongly recommend this
approach for run-time mixins since it approximates a compile-time
composition at least for the new roles involved.

A role applied with C<does> may be parameterized with an initializer
in parentheses, but only if the role supplies exactly one attribute
to the mixin class:

    $fido does Wag($tail);
    $line does taint($istainted);

The supplied initializer will be coerced to type of the attribute.
Note that this initializer is in addition to any parametric type
supplied in square brackets, which is considered part of the actual
type name:

    $myobj does Array[:of(Int)](@initial)

The C<but> operator creates a copy and works on that.  It also knows
how to generalize a particular enumerated value to its role.  So

    0 but True

is short for something like:

    0 but Bool::True

A property is defined by a role like this:

    role SomeRole {
        has SomeType $.prop is rw = 1;
    }

You can declare a property with

    my int property answer;

and that declares a role whose name is the same as the accessor:

    my role answer {
        has int $.answer is rw;
    }

Then you can say

    $a = 0 but answer(42)

Note that the parenthesized form is I<not> a subroutine or method call.
It's just special initializing syntax for roles that contain a single
property.  The above really means something like:

    $a = ($anonymous = 0) does answer(42);

which really means:

    (($anonymous = 0) does answer).answer = 42;
    $a = $anonymous;

Which is why there's a C<but> operator.

=head1 Traits

Traits are just properties (roles) applied to declared items like
containers or classes.  It's the declaration of the item itself that
makes traits seem more permanent than ordinary properties.  In addition
to adding the property, a trait can also have side effects.

Traits are generally applied with the "is" keyword, though not always.
To define a trait handler for an "is xxx" trait, define one or
more multisubs into a property role like this:

    role xxx {
        has Int $.xxx;
        multi trait_auxiliary:is(xxx $trait, Class $container; $arg?) {...}
        multi trait_auxiliary:is(xxx $trait, Any $container; $arg?) {...}
    }

Then it can function as a trait.  A well-behaved trait handler will say

    $container does xxx($arg);

somewhere inside to set the metadata on the container correctly.
Since a class can function as a role when it comes to parameter type
matching, you can also say:

    class MyBase {
        multi trait_auxiliary:is(MyBase $base, Class $class; $arg?) {...}
        multi trait_auxiliary:is(MyBase $tied, Any $container; $arg?) {...}
    }

These capture control if C<MyBase> wants to capture control of how it gets
used by any class or container.  But usually you can just let it call
the generic defaults:

    multi *trait_auxiliary:is(Class $base, Class $class; $arg?) {...}

which adds C<$base> to the "isa" list of C<$class>, or

    multi *trait_auxiliary:is(Class $tied, Any $container; $arg?) {...}

which sets the "tie" type of the container to the implementation type
in C<$tied>.

Most traits are introduced by use of a "helping verb", which could
be something like "C<is>", or "C<will>", or "C<can>", or "C<might>",
or "C<should>", or "C<does>".  We call these helping verbs "trait
auxiliaries". Here's "C<will>", which (being syntactic sugar) merely
delegates to back to "is":

    multi sub *trait_auxiliary:will($trait, $container; &arg) {
        trait_auxiliary:is($trait, $container, &arg);
    }

Other traits are applied with a single word, and we call one of those a
"trait verb".  For instance, the "C<as>" trait
is defined something like this:

    role as {
        has ReturnType $.as;
        multi sub trait_verb:as($container; ReturnType $arg) {
            $container does as($arg);
        }
        ...
    }

Unlike compile-time roles, which all flatten out in the same class,
compile-time traits are applied one at a time, like mixin roles.
You can, in fact, apply a trait to a container at run time, but
if you do, it's just an ordinary mixin role. You have to call the
appropriate C<trait_auxiliary:is()> routine yourself if you want it to
do any extra shenanigans.  The compiler won't call it for you at run
time like it would at compile time.

=head1 Delegation

Delegation lets you pretend that some other object's methods are your own.
Delegation is specified by a C<handles> trait verb with an argument
specifying one or more method names that the current object and the
delegated object will have in common:

    has $tail handles 'wag';

Since the method name (but nothing else) is known at class construction
time, the following C<.wag> method is autogenerated for you:

    method wag (|$args) { $!tail.wag(|$args) }

You can specify multiple method names:

    has $.legs handles <walk run lope shake lift>;

It's illegal to call the outer method unless the attribute
has been initialized to an object of a type supporting the method,
such as by:

    has Tail $.tail handles 'wag' .= new(|%_);

Note that putting a C<Tail> type on the attribute does not necessarily
mean that the method is always delegated to the C<Tail> class.
The dispatch is still based on the I<run-time> type of the object,
not the declared type.

Any other kind of argument to C<handles> is considered to be a
smartmatch selector for method names.  So you can say:

    has $.fur is rw handles /^get_/;

If you say

    has $.fur is rw handles Groomable;

then you get only those methods available via the C<Groomable> role
or class.  To delegate everything, use the C<Whatever> matcher:

    has $the_real_me handles *;

Wildcard matches are evaluated only after it has been determined that
there's no exact match to the method name anywhere.  When you have
multiple wildcard delegations to different objects, it's possible
to have a conflict of method names.  Wildcard method matches are
evaluated in order, so the earliest one wins.  (Non-wildcard method
conflicts can be caught at class composition time.)

If, where you would ordinarily specify a string, you put a pair, then
the pair maps the method name in this class to the method name in the
other class.  If you put a hash, each key/value pair is treated as
such a mapping.  Such mappings are not considered wildcards.

    has $.fur handles { :shakefur<shake> :scratch<get_fleas> };

You I<can> do a wildcard renaming, but not with pairs.  Instead do smartmatch
with a substitution:

    has $.fur handles (s/^furget_/get_/);

Ordinarily delegation is based on an attribute holding an object, but it can
also be based on the return value of a method:

    method select_tail handles <wag hang> {...}

If your delegation object happens to be an array:

    has @handlers handles 'foo';

then Perl 6 assumes that your array contains a list of potential
handlers, and you just want to call the I<first> one that succeeds.
This is not considered a wildcard match unless the "handles" argument
forces it to be.

[Conjectural: the hash syntax is reserved until we figure out the
semantics we really want, and whether this actually buys us anything
over normal polymorphism.] If your delegation object happens to be
a hash:

    has %objects handles 'foo';

then the hash provides a mapping from a set of Selectors specified as Pair
keys to the object specified as the Pair value that should be delegated to:

    has %barkers handles "bark" =
                (Chihauhau => $yip,
                    Beagle => $yap,
                   Terrier => $arf,
                 StBernard => $woof,
                         * => $ruff,
                );

If the current object matches no Selector, a "C<nextsame>" is
automatically performed.

=head1 Types and Subtypes

The type system of Perl consists of roles, classes, and subtypes.
You can declare a subtype like this:

    my subset Str_not2b of Str where /^[isnt|arent|amnot|aint]$/;

or this:

    my Str subset Str_not2b where /^[isnt|arent|amnot|aint]$/;

An anonymous subtype looks like this:

    Str where /^[isnt|arent|amnot|aint]$/;

A C<where> clause implies future smartmatching of some kind: the as-yet
unspecified object of the type on the left must match the selector on
the right.  Our example is roughly equivalent to this closure:

    { $_.does(Str) and $_ ~~ /^[isnt|arent|amnot|aint]$/; }

except that a subtype knows when to call itself.

A subtype is not a subclass.  Subclasses add capabilities, whereas
a subtype adds constraints (takes away capabilites).  A subtype is
primarily a handy way of sneaking smartmatching into multiple dispatch.
Just as a role allows you to specify something more general than a
class, a subtype allows you to specify something more specific than
a class.  A subtype specifies a subset of the values that the original
type specified, which is why we use the C<subset> keyword for it.

While subtypes are primarily intended for restricting parameter types
for multiple dispatch, they also let you impose preconditions on
assignment.  If you declare any container with a subtype,
Perl will check the constraint against any value you might try to
bind or assign to the container.

    subset Str_not2b of Str where /^[isnt|arent|amnot|aint]$/;
    subset EvenNum   of Num where { $^n % 2 == 0 }

    my Str_not2b $hamlet;
    $hamlet = 'isnt';   # Okay because 'isnt' ~~ /^[isnt|arent|amnot|aint]$/
    $hamlet = 'amnt';   # Bzzzzzzzt!   'amnt' !~~ /^[isnt|arent|amnot|aint]$/

    my EvenNum $n;
    $n = 2;             # Okay
    $n = -2;            # Okay
    $n = 0;             # Okay
    $n = 3;             # Bzzzzzzzt

It's legal to base one subtype on another; it just adds an additional
constraint.  That is, it's a subset of a subset.

You can use an anonymous subtype in a signature:

    sub check_even (Num where { $^n % 2 == 0 } $even) {...}

That's a bit unwieldy, but by the normal type declaration rules you
can turn it around to get the variable out front:

    sub check_even ($even of Num where { $^n % 2 == 0 }) {...}

and just for convenience we also let you write it:

    sub check_even (Num $even where { $^n % 2 == 0 }) {...}

since all the type constraints in a signature parameter are just
anded together anyway.

Subtype constraints are used as tiebreakers in multiple dispatch:

    use Rules::Common :profanity;

    multi sub mesg ($mesg of Str where /<profanity>/ is copy) {
        $mesg ~~ s:g/<profanity>/[expletive deleted]/;
        print $MESG_LOG: $mesg;
    }

    multi sub mesg ($mesg of Str) {
        print $MESG_LOG: $mesg;
    }

For multi dispatch, a long name with a matching constraint is preferred over
an equivalent one with no constraint.  So the first C<mesg> above is
preferred if the constraint matches, and otherwise the second is
preferred.

=head1 Enums

An enum is a low-level class that can function as a role or property.
A given enum value can function as a subtype, a method, or as an ordinary
value (an argumentless sub).  The names of the values are specified as a list:

    my enum Day ['Sun','Mon','Tue','Wed','Thu','Fri','Sat'];
    my enum Day <Sun Mon Tue Wed Thu Fri Sat>;

If the first value is unspecified, it defaults to 0.  To specify the
first value, use pair notation (see below).

If the declared type name begins with an uppercase letter, the default
type is C<Int> or C<Str>, depending on the type of the first value.
If the declared type is lowercase, the default return type is C<int> or C<buf>.

The type can be specified:

    my bit enum maybe <no yes>;
    my Int enum day ['Sun','Mon','Tue','Wed','Thu','Fri','Sat'];
    my enum day of uint4 <Sun Mon Tue Wed Thu Fri Sat>;

An anonymous enum just makes sure each string turns into a pair with
sequentially increasing values, so:

    %e = enum < ook! ook. ook? >;

is equivalent to:

    %e = ();
    %e<ook!> = 0;
    %e<ook.> = 1;
    %e<ook?> = 2;

The enum installer inspects list values for pairs, where the value
of the pair sets the next value explicitly.  Non-pairs C<++> the
previous value.  (Str and buf types increment like Perl 5 strings.)
Since the C<«...»> quoter automatically recognizes
pair syntax along with interpolations, we can simply say:

    my enum DayOfWeek «:Sun(1) Mon Tue Wed Thu Fri Sat»;

    our Str enum Phonetic «:Alpha<A> Bravo Charlie Delta
                            Echo Foxtrot Golf Hotel India Juliet
                            Kilo Lima Mike November Oscar Papa
                            Quebec Romeo Sierra Tango Uniform
                            Victor Whiskey X-ray Yankee Zulu»;

    enum roman [i => 1,   v => 5,
                x => 10,  l => 50,
                c => 100, d => 500,
                m => 1000];

    my Item enum hex «:zero(0) one two three four five six seven eight nine
                      :ten<a> eleven twelve thirteen fourteen fifteen»;

You may import enum types; only non-colliding values are imported.
Colliding enum values are hidden and must be disambiguated with the
type name.  Any attempt to use the ambiguous name will result in a fatal
compilation error.  (All colliding values are hidden, not just the new one,
or the old one.)  Any explicit sub or type definition hides all imported
enum values of the same name but will produce a warning unless
C<is redefined> is included.  Note that true() is a built-in function,
while True is short for C<Bool::True>.

Enum values may be used as a property on the right side
of a C<but>, and the enum type will be intuited from the value to make
sure the object in question has the right semantics mixed in:

    $x = "Today" but Tue;

is the same as

    $x = "Today" but day::Tue;

or pseudo-hash form:

    $x = "Today" but day<Tue>;

which is short for something like:

    $x = "Today";
    $x does day;
    $x.day = &day::('Tue');

There's also a pseudo-functional form:

    $x = "Today" but day(Tue);

which lets you cheat:

    $x = "Today" but day(3);

After any of those

    $x.day

returns C<day::Tue> (that is, 3), and

    $x ~~ day
    $x ~~ Tue
    $x.does(Tue)
    $x.does(day)
    $x.day == Tue
    day($x) == Tue
    Tue($x)
    $x.Tue

all return true, and

    $x.does(Wed)
    $x.Wed
    $x.day == Wed
    8.does(day)
    8 ~~ day

all return false.

Two built-in enums are:

    our bit enum Bool <False True>;
    our bit enum Taint <Untainted Tainted>;

Note that C<Bool> and C<Taint> are really role names, and the enum
values are really subset types of the C<bit> integer type.  You can
call the C<.Bool> coercion or the C<true> function or the C<?>
prefix operator on any built-in type, but the value returned is of
type C<bit>.  Never compare a value to "C<true>", or even "C<True>".
Just use it in a boolean context.

Enum types (and perhaps certain other finite, enumerable types such
as finite ranges) define a C<.pick> method on the protoobject of
that type.  Hence:

    my enum CoinFace <Heads Tails>;
    CoinFace.pick

returns C<Heads> or C<Tails> with equal probability, and

    Month.pick(*)

will return the months in random order.  Presumably

    StandardPlayingCards.pick(5)

might return a Royal Flush, but a Full House is much
more likely.  It can never return Five Aces, since the pick
is done without replacement.  (If it I<does> return Five Aces,
it's time to walk away.  Or maybe run.)

=head1 Open vs Closed Classes

By default, all classes in Perl are non-final, which means
you can potentially derive from them.  They are also open, which means
you can add more methods to them, though you have to be explicit that
that is what you're doing:

    class Object is also {
        method wow () { say "Wow, I'm an object." }
    }

Otherwise you'll get a class redefinition error.  (Also, to completely
replace a definition, use "C<is instead>" instead of "C<is also>"...but
don't do that.)

In order to discourage casual misuse of these traits, they are not
allowed on global classes unless you put a special declaration at the top:

    use MONKEY_PATCHING;

For optimization purposes, Perl 6 gives the top-level application the
right to close and finalize classes by the use of C<oo>, a pragma for
selecting global semantics of the underlying object-oriented engine:

    use oo :closed :final;

This merely changes the application's default to closed and final,
which means that at the end of the main compilation (C<CHECK> time)
the optimizer is allowed to look for candidate classes to close or
finalize.  But anyone (including the main application) can request
that any class stay open or nonfinal, and the class closer/finalizer
must honor that.

    use class :open<Mammal Insect> :nonfinal<Str>

These properties may also be specified on the class definition:

    class Mammal is open {...}
    class Insect is open {...}
    class Str is nonfinal {...}

or by lexically scoped pragma around the class definition:

    {
        use class :open;
        class Mammal {...}
        class Insect {...}
    }
    {
        use class :nonfinal;
        class Str {...}
    }

There is I<no> syntax for declaring individual classes closed or final.
The application may only request that the optimizer close and finalize
unmarked classes.

=head1 Interface Consistency

By default, all methods and submethods that do not declare an explicit
C<*%> parameter will get an implicit C<*%_> parameter declared for
them whether they like it or not.  In other words, all methods allow
unexpected named arguments, so that C<nextsame> semantics work
consistently.

If you mark a class "C<is hidden>", it hides the current class
from "C<nextsame>" semantics, and incidentally suppresses the
autogeneration of C<*%_> parameters.  Hidden classes may be visited
as C<SUPER::>, but not via "C<next>".

A similar effect can be achieved from the derived class by saying
C<hides Base> instead of C<is Base>.

=head1 Introspection

Metamethods for objects are named with interrogative pronouns in uppercase:

    WHAT        the protoobject of the type, stringifies to short name ~ '()'
    WHICH       the object's identity value
    WHO         the package supporting the object, stringifies to long name
    WHERE       the memory address of the object
    HOW         the metaclass object: "Higher Order Workings"
    WHEN        (reserved for events?)
    WHY         (reserved for documentation?)
    WHENCE      autovivification closure

These may be used either as methods or as unary operators:

    $obj.WHAT   # method form of P5's ref
    WHAT $obj   # unary form of P5's ref

These are all actually macros, not true operators or methods.  If you get
a foreign object from another language and need to call its C<.WHERE> method,
you can say:

    $obj."WHERE"

And if you don't know the method name in advance, you'd be using the
variable form anyway:

    $obj.$somemeth

which also bypasses the macros.

For now Perl 6 reserves the right to change how all these macros
and the corresponding C<^> forms are defined in terms of each other.
In particular, the C<.^> forms will automatically supply the invocant
as the first argument to methods of the metaclass, while the other
forms require you to pass this explicitly.

Note that WHAT appends C<()> to indicate emptiness.  Use C<.perl> to get the
bare name from a protoobject.

In general, use of these in ordinary code should be a red flag that
Something Very Strange is going on.  (Hence the allcaps.)  Most code
should use Perl 6's operators that make use of this information
implicitly.  For instance, instead of

    $obj.WHAT eq 'Dog()'
    $x.WHICH === $y.WHICH
    $obj.WHAT.bless(%args)

you usually just want:

    $obj ~~ Dog
    $x === $y
    $obj.bless(%args)

Every class has a C<HOW> function/method that lets you get at the
class's metaobject, which lets you get at all the metadata properties
for the class (or other metaobject protocol) implementing the objects
of the class:

    MyClass.methods()           # call MyClass's .methods method (error?)
    MyClass.HOW.methods($obj)       # get the method list of MyClass

The C<^> metasyntax is equivalent to C<.HOW>:

    MyClass.HOW.methods($obj)   # get the method list of MyClass
    ^MyClass.methods($obj)      # get the method list of MyClass
    MyClass.^methods()          # get the method list of MyClass

Each object of the class also has a C<.HOW> or C<.^> method:

    $obj.HOW.methods($obj);
    $obj.^methods();

Class traits may include:

    identifier  { :name<Dog> :ver<1.2.1> :auth<http://www.some.com/~jrandom> } 
        name      Dog
        version   1.2.1
        authority http://www.some.com/~jrandom
    author        Joe Random
    description   This class implements camera obscura.
    subject       optics, boxes
    language      ja_JP
    licensed      Artistic|GPL
    isa           list of parent classes
    roles         list of roles
    disambig      how to deal with ambiguous method names from roles
    layout        P6opaque, P6hash, P5hash, P5array, PyDict, Cstruct, etc.

These are for the standard Perl 6 Meta-Object Protocol, but other MOPs
may define other traits.  The identifier should probably be accessed
through the C<.WHO> object in any case, which may have its own object
methods depending on how type namespaces evolve over time.  Which of
these items are actually part of the C<.HOW> object and which are
delegated back to the package and prototype objects is up to the MOP.
(Note also that anonymous classes may have anonymous packages and
prototype objects, in which case stringification is not likely to
produce something of interest to non-gurus.)

The C<.^methods> method returns method-descriptors containing:

    name                the name of the method
    signature           the parameters of the method
    as                  the coercion type of the method
    multi               whether duplicate names are allowed
    do                  the method body

The C<.^methods> method has a selector parameter that lets you
specify whether you want to see a flattened or hierarchical view,
whether you're interested in private methods, and so forth.

The C<.^attributes> method returns a list of attribute descriptors
that have traits like these:

    name
    type
    scope
    rw
    private
    accessor
    build
    readonly

Strictly speaking, metamethods like C<.isa()>, C<.does()>, and C<.can()>
should be called through the meta object:

    $obj.HOW.can($obj, "bark")
    $obj.HOW.does($obj, Dog)
    $obj.HOW.isa($obj, Mammal)

or

    $obj.^can("bark")
    $obj.^does(Dog)
    $obj.^isa(Mammal)

But C<Any> gives you shortcuts to those:

    $obj.can("bark")
    $obj.does(Dog)
    $obj.isa(Mammal)

These, may, of course, be overridden in a subclass, so don't use the
short form unless you wish to allow for overrides.  In general, C<Any>
will delegate only those metamethods that read well when reasoning
about an individual object.  Infrastructural methods like C<.^methods>
and C<.^attributes> are not delegated, so C<$obj.methods> fails.

The smartmatch:

    $obj ~~ Dog

actually calls:

    $obj.HOW.does($obj, Dog)

which is true if C<$obj> either "does" or "isa" C<Dog> (or "isa"
something that "does" C<Dog>).  If C<Dog> is a subset, any additional
C<where> constraints must also evaluate true.

Unlike in Perl 5 where C<.can> returns a single C<Code> object,
Perl 6's version of C<.^can> returns a "WALK" iterator for a
set of routines that match the name, including all autoloaded and
wildcarded possibilities.  In particular, C<.^can> interrogates
any class package's C<CANDO> method for names that are to be considered autoloadable methods
in the class, even if they haven't been declared yet.  Role composition
sometimes relies on this ability to determine whether a superclass supplies
a method of a particular name if it's required and hasn't been supplied
by the class or one of its roles.

=head1 Autovivifying objects

The C<WHENCE> property of an object is its autovivifying closure.
Any undefined prototype object may carry such a closure that can
lazily create an object of the appropriate type.  When the closure
is eventually evaluated it is expected to return an argument list
corresponding to the arguments to a C<.bless> call.  For instance,
a C<CANDO> routine, instead of creating a C<Dog> object directly,
could instead return something like:

    Dog but WHENCE({ :name<Fido> })

which runs the closure if the object ever needs to be autovivified.
The closure can capture whatever initializers were available in the
original lexical scope.

The short form of the above is simply:

    Dog{ :name<Fido> }

This form is also lazily evaluated:

    my $dog = Dog{ :name<Fido> };
    defined $dog or say "doesn't exist";  # Fido doesn't exist
    $dog.wag()                            # Fido wags his tail

When the typename happens to be a role, autovivifying it involves
attempting to create a punned class of the same name as the role.
Whether this succeeds or not depends on whether the role is
sufficiently complete to serve as a class on its own.  Regardless of
whether such an attempt would succeed, it is always perfectly fine to
define a lazy protoobject for a role just as long as it's only ever
used as an argument to C<bless>, since C<bless> will only be using
its closure to construct the role's C<BUILD> arguments in the context
of the complete new class.  (Of course, an inconsistent or incomplete
class composition may subsequently fail, and in fact the incomplete
role autovivification mentioned above is likely to be implemented by
failing at the point of class composition.)

Note that when used as an argument to a method like C<bless>,
the protoobject is sufficiently lazy that autovivifying is done
only by the appropriate C<BUILD> routine.  It does not waste energy
creating a C<Dog> object when that object's attributes would later
have to be copied into the actual object.  (On top of which, such
an implementation would make it impossible to use protoobjects to
initialize incomplete roles.)

The object autovivification syntax works only for literal named types,
so any indirection must be written more explicitly:

    ::($dogproto){ :name<Fido> }
    $dogproto but WHENCE({ :name<Fido> })
    $dogproto.WHAT{ :name<Fido> }

Note that in contrast to this syntax, a lookup of a symbol in the C<Dog>
package requires a final C<::> before the subscript:

    Dog::{$varname}

=for vim:set expandtab sw=4: