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NAME ^

Synopsis_12 - Objects

AUTHOR ^

Larry Wall <larry@wall.org>

VERSION ^

  Maintainer: Larry Wall <larry@wall.org>
  Date: 27 Oct 2004
  Last Modified: 23 Feb 2008
  Number: 12
  Version: 58

Overview ^

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

Classes ^

A class is a module declared with the 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 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 ... 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 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 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 :: type sigil. In an rvalue context the :: prefix is a no-op, but in a declarational context, it binds a new type name within its declared scope.

Without a my or other scoping declarator, a bare class declarator declares an our declarator, that is, a name within the current package. Since class files begin parsing in the * (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 our class or just class. To declare a lexically scoped class, use my class. Class 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). So the outward search can look in children of the searched namespaces.

Class traits are set using 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 is modifiers:

    class Dog is Mammal is Pet {...}

Roles use does instead of 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 HOW method. 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 HOW syntax. So when you say "Dog", you're referring to both a package and a protoobject, that latter of which points to the actual object representing the class via 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 .defined method.

The notation ^Dog is syntactic sugar for Dog.HOW, so ^ 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 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.

Methods ^

Methods are routines declared in a class with the 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 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 ::?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 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 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 ! in place of the . 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

If the method was not found, it will fall back to a subroutine call instead, with the invocant becoming the first positional argument.

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 IO class defines method close is export (), which puts a multi sub close (IO) in scope by default. Thus if the $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 $_:

    .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 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 . concatenation, it cannot be parsed as a method call at all; instead if fails at compile time because standard Perl�6 has a pseudo 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 Code object, 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 method declarator on an anonymous method has the primary effect of making the declaration of the invocant optional. (It also makes it an official Routine that can be returned from, just as if you'd used 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, each array element must be a Code object, but the list is treated as a list of candidates to call. Note also that the

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

form may dispatch to a list of candidates if $candidates is a special Code object representing a partial dispatch to a list of candidates.

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 is rw. You can use an argumentless rw method anywhere you can use a variable, including in temp and let statements. (In fact, you can use an 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 Array, Hash or Code object bound to the variable:

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

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

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

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

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

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

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

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

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

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

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 "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 metaclass method is always delegated to the HOW object just as methods like .does are, so it's possible to call this as Dog.count or $dog.count. However, best practice is probably to call such a class method as Dog.^count or $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.

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 Object class already supplies a default BUILD and new.)

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 my, use 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 our variables, you may declare a lexically scoped alias to the private variable by saying:

        has $brain;     # also declares $!brain;

As with the ! 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 ! form. If you declare with the ! form, you must use that form consistently everywhere. If you declare with the . form, you also get the private ! form as a non-virtual name for the actual storage location, and you may use either ! or . form anywhere within the class, even if the class is reopened. Outside the class you must use the public . 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 rw trait causes the generated accessor to be declared rw, making it an lvalue method. The default is a read-only accessor.

If you declare the class as rw, then all the class's attributes default to 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 dot declaration also declares a corresponding private 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 $!foo should generally be restricted to submethods. Ordinary methods should stick to the $.foo form.

In fact, within submethods, use of the $.foo form on attributes that are available as $!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 $.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 BUILDALL call's the parent classes' BUILD routines first). If you attempt to get around this by declaring BUILD as a method rather than a submethod, that will also be flagged as a dire (but suppressible) compile-time warning. (It is possible to define an inheritable 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 submethod to method.)

Because $.foo, @.foo, %.foo, &.foo are just shorthands of self.foo with different contexts, the class does not need to declare any of those as an attribute -- a 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 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 BUILD only if the attribute is not otherwise initialized in either the signature or the body of the BUILD.

Class attributes are declared with either my or our. The only difference from ordinary my or 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

Construction and Initialization ^

All classes inherit a default new constructor from Object. It expects all arguments to be named parameters initializing attributes of the same name. You may write your own 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 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 CREATE:

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

(The default CREATE makes an opaque object.)

In addition to the candidate position argument, 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 bless must be named arguments, not positional. Hence, the main purpose of custom constructors is to turn positional arguments into named arguments for bless. The 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 .clone instead of .bless if that's what you mean.

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

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

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

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

The default BUILD and BUILDALL are inherited from Object, so you need to write initialization routines only if you wish to modify the default behavior. The bless function automatically passes the appropriate argument list to the 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 BUILD. For the final BUILD of the current object, all the arguments to bless are passed to the 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 Animal protoobject to specify what the arguments to Animal's BUILD routine should look like. (It does not actually autovivify an 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 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 BUILD or not, at the end of the BUILD, any default attribute values are implicitly copied into any attributes that haven't otherwise been initialized.

Mutating methods ^

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

    @array .= sort;

If there is a self:sort operator defined, that will be used. Otherwise one will be autogenerated from the ordinary 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>);

Pair query methods ^

Certain classes such as filehandles allow colon pairs to be used as if they were methods. Method names beginning with a colon:

    $filehandle.:e
    $filehandle.:!x

are expected to return a value that can be used as a boolean. While this is primarily intended for use by file tests, other classes may define such methods to provide a similar mechanism for interrogating properties. (Also note that syntactic category names are reserved for calling operators as if they were methods.)

Depending on the class, the pairs in question may have arguments. The Hash class in particular makes use of pair syntax for subscript modifiers:

    %hash.:exists{$key}
    %hash.:delete{$key}

This has the advantage that the pair's argument is actually parsed exactly as a subscript would be. A .exists() method could not easily make such a guarantee about its arguments. Plus you can say:

    %hash.:exists<foo>
    %hash.:delete<foo>

Conjecture, the trailing subscript adverbs could be brought up front as well:

    @array.:kv[@slice]
    %hash.:pairs{@slice}

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 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 callsame, callwith, nextsame, and 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 . and .?, the return value is the 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 .* and .+ is a list of 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, 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 Capture in the returned list.

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 @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.

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 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 multi. (Putting proto on the first such declaration has the same effect, but usually you want to put the multi explicitly anyway for documentation.) If a sub (or method) is not marked with multi and it is not within the package or lexical scope of a proto of the same short name, it is considered unique, an only sub. You may mark a sub explicitly as only if you're worried it might be within the scope of a proto, and you want to suppress any other declarations within this scope. An 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 proto declarations provided by Perl from the global scope are not automatically propagated to the user's scope unless explicitly imported, so a sub declaration there that happens to be the same as a global multi is considered only unless explicitly marked multi. In the absence of such an explicit 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 proto since any user-defined only version of it must of necessity be declared earlier in the user's lexical scope or not at all.)

A 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 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 proto are also shared by the routines within its scope, so a proto definition can be used to factor out common traits. This is particularly useful for establishing grammatical categories in a grammar by declaring a proto token or proto rule. (Perl�6's grammar does this, for instance.)

You can have multiple 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 proto declaration at the top, and leaving the 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, 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 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 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 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 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, multi submethod is visible to both method-dispatch and subroutine-dispatch. A 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 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 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.)

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 first considers methods (including multi-methods and submethods) from the class hierarchy of its invocant, and fails over to the subroutine dispatcher as a last resort only if no method can be found in the class hierarchy.

A subroutine call considers only visible subroutines (including submethods) of that name. There is no fail-over from subroutine to method dispatch. However, you may use 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 IO::close and 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 use the class in your scope, which will import the multi sub lexically, after which you can call it using normal subroutine call syntax.

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 proto declaration.

Within a multiple dispatch, 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 MyClass is $obj!MyClass::meth().

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

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

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 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 ::?CLASS) is generic, as is any reference to 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 has declarator always indicates the declaration from the viewpoint of the class. Therefore a private attribute declared using 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 self!attr() access to the role's $!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 self!attr() or $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 $groomer. In contrast, "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 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 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 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 does and but. The does binary operator is a mutator that derives a new anonymous class (if necessary) and binds the object to it:

    $fido does Sentry

The 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 .chase method from TailChasing hides a .chase method from 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 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 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 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 but operator.

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 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 $base to the "isa" list of $class, or

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

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

Most traits are introduced by use of a "helping verb", which could be something like "is", or "will", or "can", or "might", or "should", or "does". We call these helping verbs "trait auxiliaries". Here's "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 "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 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.

Delegation ^

Delegation lets you pretend that some other object's methods are your own. Delegation is specified by a 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 .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 Tail type on the attribute does not necessarily mean that the method is always delegated to the Tail class. The dispatch is still based on the run-time type of the object, not the declared type.

Any other kind of argument to 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 Groomable role or class. To delegate everything, use the 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 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 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 "nextsame" is automatically performed.

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 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 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 mesg above is preferred if the constraint matches, and otherwise the second is preferred.

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 Int or Str, depending on the type of the first value. If the declared type is lowercase, the default return type is int or 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 ++ the previous value. (Str and buf types increment like Perl�5 strings.) Since the «...» 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 is redefined is included. Note that true() is a built-in function, while True is short for bool::True.

Enum values may be used as a property on the right side of a 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 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 bool and taint are really role names. You can call .bool on any built-in type, but the value returned is of type bit. Never compare a value to "true", or even "True". Just use it in a boolean context.

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 "is instead" instead of "is also"...but don't do that.)

For optimization purposes, Perl�6 gives the top-level application the right to close and finalize classes by the use of 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 (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 no syntax for declaring individual classes closed or final. The application may only request that the optimizer close and finalize unmarked classes.

Interface Consistency ^

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

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

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

Introspection ^

Metamethods for objects are named with interrogative pronouns in uppercase:

    WHAT        the prototype object 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 .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 ^ forms are defined in terms of each other.

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 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()       # get the method list of MyClass

The ^ metasyntax is equivalent to .HOW:

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

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

    $obj.HOW.methods();
    $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 .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 .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 .HOW.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 .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 .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 .isa(), .does(), and .can() should be called through the meta object:

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

or

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

But 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, Any will delegate only those metamethods that read well when reasoning about an individual object. Infrastructural methods like .methods and .attributes are not delegated, so $obj.methods fails.

The smartmatch:

    $obj ~~ Dog

actually calls:

    $obj.HOW.does(Dog)

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

Unlike in Perl�5 where .can returns a single Code object, Perl�6's version of .HOW.can returns a "WALK" iterator for a set of routines that match the name, including all autoloaded and wildcarded possibilities. In particular, .can interrogates any class package's 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.

Autovivifying objects ^

The 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 .bless call. For instance, a CANDO routine, instead of creating a 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 bless, since bless will only be using its closure to construct the role's 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 bless, the protoobject is sufficiently lazy that autovivifying is done only by the appropriate BUILD routine. It does not waste energy creating a 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 Dog package requires a final :: before the subscript:

    Dog::{$varname}
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