Provided by: libclass-multimethods-perl_1.701-3_all 
NAME
Class::Multimethods - Support multimethods and function overloading in Perl
VERSION
This document describes version 1.701 of Class::Multimethods released April 9, 2000.
SYNOPSIS
# IMPORT THE multimethod DECLARATION SUB...
use Class::Multimethods;
# DECLARE VARIOUS MULTIMETHODS CALLED find...
# 1. DO THIS IF find IS CALLED WITH A Container REF AND A Query REF...
multimethod find => (Container, Query)
=> sub { $_[0]->findquery($_[1]) };
# 2. DO THIS IF find IS CALLED WITH A Container REF AND A Sample REF...
multimethod find => (Container, Sample)
=> sub { $_[0]->findlike($_[1]) };
# 3. DO THIS IF find IS CALLED WITH AN Index REF AND A Word REF...
multimethod find => (Index, Word)
=> sub { $_[0]->lookup_word($_[1]) };
# 4. DO THIS IF find IS CALLED WITH AN Index REF AND A qr// PATTERN
multimethod find => (Index, Regexp)
=> sub { $_[0]->lookup_rx($_[1]) };
# 5. DO THIS IF find IS CALLED WITH AN Index REF AND A NUMERIC SCALAR
multimethod find => (Index, '#')
=> sub { $_[0]->lookup_elem($_[1]) };
# 6. DO THIS IF find IS CALLED WITH AN Index REF AND A NON-NUMERIC SCALAR
multimethod find => (Index, '$')
=> sub { $_[0]->lookup_str($_[1]) };
# 7. DO THIS IF find IS CALLED WITH AN Index REF AND AN UNBLESSED ARRAY REF
# (NOTE THE RECURSIVE CALL TO THE find MULTIMETHOD)
multimethod find => (Index, ARRAY)
=> sub { map { find($_[0],$_) } @{$_[1]} };
# SET UP SOME OBJECTS...
my $cntr = new Container ('./datafile');
my $indx = $cntr->get_index();
# ...AND SOME INHERITANCE...
@BadWord::ISA = qw( Word );
my $badword = new BadWord("fubar");
# ...AND EXERCISE THEM...
print find($cntr, new Query('cpan OR Perl')); # CALLS 1.
print find($cntr, new Example('by a committee')); # CALLS 2.
print find($indx, new Word('sugar')); # CALLS 3.
print find($indx, $badword); # CALLS 3.
print find($indx, qr/another brick in the Wall/); # CALLS 4.
print find($indx, 7); # CALLS 5.
print find($indx, 'But don't do that.'); # CALLS 6.
print find($indx, [1,"one"]); # CALLS 7,
# THEN 5 & 6.
DESCRIPTION
The Class:Multimethod module exports a subroutine (&multimethod) that can be used to
declare other subroutines that are dispatched using a algorithm different from the normal
Perl subroutine or method dispatch mechanism.
Normal Perl subroutines are dispatched by finding the appropriately-named subroutine in
the current (or specified) package and calling that. Normal Perl methods are dispatched by
attempting to find the appropriately-named subroutine in the package into which the
invoking object is blessed or, failing that, recursively searching for it in the packages
listed in the appropriate @ISA arrays.
Class::Multimethods multimethods are dispatched quite differently. The dispatch mechanism
looks at the classes or types of each argument to the multimethod (by calling "ref" on
each) and determines the "closest" matching variant of the multimethod, according to the
argument types specified in the variants' definitions (see "Finding the "nearest"
multimethod" for a definition of "closest").
The result is something akin to C++'s function overloading, but more intelligent, since
multimethods take the inheritance relationships of each argument into account. Another way
of thinking of the mechanism is that it performs polymorphic dispatch on every argument of
a method, not just the first.
Defining multimethods
The Class::Multimethods module exports a subroutine called "multimethod", which can be
used to specify multimethod variants with the dispatch behaviour described above. The
"multimethod" subroutine takes the name of the desired multimethod, a list of class names,
and a subroutine reference, and generates a corresponding multimethod variant within the
current package.
For example, the declaration:
package LargeInt; @ISA = (LargeNumeric);
package LargeFloat; @ISA = (LargeNumeric);
package LargeNumeric;
use Class::Multimethods;
multimethod divide => (LargeInt, LargeInt) => sub
{
LargeInt::divide($_[0],$_[1]);
};
multimethod divide => (LargeInt, LargeFloat) => sub
{
LargeFloat::divide($_[0]->AsLargeFloat(),$_[1]));
};
creates a (single!) multimethod &LargeNumeric::divide with two variants. If the
multimethod is called with two references to "LargeInt" objects as arguments, the first
variant (i.e. anonymous subroutine) is invoked. If the multimethod is called with a
"LargeInt" reference and a "LargeFloat" reference, the second variant is called.
Note that if you're running under "use strict", the list of bareword class names in each
variant definition will cause problems. In that case you'll need to say:
multimethod divide => ('LargeInt', 'LargeInt') => sub
{
LargeInt::divide($_[0],$_[1]);
};
multimethod divide => ('LargeInt', 'LargeFloat') => sub
{
LargeFloat::divide($_[0]->AsLargeFloat(),$_[1]));
};
or better still:
multimethod divide => qw( LargeInt LargeInt ) => sub
{
LargeInt::divide($_[0],$_[1]);
};
multimethod divide => qw( LargeInt LargeFloat ) => sub
{
LargeFloat::divide($_[0]->AsLargeFloat(),$_[1]));
};
or best of all (;-):
{
no strict;
multimethod divide => (LargeInt, LargeInt) => sub
{
LargeInt::divide($_[0],$_[1]);
};
multimethod divide => (LargeInt, LargeFloat) => sub
{
LargeFloat::divide($_[0]->AsLargeFloat(),$_[1]));
};
}
Calling the multimethod with any other combination of "LargeNumeric" reference arguments
(e.g. a reference to a "LargeFloat" and a reference to a "LargeInt", or two "LargeFloat"
referencess) results in an exception being thrown, with the message:
No viable candidate for call to
multimethod LargeNumeric::divide at ...
To avoid this, we could provide a "catch-all" variant:
multimethod divide => (LargeNumeric, LargeNumeric) => sub
{
LargeFloat::divide($_[0]->AsLargeFloat(),$_[1]->AsLargeFloat));
}
Now, calling &LargeNumeric::divide with either a "LargeFloat" reference and a "LargeInt"
reference or two "LargeFloat" references results in this third variant being invoked. Note
that, adding this third alternative doesn't affect calls to the other two, since
Class::Multimethods always selects the "nearest" match (see "Finding the "nearest"
multimethod" below for details of what "nearest" means).
This "best fit" behaviour is extremely useful, because it means you can code the specific
cases you want to handle, and the one or more "catch-all" cases to deal with any other
combination of arguments.
Finding the "nearest" multimethod
Of course, the usefulness of the entire system depends on how intelligently
Class::Multimethods decides which version of a multimethod is "nearest" to the set of
arguments you provided. This decision process is called "dispatch resolution", and
Class::Multimethods does it like this:
1. If the types of the arguments given (as determined by "ref") exactly match the types
specified in any variant of the multimethod, that variant is the one called.
2. Otherwise, Class::Multimethods compiles a list of "viable targets". A viable target is
a variant of the multimethod with the correct number of parameters, such that for each
parameter the specified parameter type is a base class of the actual type of the
corresponding argument in the actual call.
3. If there is only one viable target, it is immediately called. if there are no viable
targets, an exception is thrown indicating the fact.
4. Otherwise, Class::Multimethod examines each viable target and computes its "distance"
to the actual set of arguments. The distance of a target is the sum of the distances
of each of its parameters. The distance of an individual parameter is the number of
inheritance steps between its class and the actual class of the corresponding
argument.
Hence, if a specific argument is of the same class as the corresponding parameter
type, the distance to that parameter is zero. If the argument is of a class that is
an immediate child of the parameter type, the distance is 1. If the argument is of a
class which is a "grandchild" of the parameter type, the distance is 2. Et cetera.
5. Class::Multimethod then chooses the viable target with the smallest "distance" as the
"final target". If there is more than one viable target with an equally smallest
distance, an exception is thrown indicating that the call is ambiguous. If there is
only a single final target Class::Multimethod records its identity (so the distance
computations don't have to be repeated next time the same set of argument types is
used), and then calls that final target.
Where to define multimethods
Class::Multimethods doesn't care which packages the individual variants of a multimethod
are defined in. Every variant of a multimethod is visible to the underlying multimethod
dispatcher, no matter where it was defined.
For example, the three variants for the "divide" multimethod shown above could all be
defined in the LargeNumeric package, or the LargeFloat package or the LargeInt package, or
in "main", or in a separate package of their own.
Of course, to make a specific multimethod visible within a given package you still need to
tell that package about it. That can be done by specifying the name of the multimethod
only (i.e. no argument list or variant code):
package Some::Other::Package::That::Wants::To::Use::divide;
use Class::Multimethods;
multimethod "divide";
For convenience, the declaration itself can be abbreviated to:
package Some::Other::Package::That::Wants::To::Use::divide;
use Class::Multimethods "divide";
Similarly, Class::Multimethod doesn't actually care whether multimethods are called as
methods or as regular subroutines. This is quite different from the behaviour of normal
Perl methods and subroutines, where how you call them, determines how they are dispatched.
With multimethods, since all arguments participate in the polymorphic resolution of a call
(instead of just the first), it make no difference whether a multimethod is called as a
subroutine:
numref3 = divide($numref1, $numref2);
or a method:
numref3 = $numref1->divide($numref2);
(so long as the multimethod has been declared in the appropriate place: the current
package for subroutine-like calls, or the invoking object's package for method-like
calls).
In other words, Class::Multimethods also provides general subroutine overloading. For
example:
package main;
use IO;
use Class::Multimethods;
multimethod debug => (IO::File) => sub
{
print $_[0] "This should go in a file\n";
}
multimethod debug => (IO::Pipe) => sub
{
print $_[0] "This should go down a pipe\n";
}
multimethod debug => (IO::Socket) => sub
{
print $_[0] "This should go out a socket\n";
}
# and later
debug($some_io_handle);
Non-class types as parameters
Yet another thing Class::Multimethods doesn't care about is whether the parameter types
for each multimethod variant are the names of "real" classes or just the identifiers
returned when raw Perl data types are passed to the built-in "ref" function. That means
you could also define multimethod variants like this:
multimethod stringify => (ARRAY) => sub
{
my @arg = @{$_[0]};
return "[" . join(", ",@arg) . "]";
}
multimethod stringify => (HASH) => sub
{
my %arg = %{$_[0]};
return "{" . join(", ", map("$_=>$arg{$_}",keys %arg)) . "}";
}
multimethod stringify => (CODE) => sub
{
return "sub {???}";
}
# and later
print stringify( [1,2,3] ), "\n";
print stringify( {a=>1,b=>2,c=>3} ), "\n";
print stringify( $array_or_hash_ref ), "\n";
Provided you remember that the parameter types ARRAY, HASH, and CODE really mean
"reference to array", "reference to hash", and "reference to subroutine", the names of
built-in types (i.e. those returned by "ref") are perfectly acceptable as multimethod
parameters.
That's a nice bonus, but there's a problem. Because "ref" returns an empty string when
given any literal string or numeric value, the following code:
print stringify( 2001 ), "\n";
print stringify( "a multiple dispatch oddity" ), "\n";
will produce a nasty surprise:
No viable candidate for call to multimethod stringify() at line 1
That's because the dispatch resolution process first calls "ref(2001)" to get the class
name for the first argument, and therefore thinks it's of class "". Since there's no
"stringify" variant with an empty string as its parameter type, there are no viable
targets for the multimethod call. Hence the exception.
To overcome this limitation, Class::Multimethods allows three special pseudo-type names
within the parameter lists of multimethod variants. The first pseudo-type - "$" - is the
class that Class::Multimethods pretends that any scalar value (except a reference) belongs
to. Hence, you can make the two recalcitrant stringifications of scalars work by defining:
multimethod stringify => ("$")
=> sub { return qq{"$_[0]"} }
With that definition in place, the two calls:
print stringify( 2001 ), "\n";
print stringify( "a multiple dispatch oddity" ), "\n";
would produce:
"2001"
"a multiple dispatch oddity"
That solves the problem, but not as elegantly as it might. It would be better if numeric
values were left unquoted. To this end, Class::Multimethods offers a second pseudo-type -
"#" - which is the class it pretends numeric scalar values belong to (where a scalar value
is "numeric" if it's truly a numerical value (without implicit coercions):
$var = 0 # numeric --> '$'
$var = 0.0 # numeric --> '$'
$var = "0"; # string --> '#'
Hence you could now also define:
multimethod stringify => ("#")
=> sub { return "+$_[0]" }
the two calls to &stringify now produce:
+2001
"a multiple dispatch oddity"
The final pseudo-type - "*" - is a wild-card or "don't care" type specifier, which matches
any argument type exactly. For example, we could provide a "catch-all" "stringify" variant
(to handle "GLOB" or "IO" references, for example):
multimethod stringify => ("*")
=> sub { croak "can't stringify a " . ref($_[0]) }
The "*" pseudo-type can also be used in multiple-argument multimethods. For example:
# General case...
multimethod handle => (Window, Event, Mode)
=> sub { ... }
# Special cases...
multimethod handle => (MovableWindow, MoveEvent, NormalMode)
=> sub { ... }
multimethod handle => (ScalableWindow, ResizeEvent, NormalMode)
=> sub { ... }
# Very special case
# (ignore any event in any window in PanicMode)
multimethod handle => ("*", "*", PanicMode)
=> sub { ... }
Resolving ambiguities and non-dispatchable calls
It's relatively easy to set up a multimethod such that particular combinations of argument
types cannot be correctly dispatched. For example, consider the following variants of a
multimethod called "put_peg":
multimethod put_peg => (RoundPeg,Hole) => sub
{
print "a round peg in any old hole\n";
};
multimethod put_peg => (Peg,SquareHole) => sub
{
print "any old peg in a square hole\n";
};
multimethod put_peg => (Peg,Hole) => sub
{
print "any old peg in any old hole\n";
};
If "put_peg" is called like so:
put_peg( RoundPeg->new(), SquareHole->new() );
then Class::Multimethods can't dispatch the call, because it cannot decide between the
"(RoundPeg,Hole)" and "(Peg,SquareHole)" variants, each of which is the same "distance"
(i.e. 1 derivation) from the actual arguments.
The default behaviour is to throw an exception (i.e. die) like this:
Cannot resolve call to multimethod put_peg(RoundPeg,SquareHole).
The multimethods:
put_peg(RoundPeg,Hole)
put_peg(Peg,SquareHole)
are equally viable at ...
Sometimes, however, the more specialized variants are only optimizations, and a more
general case (e.g. the "(Peg,Hole)" variant) would suffice as a default where such an
ambiguity exists. If that is the case, it's possible to tell Class::Multimethods to
resolve the ambiguity by calling that variant, using the "resolve_ambiguous" subroutine.
"resolve_ambiguous" is automatically exported by Class::Multimethods and is used like
this:
resolve_ambiguous put_peg => (Peg,Hole);
That is, you specify the name of the multimethod being disambiguated, and the signature of
the variant to be used in ambiguous cases. Of course, the specified variant must actually
exist at the time of the call. If it doesn't, Class::Multimethod ignores it and throws the
usual exception.
Alternatively, if no variant is suitable as a default, you can register a reference to a
subroutine that is to be called instead:
resolve_ambiguous put_peg => \&disambiguator;
Now, whenever "put_peg" can't dispatch a call because it's ambiguous, "disambiguator" will
be called instead, with the same argument list as "put_peg" was given.
Of course, "resolve_ambiguous" doesn't care what subroutine it's given a reference to, so
you can also use an anonymous subroutine:
resolve_ambiguous put_peg
=> sub
{
print "can't put a ", ref($_[0]),
" into a ", ref($_[1]), "\n";
};
Dispatch can also fail if there are no suitable variants available to handle a particular
call. For example:
put_peg( JPEG->new(), Loophole->new() );
which would normally produce the exception:
No viable candidate for call to
multimethod put_peg(JPeg,Loophole) at ...
since classes JPEG and Loophole are't in the Peg and Hole hierarchies, so there's no
inheritance path back to a more general variant.
To handle cases like this, you can use the <resolve_no_match> subroutine, which is also
exported from Class::Multimethods. "resolve_no_match" registers a multimethod variant, or
a reference to some other subroutine, that is then used whenever the dispatch mechanism
can't find a suitable variant for a given multimethod call.
For example:
resolve_no_match put_peg
=> sub
{
put_jpeg(@_)
if ref($_[0]) eq 'JPEG';
shift()->hang(@_)
if ref($_[0]) eq 'ClothesPeg';
hammer(@_)
if ref($_[0]) eq 'TentPeg';
# etc.
};
As with "resolve_ambiguous" the registered variant or subroutine is called with the same
set of arguments that were passed to the original multimethod call.
Redispatching multimethod calls
Sometimes a polymorphic method in a derived class is used to add functionality to an
inherited method. For example, a derived class's "print_me" method might call it's base
class's "print_me", making use of Perl's special "$obj-"SUPER::method()> construct:
class Base;
sub print_me
{
my ($self) = @_;
print "Base stuff\n";
}
class Derived; @ISA = qw( Base );
sub print_me
{
my ($self) = @_;
$self->SUPER::print_me(); # START LOOKING IN ANCESTORS
print "Derived stuff\n";
}
If the "print_me" methods are implemented as multimethods, it's still possible to reinvoke
an "ancestral" method, using the automatically exported "Class::Multimethods::superclass"
subroutine:
use Class::Multimethods;
multimethod print_me => (Base) => sub
{
my ($self) = @_;
print "Base stuff\n";
}
multimethod print_me => (Derived) => sub
{
my ($self) = @_;
print_me( superclass($self) ); # START LOOKING IN ANCESTORS
print "Derived stuff\n";
}
}
Applying "superclass" to the multimethod argument tells Class::Multimethod to start
looking for parameter types amongst the ancestors of Derived.
It's also possible in regular Perl to explcitly tell the polymorphic dispacther where to
start looking, by explicitly qualifying the method name:
sub Derived::print_me
{
my ($self) = @_;
$self->Base::print_me(); # START LOOKING IN Base CLASS
print "Derived stuff\n";
}
The same is possible with multimethods. "superclass" takes an optional second argument
that tells Class::Multimethods exactly where to start looking:
multimethod print_me => (Derived) => sub
{
my ($self) = @_;
print_me( superclass($self => Base) ); # START LOOKING IN Base
print "Derived stuff\n";
}
Note that, unlike regular method calls, with multimethods you can apply the "superclass"
subroutine to any or all of a multimethod's arguments. For example:
multimethod handle => (MovableWindow, MoveEvent, NormalMode) => sub
{
my ($w, $e, $m) = @_;
# Do any special stuff,
# then redispatch to more general handler...
handle(superclass($w), $e, superclass($m => Mode) );
}
In this case the redispatch would start looking for variants which matched "(any of
MovableWindow's ancestors, MoveEvent, Mode)".
It's also important to remember that, as with regular methods, the class of the actual
arguments doesn't change just because we subverted the dispatch sequence. That means if
the above redispatch called the handle variant that takes arguments (Window, MoveEvent,
Mode), the actual arguments would still be of types (MovableWindow, MoveEvent,
NormalMode).
DIAGNOSTICS
If you call "multimethod" and forget to provide a code reference as the last argument, it
"die"s with the message:
"multimethod: last arg must be a code reference at %s"
If the dispatch mechanism cannot find any multimethod with a signature matching the actual
arguments, it "die"s with the message:
"No viable candidate for call to multimethod %s at %s"
If the dispatch mechanism finds two or more multimethods with signatures equally "close"
to the actual arguments (see "The dispatch resolution process"), it "die"s with the
message:
"Cannot resolve call to multimethod %s. The multimethods:
%s
are equally viable at %s"
If you specify two variants with the same parameter lists, Class::Multimethods warns:
"Multimethod %s redefined at %s"
but only if $^W is true (i.e. under the "-w" flag).
AUTHOR
Damian Conway (damian@conway.org)
BUGS AND IRRITATIONS
There are undoubtedly serious bugs lurking somewhere in code this complex :-) Bug reports
and other feedback are most welcome.
Ongoing annoyances include:
• The module uses qr// constructs to improve performance. Hence it won't run under Perls
earlier than 5.005.
• Multimethod dispatch is much slower than regular dispatch when the resolution has to
resort to the more generic cases (though it's actually as very nearly as fast as doing
the equivalent type resolution "by hand", and certainly more reliable and
maintainable)
• The cache management is far too dumb. Adding any new multimethod clobbers the entire
cache, when it should only expunge those entries "upstream" from the the new
multimethod's actual parameter types.
It's unclear, however, under what circumstances the expense of a more careful cache
correction algorithm would ever be recouped by the savings in dispatch (well,
obviously, when the installion of multimethods is a rare event and multimethod
dispatching is frequent, but where is the breakeven point?)
COPYRIGHT
Copyright (c) 1998-2000, Damian Conway. All Rights Reserved.
This module is free software. It may be used, redistributed
and/or modified under the terms of the Perl Artistic License
(see http://www.perl.com/perl/misc/Artistic.html)