Dispatch múltiple

Dispatch múltiple, en ciència de la computació, és una característica d'alguns llenguatges de programació en els que una funció o mètode pot ser cridat en temps d'execució en relació a una sèrie d'atributs o arguments. El dispatch múltiple és un multimètode o polimorfisme on la crida a la funció o mètode és servida dinàmicament depenent del tipus d'objecte.[1][2][3]

Exemple

En aquest exemple hi ha un mètode amb 4 formes que es cridaran en funció dels paràmetres que es passin :

Llenguatge Common Lisp

 (defmethod collide-with ((x asteroid) (y asteroid))
 ;; deal with asteroid hitting asteroid
)
 (defmethod collide-with ((x asteroid) (y spaceship))
 ;; deal with asteroid hitting spaceship
)
 (defmethod collide-with ((x spaceship) (y asteroid))
 ;; deal with spaceship hitting asteroid
)
 (defmethod collide-with ((x spaceship) (y spaceship))
 ;; deal with spaceship hitting spaceship
)

Perl 6

subset Mass of Real where 0 ^..^ Inf; 
role Stellar-Object {
 has Mass $.mass is required;
 method name () returns Str {...};
}
class Asteroid does Stellar-Object {
 method name () { 'an asteroid' }
}
class Spaceship does Stellar-Object {
 has Str $.name = 'some unnamed spaceship';
}
my Str @destroyed = < obliterated destroyed mangled >;
my Str @damaged = « damaged 'collided with' 'was damaged by' »;

# We add multi candidates to the numeric comparison operators because we are comparing them numerically,
# but doesn't make sense to have the objects coerce to a Numeric type.
# (If they did coerce we wouldn't necessarily need to add these operators.)
# We could have also defined entirely new operators this same way.
multi sub infix:« <=> » (Stellar-Object:D $a, Stellar-Object:D $b) { $a.mass <=> $b.mass }
multi sub infix:« < » (Stellar-Object:D $a, Stellar-Object:D $b) { $a.mass < $b.mass }
multi sub infix:« > » (Stellar-Object:D $a, Stellar-Object:D $b) { $a.mass > $b.mass }
multi sub infix:« == » (Stellar-Object:D $a, Stellar-Object:D $b) { $a.mass == $b.mass }

# Define a new multi dispatcher, and add some type constraints to the parameters.
# If we didn't define it we would have gotten a generic one that didn't have constraints.
proto sub collide (Stellar-Object:D $, Stellar-Object:D $) {*}

# No need to repeat the types here since they are the same as the prototype.
# The 'where' constraint technically only applies to $b not the whole signature.
# Note that the 'where' constraint uses the `<` operator candidate we added earlier.
multi sub collide ($a, $b where $a < $b) {
 say "$a.name() was @destroyed.pick() by $b.name()";
}
multi sub collide ($a, $b where $a > $b) {
 # redispatch to the previous candidate with the arguments swapped
 samewith $b, $a;
}

# This has to be after the first two because the other ones
# have 'where' constraints, which get checked in the
# order the subs were written. (This one would always match.)
multi sub collide ($a, $b){
 # randomize the order
 my ($n1,$n2) = ($a.name, $b.name).pick(*);
 say "$n1 @damaged.pick() $n2";
}

# The following two candidates can be anywhere after the proto,
# because they have more specialized types than the preceding three.

# If the ships have unequal mass one of the first two candidates gets called instead.
multi sub collide (Spaceship $a, Spaceship $b where $a == $b){
 my ($n1,$n2) = ($a.name, $b.name).pick(*);
 say "$n1 collided with $n2, and both ships were ",
 (@destroyed.pick, 'left damaged').pick;
}

# You can unpack the attributes into variables within the signature.
# You could even have a constraint on them `(:mass($a) where 10)`.
multi sub collide (Asteroid $ (:mass($a)), Asteroid $ (:mass($b))){
 say "two asteroids collided and combined into one larger asteroid of mass { $a + $b }";
}

my Spaceship $Enterprise .= new(:mass(1),:name('The Enterprise'));
collide Asteroid.new(:mass(.1)), $Enterprise;
collide $Enterprise, Spaceship.new(:mass(.1));
collide $Enterprise, Asteroid.new(:mass(1));
collide $Enterprise, Spaceship.new(:mass(1));
collide Asteroid.new(:mass(10)), Asteroid.new(:mass(5));

Python

from multimethods import Dispatch
from game_objects import Asteroid, Spaceship
from game_behaviors import ASFunc, SSFunc, SAFunc
collide = Dispatch()
collide.add_rule((Asteroid, Spaceship), ASFunc)
collide.add_rule((Spaceship, Spaceship), SSFunc)
collide.add_rule((Spaceship, Asteroid), SAFunc)
def AAFunc(a, b):
 """Behavior when asteroid hits asteroid"""
 # ...define new behavior...
collide.add_rule((Asteroid, Asteroid), AAFunc)

Suport en llenguatge de programació

• LLenguatges de programació que suporten multi dispatch :
  • C# 4.0
  • Cecil
  • Clojure
  • Common Lisp (via the Common Lisp Object System)
  • Dylan
  • Elixir
  • Fortress
  • Groovy
  • Haskell via multi-parameter type classes
  • Julia
  • Lasso
  • Nice
  • Nim
  • Perl 6
  • R
  • Scala,^[citation needed] també via classes de tipus multiparàmetre
  • Seed7
  • TADS
  • Wolfram Language via symbolic pattern matching
  • Xtend Multimètodes en altres llenguatges de programació mitjançant extensions :
  • .NET (via la biblioteca MultiMethods.NET)
  • C (via la biblioteca C Object System)
  • C# (via la biblioteca multimethod-sharp^[Enllaç no actiu])
  • C++ (via la biblioteca yomm11 Arxivat 2016-06-02 a Wayback Machine.)
  • Factor (via l'estàndard multi-methods vocabulary)
  • Java (usant l'extensió MultiJava)
  • Perl (via el mòdule Class::Multimethods)
  • Python (via PEAK-Rules, RuleDispatch, gnosis.magic.multimethods, PyMultimethods, or multipledispatch)
  • Ruby (via la biblioteca The Multiple Dispatch Library and Multimethod Package and Vlx-Multimethods Package)
  • Scheme (via ex: TinyCLOS) També, Haskell i Scala via classes de tipus multiparàmetre

Referències

1. «What's the difference between Polymorphism and Multiple Dispatch?» (en anglès). https://stackoverflow.com.+[Consulta: 24 maig 2017].
2. «A polyglot's guide to multiple dispatch - Eli Bendersky's website» (en anglès). http://eli.thegreenplace.net.+[Consulta: 24 maig 2017].
3. «Single, Dynamic, Multiple and Double Dispatching» (en anglès). http://isagoksu.com. Arxivat de l'original el 2017-05-22. [Consulta: 24 maig 2017].