Dispatch múltiple

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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] [4]

Exemple[modifica]

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

Llenguatge Common Lisp[modifica]

 (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[modifica]

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[modifica]

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[modifica]

Referències[modifica]