多分派

多分派或译多重派发（multiple dispatch）或多方法（multimethod），是某些编程语言的一个特性，其中的函数或者方法，可以在运行时间（动态的）基于它的实际参数的类型，或在更一般的情况下于此之外的其他特性，来动态分派^[1]。这是对单分派多态的推广，那里的函数或方法调用，基于在其上调用方法的对象的派生类型，而动态分派。多分派使用一个或多个实际参数的组合特征，路由动态分派至实现函数或方法。

理解分派[编辑]

软件工程师通常把代码写进代码块中，代码块通常称作过程，函数，方法。代码通过被调用来执行，调用时将控制权传入函数中，当函数执行完成后将控制权返回给调用者。

函数名通常用来描述函数的目的。有时会将多个函数起同样的名称。比如同名函数在逻辑上处理相同的任务，但是操作在不同类型的输入值上。在这种情况下，无法单单通过函数名来判断目标代码块。那么，函数的实际参数的个数和类型也就被用来判断。

通常，单分派面向对象语言，在调用一个方法时，方法参数中一个参数会被特殊对待并用来决定哪一个方法（如果有多个同名方法）会被调用。在许多语言中，这个特殊的参数是在语法上指明的，许多编程语言在调用方法时把特殊参数放在小圆点（.）之前。例如 special.method(other, arguments, here)，这样 lion.sound() 将会发出狮吼，同时 sparrow.sound() 只会吱吱地叫。一般来说，对于面向对象的编程语言，这个小圆点之前的参数（上例中的lion和sparrow）被称为接收者^[2]。

相反，在实现了多分派的语言中，被调用的函数即是那些参数个数一样多并且类型也匹配的调用。在调用中并没有特殊参数来决定那个方法被调用。也就是说，所有参数的运行时类型都参与分派。CLOS是早期和著名的多分派语言。

数据类型[编辑]

对于编译时间可以区分数据类型的编程语言，在轮选（alternative）函数中进行选择可以发生在编译时间，创建轮选函数用于编译时间选择的活动通常被叫做函数重载。

在有些编程语言中，这种数据类型的识别可以被延后至运行时间（后期绑定（英语：Late binding））。轮选函数的选择发生在运行时间，并依据动态确定的函数实际参数的类型。以这种方式选择轮选函数的函数通常被称为多方法。

例子[编辑]

可以通过例子更加清晰的区分多分派和单一分派。假想一个游戏，它有两种（用户可见的）物体，飞船和小行星。当两个物体要相撞的时候，程序需要依据什么物体要相撞而做不同的事情。

具有内建多分派的语言[编辑]

Common Lisp[编辑]

在具有多分派的语言比如Common Lisp中，可以在Common Lisp对象系统中如下这样实现：

(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
  )

并且对其他方法也是类似的。没有使用显式测试和“动态转换”。

由于多分派的存在，方法要定义在类中并包含在对象中的传统想法变得不再吸引人了，上述每个collide-with方法都附属于两个不同的类而非一个类。因此方法调用的特殊语法一般会消失，从而方法调用看起来完全就像正常的函数调用，并且方法被组织入泛化函数而非类中。

Julia[编辑]

Julia有内建的多分派，并且它是语言设计的中心^[3]。Julia版本的例子如下：

collide_with(x::Asteroid, y::Asteroid) = ... # deal with asteroid hitting asteroid
collide_with(x::Asteroid, y::Spaceship) = ... # deal with asteroid hitting spaceship
collide_with(x::Spaceship, y::Asteroid) = ... # deal with spaceship hitting asteroid
collide_with(x::Spaceship, y::Spaceship) = ... # deal with spaceship hitting spaceship

用多分派库扩展的语言[编辑]

在不于语言定义或语法层次支持多分派的语言中，可能经常使用库扩展来增加多分派。

JavaScript[编辑]

JavaScript和TypeScript不在语言语法层次上支持多方法，但可以通过库来增加多分派。例如，使用multimethod包^[4]，它提供了多分派、泛化函数的实现。

JavaScript的动态类型版本：

import { multi, method } from '@arrows/multimethod'

class Asteroid {}
class Spaceship {}

const collideWith = multi(
  method([Asteroid, Asteroid], (x, y) => {
    // deal with asteroid hitting asteroid
  }),
  method([Asteroid, Spaceship], (x, y) => {
    // deal with asteroid hitting spaceship
  }),
  method([Spaceship, Asteroid], (x, y) => {
    // deal with spaceship hitting asteroid
  }),
  method([Spaceship, Spaceship], (x, y) => {
    // deal with spaceship hitting spaceship
  }),
)

TypeScript的静态类型版本：

import { multi, method, Multi } from '@arrows/multimethod'

class Asteroid {}
class Spaceship {}

type CollideWith = Multi & {
  (x: Asteroid, y: Asteroid): void
  (x: Asteroid, y: Spaceship): void
  (x: Spaceship, y: Asteroid): void
  (x: Spaceship, y: Spaceship): void
}

const collideWith: CollideWith = multi(
  method([Asteroid, Asteroid], (x, y) => {
    // deal with asteroid hitting asteroid
  }),
  method([Asteroid, Spaceship], (x, y) => {
    // deal with asteroid hitting spaceship
  }),
  method([Spaceship, Asteroid], (x, y) => {
    // deal with spaceship hitting asteroid
  }),
  method([Spaceship, Spaceship], (x, y) => {
    // deal with spaceship hitting spaceship
  }),
)

Python[编辑]

可以使用库扩展来向Python增加多分派。例如，模块multimethod.py^[5]，还有模块multimethods.py^[6]，它为Python提供了CLOS风格的多方法而不用变更语言的底层语法或关键字。

from multimethods import Dispatch
from game_objects import Asteroid, Spaceship
from game_behaviors import as_func, ss_func, sa_func
collide = Dispatch()
collide.add_rule((Asteroid, Spaceship), as_func)
collide.add_rule((Spaceship, Spaceship), ss_func)
collide.add_rule((Spaceship, Asteroid), sa_func)
def aa_func(a, b):
    """Behavior when asteroid hits asteroid."""
    # ...define new behavior...
collide.add_rule((Asteroid, Asteroid), aa_func)

# ...later...
collide(thing1, thing2)

在功能上，这非常类似于CLOS例子，但是语法是常规Python的。

使用Python 2.4介入的修饰器（decorator），Guido van Rossum出品了具有简化了的语法的多方法的一个简单实现^[7]：

@multimethod(Asteroid, Asteroid)
def collide(a, b):
    """Behavior when asteroid hits a asteroid."""
    # ...define new behavior...
@multimethod(Asteroid, Spaceship)
def collide(a, b):
    """Behavior when asteroid hits a spaceship."""
    # ...define new behavior...
# ... define other multimethod rules ...

并且接着继续定义multimethod修饰器。

PEAK-Rules包提供语法类似上述例子的多分派^[8]。它后来替代为PyProtocols^[9]。

Reg库也支持多分派和谓词分派^[10]。

模拟多分派[编辑]

C[编辑]

C语言没有动态分派，所以必须以某种形式手工实现。经常使用enum来标识一个对象的子类型。动态分派可以通过在函数指针分支表（英语：branch table）中查找这个值来完成。下面是C语言的简单例子：

typedef void (*CollisionCase)(void);

void collision_AA(void) { /* handle Asteroid-Asteroid collision  */ };
void collision_AS(void) { /* handle Asteroid-Spaceship collision */ };
void collision_SA(void) { /* handle Spaceship-Asteroid collision */ };
void collision_SS(void) { /* handle Spaceship-Spaceship collision*/ };

typedef enum {
    THING_ASTEROID = 0,
    THING_SPACESHIP,
    THING_COUNT /* not a type of thing itself, instead used to find number of things */
} Thing;

CollisionCase collisionCases[THING_COUNT][THING_COUNT] = {
    {&collision_AA, &collision_AS},
    {&collision_SA, &collision_SS}
};

void collide(Thing a, Thing b) {
    (*collisionCases[a][b])();
}

int main(void) {
    collide(THING_SPACESHIP, THING_ASTEROID);
}

使用C Object System库^[11]，C可以支持类似于CLOS的动态分派。它是完全可扩展的并且不需要任何的方法的手工处理。动态消息（方法）通过COS分派器来分派，它比Objective-C更快。下面是使用COS的例子：

#include <stdio.h>
#include <cos/Object.h>
#include <cos/gen/object.h>

// classes

defclass (Asteroid)
// data members
endclass

defclass (Spaceship)
// data members
endclass

// generics

defgeneric (_Bool, collide_with, _1, _2);

// multimethods

defmethod (_Bool, collide_with, Asteroid, Asteroid)
 // deal with asteroid hitting asteroid
endmethod

defmethod (_Bool, collide_with, Asteroid, Spaceship)
 // deal with asteroid hitting spaceship
endmethod

defmethod (_Bool, collide_with, Spaceship, Asteroid)
 // deal with spaceship hitting asteroid
endmethod

defmethod (_Bool, collide_with, Spaceship, Spaceship)
 // deal with spaceship hitting spaceship
endmethod

// example of use

int main(void)
{
  OBJ a = gnew(Asteroid);
  OBJ s = gnew(Spaceship);

  printf("<a,a> = %d\n", collide_with(a, a));
  printf("<a,s> = %d\n", collide_with(a, s));
  printf("<s,a> = %d\n", collide_with(s, a));
  printf("<s,s> = %d\n", collide_with(s, s));

  grelease(a);
  grelease(s);
}

Java[编辑]

在只有单一分派的语言比如Java中，多分派可以用多层单一分派来模拟：

interface Collideable {
    void collideWith(final Collideable other);

    /* These methods would need different names in a language without method overloading. */
    void collideWith(final Asteroid asteroid);
    void collideWith(final Spaceship spaceship);
}

class Asteroid implements Collideable {
    public void collideWith(final Collideable other) {
        // Call collideWith on the other object.
        other.collideWith(this);
   }

    public void collideWith(final Asteroid asteroid) {
        // Handle Asteroid-Asteroid collision.
    }

    public void collideWith(final Spaceship spaceship) {
        // Handle Asteroid-Spaceship collision.
    }
}

class Spaceship implements Collideable {
    public void collideWith(final Collideable other) {
        // Call collideWith on the other object.
        other.collideWith(this);
    }

    public void collideWith(final Asteroid asteroid) {
        // Handle Spaceship-Asteroid collision.
    }

    public void collideWith(final Spaceship spaceship) {
        // Handle Spaceship-Spaceship collision.
    }
}

运行时间instanceof检查可以在一个或两个层次上使用。

编程语言支持[编辑]

主范型[编辑]

Julia^[12]

支持通用的多方法[编辑]

C# 4.0（英语：C# 4.0）^[13]
Cecil（英语：Cecil (programming language)）^[14]
Clojure^[15]
Common Lisp（通过Common Lisp对象系统）^[16]
Dylan^[17]
Fortress（英语：Fortress (programming language)）^[18]
Groovy^[19]
Lasso（英语：Lasso (programming language)）^[20]^[21]
Nim，在v0.20之后泛化方法被废止，使用多方法需要加编译指令^[22]
Raku^[23]
R^[24]
Seed7（英语：Seed7）^[25]
TADS（英语：TADS）^[26]
VB.Net^[27]，通过后期绑定，还通过.Net DLR^[28]
Wolfram语言^[29]，通过符号模式匹配
Xtend（英语：Xtend）^[30]

通过扩展[编辑]

任何.NET语言（通过库MultiMethods.NET^[31]）
C（通过库C Object System^[11]）
C#（通过库multimethod-sharp^[32]）
C++（通过库yomm2^[33]和multimethods^[34]）
D（通过库openmethods^[35]）
Factor（通过标准multimethods词汇表^[36])
Java（使用扩展MultiJava^[37]）
JavaScript（通过包@arrows/multimethod^[4]）
Perl（通过模块Class::Multimethods^[38]）
Python（通过PEAK-Rules^[8]、RuleDispatch^[39]、gnosis.magic.multimethods^[6]、PyMultimethods^[40]或multipledispatch^[41]）
Racket（通过库multimethod-lib^[42]）
Ruby（通过Multiple Dispatch库^[43]、Multimethod包^[44]和Vlx-Multimethods包^[45]）
Scheme（通过TinyCLOS^[46]）
TypeScript（通过包@arrows/multimethod^[4]）

引用[编辑]

^ Ranka, Sanjay; Banerjee, Arunava; Biswas, Kanad Kishore; Dua, Sumeet; Mishra, Prabhat; Moona, Rajat. Contemporary Computing: Second International Conference, IC3 2010, Noida, India, August 9–11, 2010. Proceedings. Springer. 2010-07-26. ISBN 9783642148248.
^ Igor Wojda. Programmer dictionary: Receiver. Kt.Academy. 2018-02-08 [2020-02-27] （英语）.
^ Bezanson, Jeff; Edelman, Alan; Karpinski, Stefan; Shah, Viral B. Julia: A fresh approach to numerical computing. SIAM Review. 7 February 2017, 59 (1): 65–98. arXiv:1411.1607. doi:10.1137/141000671.
^ ^4.0 ^4.1 ^4.2 @arrows/multimethod Multiple dispatch in JavaScript/TypeScript with configurable dispatch resolution by Maciej Cąderek.
^ Coady, Aric, multimethod: Multiple argument dispatching., [2021-01-28]
^ ^6.0 ^6.1 multimethods.py 互联网档案馆的存檔，存档日期2005-03-09., Multiple dispatch in Python with configurable dispatch resolution by David Mertz, et al.
^ Five-minute Multimethods in Python.
^ ^8.0 ^8.1 PEAK-Rules 0.5a1.dev. Python Package Index. [21 March 2014].
^ PyProtocols. Python Enterprise Application Kit. [26 April 2019].
^ Reg. Read the docs. [26 April 2019].
^ ^11.0 ^11.1 C Object System: A framework that brings C to the level of other high level programming languages and beyond: CObjectSystem/COS. 2019-02-19.
^ Methods. The Julia Manual. Julialang. [11 May 2014]. （原始内容存档于17 July 2016）.
^ Multimethods in C# 4.0 With 'Dynamic'. [2009-08-20].
^ Cecil Language. [2008-04-13].
^ Multimethods in Clojure. [2008-09-04].
^ Steele, Guy L. 28. Common LISP: The Language. Bedford, MA, U.S.A: Digital Press. 1990. ISBN 978-1-55558-041-4.
^ Background and Goals. [2008-04-13].
^ The Fortress Language Specification, Version 1.0 (PDF). [2010-04-23]. （原始内容 (PDF)存档于2013-01-20）.
^ Multimethods in Groovy. [2008-04-13].
^ Methods – LassoGuide 9.2. [2014-11-11].
^ Visitor Pattern Versus Multimethods. [2008-04-13].
^ Nim Manual: Multi-methods. [2020-09-11].
^ Perl 6 FAQ. [2008-04-13].
^ How S4 Methods Work (PDF). [2008-04-13].
^ Multiple Dispatch in Seed7. [2011-04-23].
^ TADS 3 System Manual. [2012-03-19].
^ VB.Net Multiple Dispatch. [2020-03-31].
^ New Features in C#4.0 and VB.Net 10.0. [2020-03-31].
^ Notes for Programming Language Experts. [2016-08-21].
^ Multiple dispatch.
^ MultiMethods.NET
^ multimethod-sharp
^ yomm2
^ multimethods
^ openmethods
^ multimethods vocabulary
^ MultiJava
^ Class::Multimethods
^ RuleDispatch
^ PyMultimethods
^ multipledispatch
^ multimethod-lib
^ The Multiple Dispatch Library
^ Multimethod Package
^ Vlx-Multimethods Package
^ TinyCLOS

外部链接[编辑]

Stroustrup, Bjarne; Solodkyy, Yuriy; Pirkelbauer, Peter. Open Multi-Methods for C++ (PDF). ACM 6th International Conference on Generative Programming and Component Engineering. 2007.
Dynamic multiple dispatch. docs.racket-lang.org. [2018-03-12].

[1] Ranka, Sanjay; Banerjee, Arunava; Biswas, Kanad Kishore; Dua, Sumeet; Mishra, Prabhat; Moona, Rajat. Contemporary Computing: Second International Conference, IC3 2010, Noida, India, August 9–11, 2010. Proceedings. Springer. 2010-07-26. ISBN 9783642148248.

[2] Igor Wojda. Programmer dictionary: Receiver. Kt.Academy. 2018-02-08 [2020-02-27] （英语）.

[julia-review-3] Bezanson, Jeff; Edelman, Alan; Karpinski, Stefan; Shah, Viral B. Julia: A fresh approach to numerical computing. SIAM Review. 7 February 2017, 59 (1): 65–98. arXiv:1411.1607. doi:10.1137/141000671.

[multimethod_package-4] 4.0 ^4.1 ^4.2 @arrows/multimethod Multiple dispatch in JavaScript/TypeScript with configurable dispatch resolution by Maciej Cąderek.

[multimethod_module-5] Coady, Aric, multimethod: Multiple argument dispatching., [2021-01-28]

[multimethods_module-6] 6.0 ^6.1 multimethods.py 互联网档案馆的存檔，存档日期2005-03-09., Multiple dispatch in Python with configurable dispatch resolution by David Mertz, et al.

[7] Five-minute Multimethods in Python.

[PEAK-Rules-8] 8.0 ^8.1 PEAK-Rules 0.5a1.dev. Python Package Index. [21 March 2014].

[9] PyProtocols. Python Enterprise Application Kit. [26 April 2019].

[10] Reg. Read the docs. [26 April 2019].

[COS-11] 11.0 ^11.1 C Object System: A framework that brings C to the level of other high level programming languages and beyond: CObjectSystem/COS. 2019-02-19.

[juliaManual-12] Methods. The Julia Manual. Julialang. [11 May 2014]. （原始内容存档于17 July 2016）.

[13] Multimethods in C# 4.0 With 'Dynamic'. [2009-08-20].

[14] Cecil Language. [2008-04-13].

[15] Multimethods in Clojure. [2008-09-04].

[16] Steele, Guy L. 28. Common LISP: The Language. Bedford, MA, U.S.A: Digital Press. 1990. ISBN 978-1-55558-041-4.

[17] Background and Goals. [2008-04-13].

[18] The Fortress Language Specification, Version 1.0 (PDF). [2010-04-23]. （原始内容 (PDF)存档于2013-01-20）.

[19] Multimethods in Groovy. [2008-04-13].

[20] Methods – LassoGuide 9.2. [2014-11-11].

[21] Visitor Pattern Versus Multimethods. [2008-04-13].

[22] Nim Manual: Multi-methods. [2020-09-11].

[23] Perl 6 FAQ. [2008-04-13].

[24] How S4 Methods Work (PDF). [2008-04-13].

[25] Multiple Dispatch in Seed7. [2011-04-23].

[26] TADS 3 System Manual. [2012-03-19].

[27] VB.Net Multiple Dispatch. [2020-03-31].

[28] New Features in C#4.0 and VB.Net 10.0. [2020-03-31].

[29] Notes for Programming Language Experts. [2016-08-21].

[30] Multiple dispatch.

[31] MultiMethods.NET

[32] ultimethod-sharp

[33] yomm2

[34] ultimethods

[35] thods

[36] ultimethods vocabulary

[37] MultiJava

[38] Class::Multimethods

[39] RuleDispatch

[40] PyMultimethods

[41] ultipledispatch

[42] ultimethod-lib

[43] The Multiple Dispatch Library

[44] Multimethod Package

[45] Vlx-Multimethods Package

[46] TinyCLOS

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多分派

目录