Generic function: Difference between revisions

Revision as of 03:35, 21 September 2019

Revision as of 17:51, 11 May 2020

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One of the early object-oriented programming extensions to Lisp is [[Flavors (programming language)|Flavors]]. It used the usual message sending paradigm influenced by [[Smalltalk]]. The Flavors syntax to send a message is:

<sourcesyntaxhighlight lang="lisp">

(send object :message)

</syntaxhighlight>

</source>

With New Flavors, it was decided the message should be a real function and the usual function calling syntax should be used:

<sourcesyntaxhighlight lang="lisp">

(message object)

</syntaxhighlight>

</source>

''message'' now is a ''generic function'', an object and function in its own right. Individual implementations of the ''message'' are called ''methods''.

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=== Common Lisp ===

Define a generic function with two parameters object-1 and object-2. The name of the generic function is ''collide''.

<sourcesyntaxhighlight lang="lisp">

(defgeneric collide (object-1 object-2))

</syntaxhighlight>

</source>

Methods belonging to the generic function are defined outside of classes.

Here we define a method for the generic function ''collide'' which is specialized for the classes asteroid (first parameter object-1) and spaceship (second parameter object-2). The parameters are used as normal variables inside the method body. There is no special namespace that has access to class slots.

<sourcesyntaxhighlight lang="lisp">

(defmethod collide ((object-1 asteroid) (object-2 spaceship))

(format t "asteroid ~a collides with spaceship ~a" object-1 object-2))

</syntaxhighlight>

</source>

Calling the generic function:

<sourcesyntaxhighlight lang="lisp">

? (collide (make-instance 'asteroid) (make-instance 'spaceship))

asteroid #<ASTEROID 4020003FD3> collides with spaceship #<SPACESHIP 40200048CB>

</syntaxhighlight>

</source>

Common Lisp can also retrieve individual methods from the generic function. FIND-METHOD finds the method from the generic function ''collide'' specialized for the classes ''asteroid'' and ''spaceship''.

<sourcesyntaxhighlight lang="lisp">

? (find-method #'collide nil (list (find-class 'asteroid) (find-class 'spaceship)))

#<STANDARD-METHOD COLLIDE NIL (ASTEROID SPACESHIP) 4150015E43>

</syntaxhighlight>

</source>

=== Comparison to other languages ===

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<!-- i moved this earlier because i think it's the more common definition. C++, Java, C#, ML , Rust,

another, completely separate definition of '''generic function''' is a function that uses [[parametric polymorphism]]. This is the definition used when working with a language like [[OCaml]]. An example of a generic function is

<sourcesyntaxhighlight lang="ocaml">

let id : 'a -> 'a = fun x -> x

</syntaxhighlight>

</source>

which takes an argument of any type and returns something of that same type. -->