common lisp 的通用函数定义 DEFGENERIC 和 DEFNETHOD

gistfile1.txt

语法
  defgeneric function-name gf-lambda-list [[option | {method-description}*]]
  defmethod function-name {method-qualifier}* specialized-lambda-list [[declaration* | documentation]] form*

参数
  option::= (:argument-precedence-order parameter-name+) | 
          (declare gf-declaration+) | 
          (:documentation gf-documentation) | 
          (:method-combination method-combination method-combination-argument*) | 
          (:generic-function-class generic-function-class) | 
          (:method-class method-class) 
  method-description::= (:method method-qualifier* specialized-lambda-list [[declaration* | documentation]] form*)

参数和值的解释
  function-name --- 函数名
  generic-function-class --- 非空的 class 名
  gf-declaration --- 一个变量声明
  gf-documentation --- 字符串, 不会被求值
  gf-lambda-list --- 一个 Generic Function Lambda Lists
  method-class --- 非空的 class 名
  
描述
  一个通用函数 (generic function) 是由一个或多个方法组成的一个函数, 方法可用 defmethod 来定义. 当一个通用函数被调用时,
  Lisp 会使用参数的类别与参数的特化匹配且优先级最高的方法。
  defgeneric 声明一个通用函数,但不提供实现. 一般来说,先用 defgeneric 声明该函数及其参数, 然后通过 defmethod 来实现该函数的方法
  defmethod 会隐式的创造一个generic function (如果没有使用defgeneric 提前定义的话, 会出现 
    "Implicitly creating new generic function COMMON-LISP-USER::COMBINE." 这个警告)


代码
  * (defmethod combine (x y)
      (list x y))
  STYLE-WARNING:
  Implicitly creating new generic function COMMON-LISP-USER::COMBINE.
  #<STANDARD-METHOD COMBINE (T T) {1004AC8B93}>

  * (defclass stuff () ((name :accessor name :initarg :name)))
    (defclass ice-cream (stuff) ())
    (defclass topping (stuff) ())
  #<STANDARD-CLASS STUFF>
  #<STANDARD-CLASS ICE-CREAM>
  #<STANDARD-CLASS TOPPING>

  * (defmethod combine ((ic ice-cream) (top topping))
      (format nil "~A ice-cream with ~A topping."
        (name ic)
        (name top)))
  #<STANDARD-METHOD COMBINE (ICE-CREAM TOPPING) {1004BC5173}>

  * (combine (make-instance 'ice-cream :name 'fig)
      (make-instance 'topping :name 'treacle))
  "FIG ice-cream with TREACLE topping."

  * (combine 23 'skiddoo)
  (23 SKIDDOO)