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  ANSI Common Lisp   7 Objects   7.6 Generic Functions and Methods   7.6.6 Method Selection and Combination

7.6.6.4 Built-in Method Combination Types

The object system provides a set of built-in method combination types. To specify that a generic function is to use one of these method combination types, the name of the method combination type is given as the argument to the :method-combination option to defgeneric or to the :method-combination option to any of the other operators that specify generic function options.

The names of the built-in method combination types are listed in the next figure.

Built-in Method Combination Types
+ append max nconc progn
and list min or standard

The semantics of the standard built-in method combination type is described in Section 7.6.6.2 Standard Method Combination. The other built-in method combination types are called simple built-in method combination types.

The simple built-in method combination types act as though they were defined by the short form of define-method-combination. They recognize two roles for methods:

The semantics of the simple built-in method combination types is as follows:

  • If there are any around methods, the most specific around method is called. It supplies the value or values of the generic function.

  • Inside the body of an around method, the function call-next-method can be used to call the next method. The generic function no-next-method is invoked if call-next-method is used and there is no applicable method to call. The function next-method-p may be used to determine whether a next method exists. When the next method returns, the around method can execute more code, perhaps based on the returned value or values.

  • If an around method invokes call-next-method, the next most specific around method is called, if one is applicable. If there are no around methods or if call-next-method is called by the least specific around method, a Lisp form derived from the name of the built-in method combination type and from the list of applicable primary methods is evaluated to produce the value of the generic function. Suppose the name of the method combination type is operator and the call to the generic function is of the form

    (generic-function a1 ... an)

  • Let M1, ... ,Mk be the applicable primary methods in order; then the derived Lisp form is

    (operator  M1  a1 ... an ... Mk a1 ... an)

  • If the expression Mi  a1 ... an is evaluated, the method Mi will be applied to the arguments a1 ... an. For example, if operator is or, the expression Mi  a1 ... an is evaluated only if Mj  a1 ... an, 1 <= j<i, returned nil.

  • The default order for the primary methods is :most-specific-first. However, the order can be reversed by supplying :most-specific-last as the second argument to the :method-combination option.

The simple built-in method combination types require exactly one qualifier per method. An error is signaled if there are applicable methods with no qualifiers or with qualifiers that are not supported by the method combination type. An error is signaled if there are applicable around methods and no applicable primary methods.


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