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  ANSI Common Lisp   7 Objects   7.6 Generic Functions and Methods

7.6.2 Introduction to Methods

Methods define the class-specific or identity-specific behavior and operations of a generic function.

A method object is associated with code that implements the method's behavior, a sequence of parameter specializers that specify when the given method is applicable, a lambda list, and a sequence of qualifiers that are used by the method combination facility to distinguish among methods.

A method object is not a function and cannot be invoked as a function. Various mechanisms in the object system take a method object and invoke its method function, as is the case when a generic function is invoked. When this occurs it is said that the method is invoked or called.

A method-defining form contains the code that is to be run when the arguments to the generic function cause the method that it defines to be invoked. When a method-defining form is evaluated, a method object is created and one of four actions is taken:

If the lambda list of a new method is not congruent with the lambda list of the generic function, an error is signaled. If a method-defining operator that cannot specify generic function options creates a new generic function, a lambda list for that generic function is derived from the lambda list of the method in the method-defining form in such a way as to be congruent with it. For a discussion of congruence, see Section 7.6.4 Congruent Lambda-lists for all Methods of a Generic Function.

Each method has a specialized lambda list, which determines when that method can be applied. A specialized lambda list is like an ordinary lambda list except that a specialized parameter may occur instead of the name of a required parameter. A specialized parameter is a list (variable-name parameter-specializer-name), where parameter-specializer-name is one of the following:

Parameter specializer names are used in macros intended as the user-level interface (defmethod), while parameter specializers are used in the functional interface.

Only required parameters may be specialized, and there must be a parameter specializer for each required parameter. For notational simplicity, if some required parameter in a specialized lambda list in a method-defining form is simply a variable name, its parameter specializer defaults to the class t.

Given a generic function and a set of arguments, an applicable method is a method for that generic function whose parameter specializers are satisfied by their corresponding arguments. The following definition specifies what it means for a method to be applicable and for an argument to satisfy a parameter specializer.

Let A1, ... , An be the required arguments to a generic function in order. Let P1, ... , Pn be the parameter specializers corresponding to the required parameters of the method M in order. The method M is applicable when each Ai is of the type specified by the type specifier Pi. Because every valid parameter specializer is also a valid type specifier, the function typep can be used during method selection to determine whether an argument satisfies a parameter specializer.

A method all of whose parameter specializers are the class t is called a default method; it is always applicable but may be shadowed by a more specific method.

Methods can have qualifiers, which give the method combination procedure a way to distinguish among methods. A method that has one or more qualifiers is called a qualified method. A method with no qualifiers is called an unqualified method. A qualifier is any non-list. The qualifiers defined by the standardized method combination types are symbols.

In this specification, the terms "primary method" and "auxiliary method" are used to partition methods within a method combination type according to their intended use. In standard method combination, primary methods are unqualified methods and auxiliary methods are methods with a single qualifier that is one of :around, :before, or :after. Methods with these qualifiers are called around methods, before methods, and after methods, respectively. When a method combination type is defined using the short form of define-method-combination, primary methods are methods qualified with the name of the type of method combination, and auxiliary methods have the qualifier :around. Thus the terms "primary method" and "auxiliary method" have only a relative definition within a given method combination type.

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