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#### 7.6.1 Introduction to Generic Functions

A generic function is a function whose behavior depends on the classes or identities of the arguments supplied to it. A generic function object is associated with a set of methods, a lambda list, a method combination_2, and other information.

Like an ordinary function, a generic function takes arguments, performs a series of operations, and perhaps returns useful values. An ordinary function has a single body of code that is always executed when the function is called. A generic function has a set of bodies of code of which a subset is selected for execution. The selected bodies of code and the manner of their combination are determined by the classes or identities of one or more of the arguments to the generic function and by its method combination.

Ordinary functions and generic functions are called with identical syntax.

Generic functions are true functions that can be passed as arguments and used as the first argument to funcall and apply.

A binding of a function name to a generic function can be established in one of several ways. It can be established in the global environment by ensure-generic-function, defmethod (implicitly, due to ensure-generic-function) or defgeneric (also implicitly, due to ensure-generic-function).

No standardized mechanism is provided for establishing a binding of a function name to a generic function in the lexical environment.

When a defgeneric form is evaluated, one of three actions is taken (due to ensure-generic-function):

`*`

  If a generic function of the given name already exists, the existing generic function object is modified. Methods specified by the current defgeneric form are added, and any methods in the existing generic function that were defined by a previous defgeneric form are removed. Methods added by the current defgeneric form might replace methods defined by defmethod, defclass, define-condition, or defstruct. No other methods in the generic function are affected or replaced.

`*`

  If the given name names an ordinary function, a macro, or a special operator, an error is signaled.

`*`

  Otherwise a generic function is created with the methods specified by the method definitions in the defgeneric form.

Some operators permit specification of the options of a generic function, such as the type of method combination it uses or its argument precedence order. These operators will be referred to as “operators that specify generic function options.”

The only standardized operator in this category is defgeneric.

Some operators define methods for a generic function. These operators will be referred to as method-defining operators ; their associated forms are called method-defining forms. The standardized method-defining operators are listed in Figure 7–2.

      defgeneric        defmethod  defclass
      define-condition  defstruct

      Figure 7–2: Standardized Method-Defining Operators

Note that of the standardized method-defining operators only defgeneric can specify generic function options. defgeneric and any implementation-defined operators that can specify generic function options are also referred to as “operators that specify generic function options.”

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