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

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:

• An around method has the keyword symbol :around as its sole qualifier. The meaning of :around methods is the same as in standard method combination. Use of the functions call-next-method and next-method-p is supported in around methods.

• A primary method has the name of the method combination type as its sole qualifier. For example, the built-in method combination type and recognizes methods whose sole qualifier is and; these are primary methods. Use of the functions call-next-method and next-method-p is not supported in primary 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 ... a_n)

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

  (operator  M₁  a₁ ... a_n ... M_k a₁ ... a_n)

• If the expression M_i  a₁ ... a_n is evaluated, the method M_i will be applied to the arguments a₁ ... a_n. For example, if operator is or, the expression M_i  a₁ ... a_n is evaluated only if M_j  a₁ ... a_n, 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.