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  ANSI Common Lisp   7 Objects   7.7 Dictionary of Objects

7.7.2 ensure-generic-function Function

Syntax:
ensure-generic-function function-name &key argument-precedence-order declare documentation environment generic-function-class lambda-list method-class method-combination
     generic-function

Arguments and Values:
function-name - a function name.

The keyword arguments correspond to the option arguments of defgeneric, except that the :method-class and :generic-function-class arguments can be class objects as well as names.

Method-combination -- method combination object.

Environment -- the same as the &environment argument to macro expansion functions and is used to distinguish between compile-time and run-time environments.

generic-function - a generic function object.

Description:
The function ensure-generic-function is used to define a globally named generic function with no methods or to specify or modify options and declarations that pertain to a globally named generic function as a whole.

If function-name is not fbound in the global environment, a new generic function is created. If (fdefinition function-name) is an ordinary function, a macro, or a special operator, an error is signaled.

If function-name is a list, it must be of the form (setf symbol). If function-name specifies a generic function that has a different value for any of the following arguments, the generic function is modified to have the new value: :argument-precedence-order, :declare, :documentation, :method-combination.

If function-name specifies a generic function that has a different value for the :lambda-list argument, and the new value is congruent with the lambda lists of all existing methods or there are no methods, the value is changed; otherwise an error is signaled.

If function-name specifies a generic function that has a different value for the :generic-function-class argument and if the new generic function class is compatible with the old, change-class is called to change the class of the generic function; otherwise an error is signaled.

If function-name specifies a generic function that has a different value for the :method-class argument, the value is changed, but any existing methods are not changed.

Affected By:
Existing function binding of function-name.

Exceptional Situations:
If (fdefinition function-name) is an ordinary function, a macro, or a special operator, an error of type error is signaled.

If function-name specifies a generic function that has a different value for the :lambda-list argument, and the new value is not congruent with the lambda list of any existing method, an error of type error is signaled.

If function-name specifies a generic function that has a different value for the :generic-function-class argument and if the new generic function class not is compatible with the old, an error of type error is signaled.

See Also:
defgeneric

Allegro CL Implementation Details:
None.

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