#### 7.7.2 ensure-generic-function [Function] ¶

`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.

[Editorial Note by KMP: What about documentation. Missing from this arguments enumeration, and confusing in description below.]

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 [Macro]