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no-next-method

no-next-method Standard Generic Function

Syntax:

no-next-method generic-function method &rest args → {result}*

Method Signatures:

no-next-method (generic-function standard-generic-function)

(method standard-method)

&rest args

Arguments and Values:

generic-function – generic function to which method belongs.

method – method that contained the call to call-next-method for which there is no next method. args – arguments to call-next-method.

result—an object.

Description:

The generic function no-next-method is called by call-next-method when there is no next method.

The generic function no-next-method is not intended to be called by programmers. Programmers may write methods for it.

Exceptional Situations:

The system-supplied method on no-next-method signals an error of type error.

See Also:

call-next-method

Expanded Reference: no-next-method​

Default Behavior​

no-next-method is called when call-next-method is invoked but there is no next method in the chain. The default system-supplied method signals an error.

(defgeneric only-one (x))

(defmethod only-one ((x integer))
  ;; This is the only method -- call-next-method will fail
  (call-next-method))

(handler-case (only-one 42)
  (error () :caught-error))
=> :CAUGHT-ERROR

Avoiding the Error with next-method-p​

The standard way to avoid no-next-method errors is to check with next-method-p before calling call-next-method.

(defgeneric safe-chain (x))

(defmethod safe-chain ((x number))
  (if (next-method-p)
      (call-next-method)
      :end-of-chain))

(safe-chain 42)
=> :END-OF-CHAIN

Defining a Custom no-next-method Handler​

You can define a method on no-next-method to customize the behavior when no next method exists.

(defgeneric chained-op (x))

(defmethod chained-op ((x integer))
  (+ x (call-next-method)))

(defmethod no-next-method ((gf (eql #'chained-op)) method &rest args)
  (declare (ignore gf method args))
  0)  ;; Return 0 as the base case

(chained-op 5)
=> 5