Skip to main content

undefined-function

undefined-function Condition Type

Class Precedence List:

undefined-function, cell-error, error, serious-condition, condition, t

Description:

The type undefined-function consists of error conditions that represent attempts to read the definition of an undefined function.

The name of the cell (see cell-error) is the function name which was funbound.

See Also:

cell-error-name

Expanded Reference: undefined-function​

Type Hierarchy​

undefined-function is a subtype of cell-error (and thus error) that is signaled when an attempt is made to call a function that has no definition.

(subtypep 'undefined-function 'cell-error)
=> T
=> T
(subtypep 'undefined-function 'error)
=> T
=> T
(subtypep 'undefined-function 'condition)
=> T
=> T

Handling undefined-function with handler-case​

When you call an undefined function, this condition is signaled. You can catch it and use cell-error-name to find out which function was missing.

(handler-case
    (funcall 'this-function-does-not-exist)
  (undefined-function (c)
    (format nil "Function ~S is not defined" (cell-error-name c))))
=> "Function THIS-FUNCTION-DOES-NOT-EXIST is not defined"

Recovering from undefined-function​

You can use handler-case to provide fallback behavior when a function might not be defined.

;; Try calling an optional function, fall back to a default
(defun try-call (fn-name &rest args)
  (handler-case
      (apply fn-name args)
    (undefined-function ()
      (format nil "~S is unavailable, using fallback" fn-name))))

(try-call 'nonexistent-processor 1 2 3)
=> "NONEXISTENT-PROCESSOR is unavailable, using fallback"

Inspecting the Condition Object​

The condition carries the name of the missing function accessible via cell-error-name.

(let ((cond (handler-case
                (funcall 'no-such-fn)
              (undefined-function (c) c))))
  (values (type-of cond)
          (cell-error-name cond)
          (typep cond 'undefined-function)))
=> UNDEFINED-FUNCTION
=> NO-SUCH-FN
=> T