lispdoc - results for constantly

(constantly value)

Function: Return a function that always returns VALUE.

Example:

(defun compute-warning-handler (handle-warnings)
  (let ((nothing (constantly nil)))
    (ecase handle-warnings
      ((t) #'show-warning)
      (:debug #'debug-warning)
      ((nil) nothing))))

Mentioned in:
HyperSpec - Function CONSTANTLY