Macro: XOR

Documentation

Evaluates the args one at a time. If more than one arg returns true evaluation stops and NIL is returned. If exactly one arg returns true that value is retuned.

Source

(defmacro xor (&rest datums)
  "Evaluates the args one at a time. If more than one arg returns true
  evaluation stops and NIL is returned. If exactly one arg returns
  true that value is retuned."
  (let ((state (gensym "XOR-state-"))
        (block-name (gensym "XOR-block-"))
        (arg-temp (gensym "XOR-arg-temp-")))
    `(let ((,state nil)
           (,arg-temp nil))
       (block ,block-name
         ,@(loop
              for arg in datums
              collect `(setf ,arg-temp ,arg)
              collect `(if ,arg-temp
                           ;; arg is T, this can change the state
                           (if ,state
                               ;; a second T value, return NIL
                               (return-from ,block-name nil)
                               ;; a first T, swap the state
                               (setf ,state ,arg-temp))))
         (return-from ,block-name ,state)))))