scratchy.lisp

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;; Macro
(defmacro deferredvar (variable value &optional (documentation nil))
  `(progn
     (defvar ,variable nil ,documentation)
     (push (lambda ()
             (setf ,variable
                   (if (and (symbolp (first ,value))
                            (fboundp (first ,value)))
                       (funcall (first ,value) (rest ,value))
                       ,value)))
           *deferred-variables*)))

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;; Macro Usage
(deferredvar fishes-count '(count-fishes :type "salmon") "this function counts the fishes")


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;; Macro Expansion
(PROGN
  (DEFVAR FISHES-COUNT NIL "this function counts the fishes")
  (PUSH (LAMBDA NIL (SETF FISHES-COUNT
                          (IF (AND (SYMBOLP (FIRST '(COUNT-FISHES "salmon")))
                                   (FBOUNDP (FIRST '(COUNT-FISHES "salmon"))))
                              (FUNCALL (FIRST '(COUNT-FISHES "salmon"))
                                       (REST '(COUNT-FISHES "salmon")))
                              '(COUNT-FISHES :TYPE "salmon"))))
        *DEFERRED-VARIABLES*))

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;; Function definition
(defun count-fishes (type-of-fish)
  ...)

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;; Problem
The value ("salmon") is not of the expected type (OR
                                                  PATHNAME
                                                  STREAM
                                                  STRING).
   [Condition of type TYPE-ERROR]

Restarts:
 0: [RETRY] Retry SLIME REPL evaluation request.
 1: [*ABORT] Return to SLIME's top level.
 2: [ABORT-BREAK] Reset this thread
 3: [ABORT] Kill this thread

Backtrace:
  0: (count-fishes ("salmon"))
  1: (#<Anonymous Function #x3020018838CF>)
  2: (INITIALIZE-DEFERRED-VARIABLES)
  3: (CCL::CALL-CHECK-REGS INITIALIZE-DEFERRED-VARIABLES)
  4: (CCL::CHEAP-EVAL (INITIALIZE-DEFERRED-VARIABLES))