unscan.lisp

(cl:in-package #:incf-cl)

;;; Copyright (c) 2008-2019 Juan M. Bello Rivas <jmbr@superadditive.com>
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(defgeneric unscan (function list &key initial-value)
  (:documentation "Semi-inverse of SCAN*.

  INCF-CL> (equal (unscan (flip #'-) (scan* #'+ '(1 2 3)) :initial-value 0)
                  '(1 2 3))
  T")
  (:argument-precedence-order list function))

(defmethod unscan ((function function) (list null) &key initial-value)
  (declare (ignore function initial-value))
  nil)

(defmethod unscan ((function function) (list list) &key initial-value)
  (destructuring-bind (c . cs) list
    (cons (funcall function initial-value c)
          (unscan function cs :initial-value c))))