simpler CL split-with implementation than in dash.el

split-with.lisp

(defun split-with (pred list)
  "splits the LIST at the first NIL result of PRED" 
  (values (loop while (and (not (null list))
                           (funcall pred (first list)))
                collect (pop list))
          list))

(defun map-while (predicate function list)
  (loop with e
        while (not (null list))
        do (setf e (pop list))
        while (funcall predicate e)
        collect (funcall function e)))

(defun map-while (predicate function list &aux e r rt)
  (do ()
      ((or (null list)
           (not (funcall predicate (first list))))
       r)
    (setf e (funcall function (pop list)))
    (if r
        (setf (rest rt) (list e)
              rt (rest rt))
      (setf r (list e)
            rt r))))

(defun take-while (predicate list)
  (values (map-while predicate
                     (lambda (e)
                       (pop list)
                       e)
                     list)
          list))