#!/usr/local/bin/sbcl --script

(defmacro with-collect (collectors &body body)
  (let ((heads (mapcar #'(lambda (x) (gensym)) collectors))
        (tails (mapcar #'(lambda (x) (gensym)) collectors)))
    `(let* ,(mapcan #'(lambda (he ta)
                        `((,he (list nil)) (,ta ,he)))
                    heads tails)
       (macrolet
           ,(mapcar #'(lambda (co ta)
                        `(,co (form)
                           `(rplacd ,',ta (setq ,',ta (list ,form)))))
                    collectors tails)
         ,@body)
       ,(if (cdr collectors)
            `(values ,@(mapcar #'(lambda (he) `(cdr ,he))
                               heads))
          `(cdr ,(car heads))))))

(defmacro -> (form . forms)
  (if (null forms)
      form
    `(-> ,(nsubst form 'it (car forms)) ,@(cdr forms))))

(defun join-string (lst sep)
  (-> (with-collect (acc)
        (acc (car lst))
        (dolist (str (cdr lst))
          (acc sep)
          (acc str)))
      (apply #'concatenate 'string it)))

(let ((x (list "a" "b" "c" "d")))
  (print (join-string x " ")))