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

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

(defun join-string (lst sep)
  (-> (loop
        :with acc := (car lst)
        :for str :in (cdr lst)
        :collect sep :into acc
        :collect str :into acc
        :finally (return acc))
      (apply #'concatenate 'string it)))

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