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

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

(defun join-string (lst sep)
  (-> (let ((acc nil))
        (dolist (str (cdr lst) (cons (car lst) (nreverse acc)))
          (push str acc)
          (push sep acc)))
      (apply #'concatenate 'string it)))

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