How can I return a list from this function given :str nil and 1 argument

Question

I could not figure out how to get a list out of this (my/tf :str nil "foo" ).

This function returns a concatenated string of directories or a list.

Function Must:

1. Return a string when one argument is given OR :str t
2. Return a list when more than 1 argument is given OR :str nil

(cl-defun my/tf (&rest rest  &key (str "_") (ex t)  &allow-other-keys )
(let ((new-list (cl-remove-if-not #'stringp rest))
      (dir user-emacs-directory ))
  (if rest (if (and  (not (equal str t)) (< 1 (length new-list) )) ; Return list when more than one arg.
              ;; Return list
               (mapcar (lambda (str-arg)
                          (if ex (expand-file-name str-arg dir )
                                 (concat (abbreviate-file-name dir) str-arg )
                              )) new-list)
              ;; Return string - Concat arguments separated by space.
               (mapconcat (lambda (str-arg)
                          (if ex (expand-file-name str-arg dir )
                                 (concat (abbreviate-file-name dir) str-arg )
                              )) new-list " "))
          (expand-file-name user-emacs-directory))))
           
;; WORKS
(my/tf "foo" "bar" ) ;("/home/rick/.config/emacs/foo" "/home/rick/.config/emacs/bar")
(my/tf "foo" ) ; ⇒ "/home/rick/.config/emacs/foo"
(my/tf  :str t "foo" ) ; ⇒ "/home/rick/.config/emacs/foo"
(my/tf :ex nil "foo" "bar" ) ; ⇒ ("~/.config/emacs/foo" "~/.config/emacs/bar")
(my/tf :ex nil :str t "foo" "bar" ) ; ⇒ "~/.config/emacs/foo ~/.config/emacs/bar"

;; DON’T WORK
;; When I use the (or (= 1 (length new-list) )(< 1 (length new-list)))
;; it messes the other ones up.
; Need a list when :str nil  ⇒ ("/home/rick/.config/emacs/foo")
(my/tf  :str nil "foo" ) ; ⇒ "/home/rick/.config/emacs/foo"

Answer 1

I ended up using cond to fix it. Here is the full working code.

(cl-defun my/tf (&rest rest  &key (str "_") (ex t)  &allow-other-keys )
  (let ((new-list (cl-remove-if-not #'stringp rest))
        (dir user-emacs-directory ))
    (if rest (cond ( (equal str nil )
                    ;; Return string
                    (mapcar (lambda (str-arg)
                              (if ex (expand-file-name str-arg dir )
                                      (concat (abbreviate-file-name dir) str-arg ))) new-list))
                  ( (and (not (equal str t)) (< 1 (length new-list) ))
                    ;; Return list
                    (mapcar (lambda (str-arg)
                              (if ex (expand-file-name str-arg dir )
                                    (concat (abbreviate-file-name dir) str-arg ))) new-list))
                ( t (mapconcat (lambda (str-arg)
                      ;; Return string
                    (if ex (expand-file-name str-arg dir)
                            (concat (abbreviate-file-name dir) str-arg ))) new-list " ")))
            (expand-file-name user-emacs-directory))))