How do I convert a list written on a text field from lstring to list?

I'm not an English native, so if there are any mistakes, please correct me. I have written a program that takes an arithmetic expression and reduces it. It takes a list and returns a list like this one: '((a + 2) * b). The user can only use addition and multiplication.

I'm trying to create a GUI that will take the list, reduce it and return the result. But the text field only returns a string value. I want to convert the string value in a list like the precedent example. If the user writes (a + (b * 1)), how do I convert it into a list (a + (b * 1))?

Here is what I have done.

(define (convert string)
  (map string->symbol          ; convert each substring into a symbol
       (string-split string))) ; split the string by its spaces

; Crée une fenêtre "Simplifier"
(define frame (new frame% [label "Simplifier"]))

; Crée un conteneur "panel"
(define panel (new vertical-panel% [parent frame]))

; Crée un message dans le conteneur "panel)
(define msg (new message%
                 [parent panel]
                 [label "Donnez une expression à simplifier.
N'oubliez pas les parenthèses!"]))

; Crée un champ de texte
(define text-field (new text-field%
                        (label "Expression : ")
                        (parent panel)))

; Crée un message qui affichera le résultat
(define message (new message%
                     (parent panel)
                     (auto-resize #t)
                     (label " ")))

; Faire le bouton "Validez"
(new button% [parent panel]
             [label "Validez"]
             ; Procédure Callback pour un clique sur le bouton "Validez":
             [callback (lambda (button event)
                         (define text (simplifier (convert (send text-field get-value))))
                         (send message set-label (slist->string text)))])

; Affiche la fenêtre
(send frame show #t)

Here the result of convert:

> (convert "(a + b)")
'(|(a| + |b)|)

Can I have a suggestion? Thanks in advance.

sorry I didn't keep my promise to update. Here how I made it work.

; Convert a list in a list of string
(define list->slist (lambda (l)
                      (cond
                        ((null? l)       '())
                        ((list? (car l)) (cons (list->slist (car l)) (list->slist (cdr l))))
                        ((Cte? (car l))  (cons (number->string (car l)) (list->slist (cdr l))))
                        (#t              (cons (symbol->string (car l)) (list->slist (cdr l))))
                        )))

; Convert a list of string in string
(define convert (lambda (slst)
                  (cond
                    ((null? slst)       "")
                    ((list? (car slst)) (string-append "(" (convert (car slst)) " " (convert (cdr slst)) ")"))
                    (#t                 (string-append (car slst) " " (convert (cdr slst))))
                    )))

It works for me. I can't see any simpler way. Thanks for the help.