(use extras data-structures)

(define (read-until c in)
  (read-token (lambda (x) (not (eq? c x))) in))

(define (read-string/eval in)
  (let loop ((s '()))
    (let* ((s (cons (read-until #\( in) s))
           (e (read in)))
      (if (eof-object? e)
          (reverse-string-append s)
          (loop (cons (->string (eval e)) s))))))

;; (call-with-input-string "foo = (+ 1 2 3)\nbar = (* (+ 2 3) 5)" read-string/eval)
;; =>
;; "foo = 6\nbar = 25"