Bf interpreter

;;;No known bugs
(defun simplify (code); code is a string
  (loop with returncode = ""
     for char across code
     if (not (position char "><+-.,[]"))
     do (progn)
     else
     do (setf returncode (concatenate 'string returncode (coerce `(,char) 'string)))
     finally (return returncode)))

(defun matching-closing (code instruction-pointer)
  (loop with counter = 0
     for char from 0 to (1- (- (length code) instruction-pointer ))
     when (equal (elt (subseq code instruction-pointer) char) #\[)
     do (incf counter)
     when (and (not (zerop counter)) (equal (elt (subseq code instruction-pointer) char) #\]))
     do (decf counter)
     when (and (zerop counter) (equal (elt (subseq code instruction-pointer) char) #\]))
     return (+ instruction-pointer char)))

(defun main (code)
  (setf code (simplify code))
  (let ((memory (make-array (expt 10 5) :adjustable t :initial-element 0)) (data-pointer (/ (expt 10 5) 2)))
    (loop with stack = '()
       :for instruction-pointer from 0 :to (1- (length code)) :do
     (case (elt code instruction-pointer)
       (#\> (incf data-pointer))
       (#\< (decf data-pointer))
       (#\+ (incf (aref memory data-pointer)))
       (#\- (decf (aref memory data-pointer)))
       (#\. (princ (coerce  `(,(code-char (aref memory data-pointer))) 'string)))
       (#\, (setf (aref memory data-pointer) (char-code (read-char))))
       (#\[ (if (zerop (aref memory data-pointer))
                (setf instruction-pointer (matching-closing code instruction-pointer))
                (push instruction-pointer stack)))
       (#\] (if (not (zerop (aref memory data-pointer)))
                (setf instruction-pointer (1- (pop stack)))
                (pop stack)))))))

(defun read-file (file) ; Just a simple function to read from a file and return all of it's contents as a string. Used to read bf programs from files.
  (let ((return-string ""))
    (with-open-file (stream file)
      (loop :for line = (read-line stream nil 'foo)
            :until (eq line 'foo)
            :do (setf return-string  (concatenate 'string return-string line))
            :finally (return return-string)))))