#lang racket
(require rackunit)
(provide subst)

; ---------------------------------------------------------------------------
; PROBLEM 2: subst                               (first version: straight SR)
; ---------------------------------------------------------------------------

;(define subst
;  (lambda (new old slist) 
;    (if (null? slist)
;        ;; handle empty list
;        ;; handle list containing a symbol-expression
;        )
;    ))

;(define subst
;  (lambda (new old slist) 
;    (if (null? slist)
;        '()
;        ;; handle list containing a symbol-expression
;        )
;    ))

;(define subst
;  (lambda (new old slist) 
;    (if (null? slist)
;        '()
;        (if (symbol? (first slist))
;            ;; handle a symbol in the first
;            ;; handle an slist in the first
;            )
;        )
;    ))

;(define subst
;  (lambda (new old slist)
;    (if (null? slist)
;        '()
;        (if (symbol? (first slist))
;            (if (eq? (first slist) old)
;                (cons new           (subst new old (rest slist)))
;                (cons (first slist) (subst new old (rest slist)))
;                )
;            ;; handle an slist in the first slot
;            )
;        )
;    ))

(define subst
  (lambda (new old slist) 
    (if (null? slist)
        '()
        (if (symbol? (first slist))
            (if (eq? (first slist) old)
                (cons new
                      (subst new old (rest slist)))
                (cons (first slist)
                      (subst new old (rest slist))))
            (cons (subst new old (first slist))
                  (subst new old (rest slist))) ))))

(check-equal? (subst 'd 'b '(a b c a b c d))
              '(a d c a d c d))
(check-equal? (subst 'a 'b '((a b) (((b g r) (f r)) c (d e)) b))
              '((a a) (((a g r) (f r)) c (d e)) a))

; ---------------------------------------------------------------------------
; This works.  But it is complex and seems dissatisfying.
; Look at all the duplication!  We can do better...
; if we admit that we didn't really follow the data.
; ---------------------------------------------------------------------------