btree_3.rkt

(define t-pre-order
  (lambda (t)
    (if (t-empty? t)
      '()
      (append (list (t-value t))
              (t-pre-order (t-left t))
              (t-pre-order (t-right t)))) ))


(define t-pre-order-r
  (lambda (t)
    (let aux ([t t] [res '()])
      (cond
        [(t-empty? t) res]
        [#t
          (aux t-empty
            (cons (t-value t)
                  (aux (t-left t)
                       (aux (t-right t) res))))])) ))

(printf "pre-order  : ~s~n" (t-pre-order my-tree))
(printf "pre-order  : ~s~n" (t-pre-order-r my-tree))

😢

optimizer: warning in: /path/to/workspace/.learn/scheme/btree.rkt:72:4: aux in module: 'btree: constant-fold attempt failed: car: contract violation
  expected: pair?
  given: #f
optimizer: warning in: /path/to/workspace/.learn/scheme/btree.rkt:72:4: aux in module: 'btree: constant-fold attempt failed: cadr: contract violation
  expected: (cons/c any/c pair?)
  given: #f
optimizer: warning in: /path/to/workspace/.learn/scheme/btree.rkt:72:4: aux in module: 'btree: constant-fold attempt failed: caddr: contract violation
  expected: (cons/c (cons/c any/c pair?) any/c)
  given: #f
optimizer: warning in: /path/to/workspace/.learn/scheme/btree.rkt:72:4: aux in module: 'btree: constant-fold attempt failed: car: contract violation
  expected: pair?
  given: #f
optimizer: warning in: /path/to/workspace/.learn/scheme/btree.rkt:72:4: aux in module: 'btree: constant-fold attempt failed: cadr: contract violation
  expected: (cons/c any/c pair?)
  given: #f
optimizer: warning in: /path/to/workspace/.learn/scheme/btree.rkt:72:4: aux in module: 'btree: constant-fold attempt failed: caddr: contract violation
  expected: (cons/c (cons/c any/c pair?) any/c)
  given: #f