add seasoned schemer exercises to cps

[lambda.git] / hints / cps_hint_4

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+This function is developed in *The Seasoned Schemer* pp. 165-177. It accepts a list `lst` and returns `#t` or `#f` depending on whether any atom appears in `lst` twice in a row. The list is interpreted as though it were flattened: all embedded lists are collapsed into the topmost level, and empty list elements are ignored. However, no flattened copy of the list is ever constructed.
+
+       #lang racket
+       
+       (define (atom? x)
+         (and (not (pair? x)) (not (null? x))))
+       
+       (define delta
+         (letrec ([yield (lambda (x) x)]
+                  [resume (lambda (x) x)]
+                  [walk (lambda (l)
+                          (cond
+                            ; this is the only case where walk terminates naturally
+                            [(null? l) '()]
+                            [(atom? (car l)) (begin
+                                               (let/cc k2
+                                                 (set! resume k2) ; now calling resume with val will ignore val
+                                                                  ; and continue with the second line of (begin ... (walk (cdr l)))
+                                                 ; when the next line is executed, yield will be bound to k1 or k3
+                                                 (yield (car l)))
+                                               ; the previous yield line will never return, but the following line will be executed when resume is called
+                                               (walk (cdr l)))]
+                            [else (begin
+                                    ; walk will only ever return when a '() is reached, and will in that case return a '()
+                                    (walk (car l))
+                                    (walk (cdr l)))]))]
+                  [next (lambda () ; next is a thunk
+                          (let/cc k3
+                            (set! yield k3) ; now calling yield with val will return val from the call to next
+                            ; when the next line is executed, resume will be bound to k2
+                            (resume 'blah)))]
+                  [check (lambda (prev)
+                           (let ([n (next)])
+                             (cond
+                               [(eq? n prev) #t]
+                               [(atom? n) (check n)]
+                               ; n will fail to be an atom iff we've walked to the end of the list, and (resume 'blah) returned naturally
+                               [else #f])))])
+           (lambda (lst)
+             (let ([fst (let/cc k1
+                          (set! yield k1) ; now calling yield with val will bind fst to val and continue with the (cond ...) block below
+                          (walk lst)
+                          ; the next line will be executed only when lst contains no atoms
+                          (yield '()))])
+               (cond
+                 [(atom? fst) (check fst)]
+                 [else #f])
+               ))))
+       
+       (delta '(((a b) ()) (c (d ())))) ; ~~> #f
+       (delta '(((a b) ()) (b (d ())))) ; ~~> #t
+       (delta '(((a b) ()) (c (d (d))))) ; ~~> #t
+       (delta '(((a b c) ()) (c (d ())))) ; ~~> #t
+       (delta '(((a b) ()) (c (d ()) c))) ; ~~> #f
+