+#lang racket
+(require racket/control) ; this tells Scheme to let us use shift and reset
+
+(define (visit yield t)
+ (cond [(pair? t) (visit yield (car t)) (visit yield (cdr t))]
+ [else (yield t)]))
+
+
+; delimcc-based implementation of coroutines, following http://okmij.org/ftp/continuations/implementations.html#caml-shift
+(define (coroutine2 main start thread)
+ (letrec ([yield (lambda (x) (shift0 k (cons x k)))]
+ [loop (lambda (curk data)
+ (let ([x (car data)] [k (cdr data)])
+ (cond
+ [(eq? k 'finished) (loop curk (curk x))]
+ [(eq? k 'exit) x]
+ [else (loop k (curk x))])))])
+ (loop (lambda (x) (reset0 (cons (thread yield x) 'finished))) (reset0 (cons (main yield start) 'exit)))))
+
+; call/cc-based, following Xavier Leroy's ocaml-callcc
+(define (coroutine2^ main start thread)
+ (let/cc initk (let* ([curk initk]
+ [yield (lambda (x) (let/cc k (let ([oldk curk]) (set! curk k) (oldk x))))])
+ (main yield (begin (thread yield (let/cc k2 (set! curk k2) start)))))))
+
+(define (proc coroutine2 max1 max2)
+ (letrec ([proc1 (lambda (yield n) (if (>= n max1) (begin (displayln "1: exit") 100) (begin (display "1: received ") (displayln n) (proc1 yield (yield (+ 1 n))))))]
+ [proc2 (lambda (yield n) (if (>= n max2) (begin (displayln "2: finished") -2) (begin (display "2: received ") (displayln n) (proc2 yield (yield (+ 1 n))))))])
+ (coroutine2 proc1 0 proc2)))
+
+; the following is meant to be a general-purpose handler with the following behavior:
+; 1. call main with start
+; 2. first yield to proc1, which yields back to main,
+; 3. then main yields to proc2, which yields back to main; and so on
+; 4. when either proc finishes, subsequent yields from main which would have gone to that procedure instead always return #f
+; 5. we stop looping only when main finishes
+(define (coroutine3 main start proc1 proc2)
+ (letrec ([yield (lambda (x) (shift0 k (cons x k)))]
+ [false (lambda (x) (reset0 (false (shift0 k (cons #f k)))))]
+ [loop (lambda (inmain curk otherk data)
+ (let ([x (car data)] [k (cdr data)])
+ (cond
+ [(eq? k 'finished) (loop #t otherk false (curk x))]
+ [(eq? k 'exit) x]
+ [inmain (loop #f k otherk (curk x))]
+ [else (loop #t otherk k (curk x))])))])
+ (loop #t (lambda (x) (reset0 (cons (proc1 yield x) 'finished)))
+ (lambda (x) (reset0 (cons (proc2 yield x) 'finished)))
+ (reset0 (cons (main yield start) 'exit)))))
+
+; the same-fringe application doesn't make use of the 'start or 'restart parameters
+; the 'blah values yielded to the leaf-iterators are ignored too
+(define (same-fringe1 tree1 tree2)
+ (letrec ([next1 (lambda (yield x) (visit yield tree1))]
+ [next2 (lambda (yield x) (visit yield tree2))]
+ [main (lambda (yield x)
+ (let* ([leaf1 (yield 'blah)]
+ [leaf2 (yield 'blah)])
+ (cond [(and leaf1 leaf2) (and (equal? leaf1 leaf2) (main yield 'blah))]
+ [(or leaf1 leaf2) #f]
+ [else #t])))])
+ (coroutine3 main 'restart next1 next2)))
+
+
+; another delimcc solution, based on Biernacki, Danvy and Shan "On the static and dynamic extents of delimited continuations" 2006, section 4.1.4
+; here, next1 = '(leaf1 . thunk_for_more_leaves); final thunk => '(finished . #f)
+(define (make-enumerator2 tree)
+ (define (yield x) (shift k (cons x k)))
+ (reset (visit yield tree) '(finished . #f)))
+
+(define (same-fringe2 tree1 tree2)
+ (define next1 (make-enumerator2 tree1))
+ (define next2 (make-enumerator2 tree2))
+ (letrec ([loop (lambda (res1 res2)
+ (let* ([leaf1 (car res1)]
+ [leaf2 (car res2)]
+ [next1 (cdr res1)]
+ [next2 (cdr res2)])
+ (cond
+ [(and next1 next2) (and (equal? leaf1 leaf2) (loop (next1) (next2)))]
+ [(or next1 next2) #f]
+ [else #t])))])
+ (loop next1 next2)))
+
+
+; call/cc solution, from http://c2.com/cgi/wiki?SameFringeProblem ("Scheme Language, using CoRoutines")
+; here, (next1) => '(1 . #t); (next1) => '(2 . #t); (next1) => '(finished . #f)
+(define (make-enumerator3 t)
+ (letrec ([resk #f]
+ [yieldk #f]
+ [resume (lambda () (let/cc k
+ (set! yieldk k)
+ (cond [(eq? resk #f)
+ (visit yield t)
+ (set! resk 'finished)
+ (yieldk (cons 'finished #f))]
+ [(eq? resk 'finished)
+ #;(error "End of generator")
+ (yieldk (cons 'finished #f))
+ ]
+ [else (resk)])))]
+ [yield (lambda (x) (let/cc k
+ (set! resk k)
+ (yieldk (cons x #t))))])
+ resume))
+
+(define (same-fringe3 tree1 tree2)
+ (define next1 (make-enumerator3 tree1))
+ (define next2 (make-enumerator3 tree2))
+ (letrec ([loop (lambda (res1 res2)
+ (let* ([leaf1 (car res1)]
+ [leaf2 (car res2)]
+ [isleaf1 (cdr res1)]
+ [isleaf2 (cdr res2)])
+ (cond
+ [(and isleaf1 isleaf2) (and (equal? leaf1 leaf2) (loop (next1) (next2)))]
+ [(or isleaf1 isleaf2) #f]
+ [else #t])))])
+ (loop (next1) (next2))))
+
+
+
+(define (test same-fringe)
+ (define tree1 '(((1 . 2) . (3 . 4)) . (5 . 6)))
+ (define tree2 '(1 . (((2 . 3) . (4 . 5)) . 6)))
+ (define tree3 '(1 . (((2 . 3) . (4 . 5)) . 7)))
+ (define tree4 '(((1 . 2) . (4 . 5)) . 7))
+ (define tree5 '(((1 . 2) . (3 . 4)) . 5))
+ (define tree6 '(((10 . 2) . (3 . 4)) . 5))
+ (define tree7 8)
+ (and (same-fringe tree1 tree2)
+ (same-fringe tree7 tree7)
+ (not (or
+ (same-fringe tree1 tree3)
+ (same-fringe tree1 tree4)
+ (same-fringe tree4 tree1)
+ (same-fringe tree5 tree1)
+ (same-fringe tree1 tree5)
+ (same-fringe tree1 tree6)
+ (same-fringe tree6 tree1)
+ (same-fringe tree6 tree7)
+ ))))
+
+#|
+
+In Lua, using CoRoutines:
+ function tree_leaves(tree)
+ if tree.leaf then
+ coroutine.yield(tree.leaf)
+ else
+ tree_leaves(tree.left)
+ tree_leaves(tree.right)
+ end
+ end
+ function same_fringe(tree1, tree2)
+ local iter1 = coroutine.wrap(tree_leaves)
+ local iter2 = coroutine.wrap(tree_leaves)
+ for node in iter1, tree1 do
+ if node ~= iter2(tree2) then
+ return false
+ end
+ end
+ return iter2() == nil
+ end
+
+In OCaml:
+# #require "delimcc";;
+# open Delimcc;;
+# type seq = End | Next of int * seq computation
+ and 'a computation = unit -> 'a;;
+# type 'a tree = Leaf of 'a | Node of 'a tree * 'a tree;;
+# let rec visit p = function Leaf i -> shift p (fun a -> Next (i, a)) | Node (t1,t2) -> let () = visit p t1 in visit p t2;;
+# let prompt mid = let p = new_prompt() in push_prompt p (mid p);;
+val prompt : ('a Delimcc.prompt -> unit -> 'a) -> 'a = <fun>
+# let make_seq t = prompt (fun p () -> let () = visit p t in End);;
+val make_seq : int tree -> seq = <fun>
+# let tree1 = Node (Node (Node(Leaf 1,Leaf 2), Node(Leaf 3,Leaf 4)), Node(Leaf 5,Leaf 6));;
+# let next1 = make_seq tree1;;
+val next1 : seq = Next (1, <fun>)
+# let next2 = match next1 with Next(_,f) -> f ();;
+val next2 : seq = Next (2, <fun>)
+# let next3 = match next2 with Next(_,f) -> f ();;
+val next3 : seq = Next (3, <fun>)
+# let next4 = match next3 with Next(_,f) -> f ();;
+val next4 : seq = Next (4, <fun>)
+# let next5 = match next4 with Next(_,f) -> f ();;
+val next5 : seq = Next (5, <fun>)
+# let next6 = match next5 with Next(_,f) -> f ();;
+val next6 : seq = Next (6, <fun>)
+# let next7 = match next6 with Next(_,f) -> f ();;
+val next7 : seq = End
+
+|#