<div dir="ltr"><div class="gmail_default" style="font-family:courier new,monospace">Having a little problem getting something working with sequences. What I'd like to do is have two sequences, call them seq-a and seq-b, which are appended to each other like (in-sequences ...), except that when seq-a runs out it passes information to seq-b in order for seq-b to know how to start. I thought that I could be sneaky and have the procedure returned by sequence-generate* return a value even though the sequence had ostensibly ended, but the docs aren't lying: it's an error to evaluate that procedure after the sequence has ended. Next I tried making the main sequence struct a subtype of another struct. But this fails in surprising ways. Simple example to show the problem: ;;;;;;;;;;;; (struct addable (x)) (struct adder addable (y) #:property prop:sequence (λ(p) (make-do-sequence (λ() (values (λ(pos) (adder-y pos)) (λ(pos) (adder (+ (adder-y pos) (adder-x pos)) (adder-x pos))) p #f #f #f))))) ;;;;;;;;;;;; adder-x: unbound identifier in module in: adder-x Unexpected. I thought this could be fixed by replacing (adder-x pos) with (addable-x pos) since this works: > (struct posn (x y)) > (struct 3d posn (z)) > (posn-x (3d 'x 'y 'z)) 'x This gives no errors but it breaks in unexpected ways. ;;;; > (struct addable (x)) > (struct adder addable (y) #:property prop:sequence (λ(p) (make-do-sequence (λ() (values (λ(pos) (adder-y pos)) (λ(pos) (adder (+ (adder-y pos) (addable-x pos)) <div class="gmail_default"> (addable-x pos))) p #f #f #f))))) > (for ((t (adder 0 2)) (i (in-range 10))) (printf "~a " t)) 2 0 2 2 4 6 10 16 26 42 </div> So there seems to be no hope that way. Any chance there is a way to pass information between two subsequences so that the overall iteration doesn't notice anything happening behind the scenes? Is mutation the only hope? Deren </div></div>