Sorry about the other message, the enter key got in the way. Consider this code: ---------------- #lang racket (define x 10) (define d (delay ((λ(x) (* x x)) x))) (define d2 (* x x)) (define d3 (apply * (list x x))) (define d4 (apply * `(,x ,x))) (define d5 (apply (λ(x) (* x x)) (list x))) (set! x 20)
(define (show) (for-each (λ(v) (displayln (force v))) (list d d2 d3 d4 d5))) ----------------- I had expected a call to (show) to either display identical values, or consider a few of them different. Instead, only the very first value yields the substituted value for the variable x; all the others have captured the value of 'x', including 'd5', which seems like it should be identical to 'd' if any of them are. Not that I make a habit of mixing mutation and delay---frankly I forget now why I even fired up DrRacket to test this---but it was quite surprising. The definition of delay says only: "Creates a promise that, when forced, evaluates the bodys to produce its value." I can't find an obvious reason why these five values aren't identical under such a definition. Deren _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
