This page is part of the web mail archives of SRFI 40 from before July 7th, 2015. The new archives for SRFI 40 contain all messages, not just those from before July 7th, 2015.
Hello, Andre. There is a problem with your proposal as it is: doing (force (lazy (delay expr))) does not update the promise returned by (delay expr) to cache the results of evaluating expr, so subsequent forces will need to reevaluate it. The following example ilustrates this problem: (let ((s (delay (begin (format #t "Eval~%") '())))) (force (lazy s)) (force s)) This will print "Eval" twice, showing that the expresion was evaluated two times. This is a problem because, according to your transformation rules (section "Correct Usage"), the bodies of procedures returning promises should be wrapped with lazy (instead of delay and force). To write a function that removes the first N elements of a stream one would thus do: (define (stream-drop str n) (lazy (if (zero? n) str (stream-drop (cdr (force str)) (- n 1))))) The result is a (lazy (delay expr)) that, when forced, won't cause the original stream to cache the result. You can rewrite my first example replacing the call to lazy with a call to (lambda (str) (stream-drop str 0)) to see the same bug. The problem is easy to find. (lazy (delay expr)) expands to: (cons 'lazy (lambda () (cons 'lazy (lambda () (cons 'eager expr))))) Unfortunately, forcing this expresion only updates the external pair,, so future forces of the internal pair will need to evaluate expr again. This bug obviously affects its current reference implementation for streams, making it problematic to work with streams that are built reading from ports, for example. SRFI-40 makes things worse by specifying no function for forcing the streams directly. In consequence, I'm posting this message to both lists, I hope that's ok. Any thoughts on how to solve this problem? I *think* you could specify that lazy should only be passed forced promises (created by means of (eager expr) or, if you don't want eager to be part of your specification, (force (delay expr))) or promises created by lazy itself and you wouldn't lose your safe-for-space properties. Thank you! Alejo. http://bachue.com/alejo Ps: Please CC to me in your reply, as I haven't managed to subscribe to the lists (waiting for a reply to my subscribe message for hours...). ---=( Comunidad de Usuarios de Software Libre en Colombia )=--- ---=( http://bachue.com/colibri )=--=( colibri@xxxxxxxxxx )=---
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