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...).
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