Hi all,
I think this (#3 below) is where I got the idea:
http://en.wikipedia.org/wiki/Lazy_evaluation
Excerpt:
---------------
Lazy evaluation refers to how expressions are evaluated when they are passed as arguments to functions and entails the following three points:[1]
1. The expression is only evaluated if the result is required by the calling function, called delayed evaluation.[2]
2. The expression is only evaluated to the extent that is required by the calling function, called Short-circuit evaluation.
3. the expression is never evaluated more than once, called applicative-order evaluation.[3]
---------------
So, I guess #3 doesn't apply to Haskell, or maybe I just misunderstood the meaning of the statement. I assumed that if f(p) = q (by some calculation) then that calculation would be replaced by q so the next time the function was called it could just return q, as occurs in memoization.
Michael
--- On Tue, 12/15/09, Gregory Crosswhite
Am Mittwoch 16 Dezember 2009 05:08:39 schrieb Gregory Crosswhite:
Not even then, necessarily. And it's not always a good idea.
f k = [1 .. 20^k]
You raise a really good point here. One can force sharing, as I understand it, by using a let clause: n = let xs = f 20 in length (xs ++ xs) If I understand correctly, this should cause xs to be first evaluated, and then cached until the full length is computed, which in this case is obviously undesirable behavior. Cheers, Greg _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe