On Mon, 27 Jun 2005, Daniel Fischer wrote:
Am Sonntag, 26. Juni 2005 21:02 schrieben Sie:
On Sun, 26 Jun 2005, Daniel Fischer wrote:
m x y = if x==0 then 0 else x*y
Plain
foldr m 1
does fine, in fact much better than
foldl' (*) 1 . upTo (== 0),
both in hugs and ghc, regarding speed and memory usage.
E.g. foldr m 1 [a,b,c]
means m a (m b (m c 1)))
That is, it is not possible for the runtime system to store interim numeric results, it can only accumulate function calls.
Well, yes, but since 'm' is lazy in the second argument, the list isn't traversed past the first zero - like foldr (&&) True.
That was not the point. The point is that "m a (m b (m c..." can't be reduced immediately to, say, "(m (m a b) c) ...", since this is completely different. The computation can only start when the end of the list is reached.