thanks Alex for this nice foldr. ;-) however this lazy-foldr can only be used when the fn can stop by itself (in this case, it's 'and' which is short-circuited) otherwise lazy-foldr will never get stop. i think for a good foldr, the evaluation of #(lazy-foldr f val xs) should be only forced when necessary?
On Nov 27, 6:44 pm, Alex Osborne <a...@meshy.org> wrote: > "nicolas.o...@gmail.com" <nicolas.o...@gmail.com> writes: > > I doubt there is a foldr that handles the infinite list. > > To do anything, it would have to read at least one element: the last. > > > Alex nice answer is a foldl. > > Actually I think you have them backwards. Wikipedia has a pair of > diagrams which I find very useful for visualising which is which: > > foldr:http://en.wikipedia.org/wiki/File:Right-fold-transformation.png > > foldl (reduce in > Clojure):http://en.wikipedia.org/wiki/File:Left-fold-transformation.png > > Look at the right side of the diagram. That's the call structure. (The > left side of the diagram is the linked list (1 2 3 4 5). The ":" just > means cons in Haskell syntax.) > > With a foldr the outermost call is given the first element of the list, > so mine is definitely a foldr. > > Let's use (and) as the combining function and use an infinite list that > begins [true true false false ...]. So that looks like: > > (lazy-foldr #(and %1 (%2)) nil (cycle [true true false false])) > > We can draw an ASCII call structure diagram in the same form as the > Wikipedia one and see this: > > and > / \ > T and > / \ > T and > / \ > F (not evaluated) > > Since (and) short-circuits when given false, only the part of the list > up to the first false is evaluated. > > There's probably not much practical use for lazy-foldr. But it was fun > to write. :-) -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
