"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
