This looks like map rather than foldr. On Sun, Nov 28, 2010 at 7:40 AM, tpeng <pengt...@gmail.com> wrote: > i have a one: > (defn lazy-foldr [f coll] > (lazy-seq > (if-let [[x & xs] coll] > (cons x (lazy-foldr f xs))))) > > it seems works so far ;-p > > On Nov 28, 4:01 pm, Laurent PETIT <laurent.pe...@gmail.com> wrote: >> Hello, >> >> clojure is not lazy on evaluation of arguments by default, so I think this >> is not possible to do this "transparently". >> >> Using delay/force could help, but this would not be "transparent" at all for >> the writer of the fn ;-) >> >> 2010/11/27 tpeng <pengt...@gmail.com> >> >> > 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<clojure%2bunsubscr...@googlegroups.com> >> > For more options, visit this group at >> >http://groups.google.com/group/clojure?hl=en > > -- > 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
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