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. :-)
>
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