Yeah, after I wrote that, it occurred to me that there are good reasons why
Clojure nudges you away from doing lazy operations on maps, although I
hadn't thought much about it.
I didn't think of walk. That is probably the most elegant out of the
approaches I have seen to this common problem.
On Tue, Feb 22, 2011 at 6:43 PM, Alan <a...@malloys.org> wrote:
> On Feb 22, 3:23 pm, rob levy <r.p.l...@gmail.com> wrote:
> > The usual intuitive options for this are reduce, zipmap, or into. You
> can
> > also write a lazily recursive solution. I wonder why there's no function
> in
> > core that lazily re-constructs the map with the results of the function?
> It
> > seems to have been discussed on the list at least once or twice. It
> seems
> > like there would have to be two versions of it, one expecting a function
> > with an arity of one (for just the value) and another expecting an arity
> of
> > two (key and value).
>
> Maps aren't lazy. They would be seqs if they were lazy, and not have
> fast lookup by key.
>
> clojure.contrib.generic.functor/fmap updates values, and clojure.walk/
> walk (and maybe post-walk) can do what you're looking for with pairs:
>
> user=> (walk (fn [[k v]] [v k]) identity {:a 1 :b 2})
> {1 :a, 2 :b}
>
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