Sorry for the double-post. group-by actually maps outputs to inputs.
But you can run into trouble even with referentially transparent
functions, because of Clojure's equality semantics:
user=> (def x [(list :a) (vector :a)])
#'user/x
user=> (zipmap x (map class x))
{(:a) clojure.lang.PersistentVector}
- Will Morgan
On Aug 16, 5:52 pm, wwmorgan <wmorga...@gmail.com> wrote:
> Note that if f is not referentially transparent, you can get different
> results depending on the order in which the collection is traversed,
> for unordered collections. See the API function group-by. It makes
> this behavior explicit by mapping each input argument to a vector of
> output arguments, in the same order as their corresponding inputs were
> traversed.
>
> - Will Morgan
>
> On Aug 16, 3:31 am, Alan <a...@malloys.org> wrote:
>
> > (defn apply-keys [f ks]
> > (zipmap ks (map f ks)))
>
> > Trivial to write, but it can be quite useful. For example:
> > (defn whatever [arg]
> > (let [some-list (make-list-from arg)
> > mapped (map myfunc some-list)]
> > (zipmap some-list mapped)))
>
> > compared to
>
> > (defn whatever [arg]
> > (apply-keys myfunc (make-list-from arg)))
>
> > I was working on my project and found myself doing this fairly often;
> > I searched high and low in clojure.core and clojure.contrib but
> > couldn't find anything similar. This amazed me, because it seems so
> > common and generic that I assumed clojure would do it for me. If this
> > isn't already in a standard library, I'd like to see it added. I'm
> > happy to give it away to someone who maintains a package that would
> > "fit" this function, or if someone wants to explain to me how to add
> > my own package I'm willing to do that.
>
> > Does this seem useful to anyone else?
>
>
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