Using the array-map sounds like a good work-around when you know there are
potential key collisions. It would also allow you to swap keys reliably using
a temporary key.
(rename-keys {:a 1 :b 2 :c 3} (array-map :a :tmp :b :a :tmp :b))
;; {:b 1, :a 2, :c 3}
The larger question is: Should rename-keys handle the collisions for you? I
think it was probably a pragmatic decision not to handle collisions in order to
have better performance for the (usual) collision-free case with a small number
of changes. It would be good if the doc for rename-keys warned that behavior
with key collisions is not defined.
On May 10, 2011, at 10:56 PM, Sunil S Nandihalli wrote:
> (defn rename-keys [mp kmap-fn]
> (into {} (map (fn [[key val]] [(kmap-fn key) val]) mp)))
By the way, the suggested implementation doesn't work for keys that don't
change (that is, when some keys that are left out of the kmap). You get a nil
for the key in that case. I think it would work if the old key was used when
you got a nil for the new key. However, you still have an issue if someone
actually wants to use nil or false as a new key. (Not likely, but you'd need to
document it.)
(defn truthy-rename-keys [mp kmap-fn]
(into {} (map (fn [[key val]] [(or (kmap-fn key) key) val]) mp)))
I did a few timings and it looks like performance is similar for rename-keys
and truthy-rename-keys when kmap has lots of key changes, but rename-keys is
much faster when there are only a couple of key changes. That makes sense as
rename-keys is reducing over the kmap, but truthy-rename-keys is mapping over
the original map.
My conclusion is that it's probably not worth "fixing" rename-keys. A little
extra documentation would be helpful. I updated the ClojureDocs.org entry with
a couple of new examples with key collisions.
Regards,
Steve Miner
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