On Mon, Jul 29, 2013 at 8:57 PM, Ryan Newton
Oops, that was sloppy. Yes, your version does get the job done without allocating. The corrected test is attached to this email.
The lens interface does look quite full featured! And it's nice to see that it consistently includes '_' variants. I cite that as additional evidence for the norm ;-).
Personally, I still want traverseWithKey_ for convenience, especially because the solution you used is non-obvious. I imagine many Data.Map users would not come up with it (as the rest of us on this thread didn't).
Sure, putting it in containers is fine. I just want to mention that these kinds of functions are very general-purpose. In particular, using just traverseWithKey and the proposed alterF, you can implement at least all of: (!) adjust adjustWithKey alter delete empty findWithDefault foldl foldl' foldlWithKey foldlWithKey' foldr foldr' foldrWithKey foldrWithKey' insert insertLookupWithKey insertWith insertWithKey lookup map mapAccum mapAccumRWithKey mapAccumWithKey mapMaybe mapMaybeWithKey mapWithKey member notMember toList update updateLookupWithKey updateWithKey With optimal asymptotic behavior and, in many cases, with just-about-optimal constant factors too (this is essentially what lens does). But adding this function directly to containers for completeness seems reasonable. Shachaf