I have to admit I'm not a huge fan of these functions. The major objections that come to mind:

* They can't be made to pass the Traversable/Traversal laws and can't be implemented much more efficiently than the naive 'dump it out to a list and read it back in' approach, so baking them into the library doesn't add much

* The names are dreadfully confusing next to combinators like traverseWithKey that do pass the laws.

* If you use fromDistinctAscList you'll get much of the benefit of the monotonic walk you're doing now. Moreover fromList basically gets almost the same performance as fromDistinctAscList these days. Did you benchmark to see how much the custom traversal helps?

Between those concerns I'm currently -1 on adding these.

-Edward

On Mon, Mar 10, 2014 at 9:24 PM, Niklas Haas <haskell@nand.wakku.to> wrote:

Hello all,

While programming a specific type of map update I came across a great
need for a 'traverseKeys' function. I ended up making it myself but it
was still less efficient than it needed to be because my traversal
function was actually monotonic.

I went ahead and added this along with some variants to Data.Map.

> traverseKeys :: (Applicative f, Ord k2) => (k1 -> f k2) -> Map k1 a -> f (Map k2 a)
> traverseKeysWith :: (Applicative f, Ord k2) => (a -> a -> a) -> (k1 -> f k2) -> Map k1 a -> f (Map k2 a)
> traverseKeysMonotonic :: (Applicative f, Ord k2) => (k1 -> f k2) -> Map k1 a -> f (Map k2 a)

A full patch can be found here:
https://github.com/nandykins/containers/commit/a8b0ebd57653bc0a309af69d732356e3572c455c

Let me know what you think,

Discussion period: 2 weeks
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