Re: Proposal: IntMap.differenceKeysSet for removing an IntSet of keys

On 6/9/11 10:17 PM, Liyang HU wrote:

On 10 June 2011 10:09, Felipe Almeida Lessa wrote:

...

If we had something like fromIntSet :: IntSet -> IntMap () then at least we could have a RULE to convert "difference m (fromIntSet s)" to "differenceIntSet m s". Then we could just hide differenceIntSet and the name wouldn't matter. Perhaps this "fromIntSet" function should be proposed as well?

Sure, that sounds quite reasonable. Have done that as patch 0005 on the ticket. I've generalised it a bit to:

fromSet :: (Key -> a) -> IntSet -> IntMap a

Which version does everyone prefer? differenceKeysSet m s, or difference m (fromSet f s) ?

One downside of using fromSet is that it clones the IntSet before differencing. Since IntSet is spine-strict, this can be quite expensive since it'll clone the whole thing no matter how much is used. I like the idea of having a fromSet function with the (Key->a) argument. But there's also room for differenceKeysSet for efficiency reasons. Perhaps you can come up with some fusion rules to get rid of the overhead of difference m (fromSet f s)? Certainly there should be the rule: map f . fromSet g = fromSet (f . g) and similar for the other mapping functions. -- Live well, ~wren