Re: Add traverseWithIndex to Data.Sequence

I just realized there *is* a way to implement traverseWithIndex reasonably efficiently with the current interface: traverseWithIndex f = sequenceA . mapWithIndex f I still think it's a good thing to add though. On Jan 19, 2016 12:24 PM, "Milan Straka" wrote:

Hi David and all,

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-----Original message----- From: David Feuer Sent: 19 Jan 2016, 11:55

Indexed traversals seem to be pretty popular these days, and sequences should be able to support them efficiently. At present, the best options are

1. Thread a counter through. This doesn't work well for weird things like lazy State.

2. Use zipWith first to add indices, then traverse. This adds a smaller, but still asymptotic, penalty to the same sorts of unusual functors, and also adds constant-factor overhead to strict ones that the counter threading wouldn't.

I propose the following, modified mechanically from mapWithIndex. It should be about as efficient as possible in all cases.

+1 from me.

BTW, we have traverseWithKey in Map and IntMap, and traverseWithIndex seems a bit analogous for (indexable) Sequences, where an index can play a role of a key.

Cheers, Milan