Foldable traversable

[treeowl]

Improve folding and traversing. Add mapMWithKey (for traversing in strict monads). Clean up cruft.

[konsumlamm]

I'm not sure I'm convinced about the need for mapMWithKey. I don't know of any similar function in other related packages. Do you have benchmarks that show a significant speedup?

[treeowl]

I'm not sure I'm convinced about the need for mapMWithKey. I don't know of any similar function in other related packages. Do you have benchmarks that show a significant speedup?

I can write one. This is a general problem; deferring data structure construction until the very end (building up chains of closures) is awful.

[treeowl]

@konsumlamm, here's a test program:

{-# language BangPatterns #-}
module Main where
import Data.PQueue.Prio.Min (MinPQueue)
import qualified Data.PQueue.Prio.Min as Q
import Control.Monad.Trans.State.Strict

glump :: MinPQueue Int Int
glump = Q.fromList [(a, a) | a <- [1..2*10^6]]

hoom :: MinPQueue Int Int -> MinPQueue Int Int
hoom = flip evalState 0 . Q.mapMWithKey
  (\x y -> do
     !old <- get
     put (old + 1)
     pure (x + y + old))

main = print $ take 3 $ Q.toAscList (hoom glump)

On my system, this runs in (give or take)

real    0m1.688s
user    0m1.519s
sys     0m0.168s

If I use traverseWithKey instead, it runs in (give or take)

real    0m2.620s
user    0m2.363s
sys     0m0.256s

That seems to me like enough to care about.

[konsumlamm]

Next time, please wait until the PR has been approved/re-reviewed before you merge, to make sure that the comments really have been addressed.