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I'm writing a monadic parser instance that transforms Data.Map.Map's:

instance (Ord a, FromEDN a, FromEDN b) => FromEDN (M.Map a b) where parseEDNv (E.Map m) = mapMmap parseEDNv parseEDN m parseEDNv v = typeMismatch "Map" v

Data.Map doesn't provide it's own mapM version like Data.Vector does, so i had to write it from scratch:

mapMmap :: (Ord a2, Monad m) => (a1 -> m a2) -> (b1 -> m b2) -> M.Map a1 b1 -> m (M.Map a2 b2) mapMmap kf vf m = do let pairsIn = M.assocs m pairsOut <- mapM fs pairsIn return $! M.fromList pairsOut where fs (k, v) = do newK <- kf k newV <- vf v return (newK, newV)

It works, but very verbose. How to trim into a more succinct INLINEable version without too much black^W monadic magic?

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  • \$\begingroup\$ Have you considered traverseWithKey? \$\endgroup\$ Commented Feb 28, 2014 at 4:51

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There is keys library which implements 

mapWithKeyM_ :: (FoldableWithKey t, Monad m) => (Key t -> a -> m b) -> t a -> m ()

and related functions for Data.Map.

There is also Data.Traversable.mapM, but it only maps over values. Mapping over both keys and values is not a mapM operation because there is Ord constraint.

If you still consider implementing your function yourself, it can be written like this:

import qualified Data.Map as M import Control.Monad mapMmap :: (Ord a2, Monad m) => (a1 -> m a2) -> (b1 -> m b2) -> M.Map a1 b1 -> m (M.Map a2 b2) mapMmap kf vf = liftM M.fromList . mapM fs . M.assocs where fs (k, v) = liftM2 (,) (kf k) (vf v)
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  • \$\begingroup\$ Ah, liftM2, that's how it is used! \$\endgroup\$ Commented Sep 14, 2012 at 16:40

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