Use alterF machinery?

[treeowl]

alterF in containers has an internal lookup function that looks up a key in a Map. If the key is present, then it returns not only the associated value but also a record of where the key was found. If the key is absent, it returns a record of where the key would be inserted. I don't think information about missing keys is likely all that useful for this package. However, for maps with static key sets, it could be interesting to have one or more additional types.

data Location ph = Location !{- Whatever exactly Data.Map uses; I forget -}
data LocatedKey ph k = LocatedKey {-# UNPACK #-} !(Location ph) !k

It's possible to write functions like

lookupLoc :: Ord k => k -> Map ph k a -> Maybe (LocatedKey ph k, a)
lookupLTLoc :: Ord k => k -> Map ph k a -> Maybe (LocatedKey ph k, a)
iixLoc :: Functor f => Location ph -> (k -> a -> f a) -> Map ph k a -> f (Map ph k a)
toLocatedList :: Map ph k a -> [(LocatedKey ph k, a)]
deletingLoc ::
            Location ph  -- ^ key location to remove
         -> Map ph k v -- ^ initial map
         -> (forall ph'. (Key ph' k -> Key ph k, Map ph' k v) -> t) -- ^ continuation
         -> t