EnumMap of EnumMaps.
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Based on containers 5.0, EnumMapMap provides an EnumMap, similar to IntMap but accepting any Enum type. It also provides an EnumMap of EnumMaps type. The Key is built using an infix operator (:&) and a base type, K. A typical use might look like this:

import           EnumMapMap.Strict (EnumMapMap, (:&)(..), K(..)
import qualified EnumMapMap.Strict as EMM

data Apple = Apple {weight :: Int, Circumference :: Int}
newtype AppleID = AppleID Int
newtype TreeID = TreeID Int
newtype OrchardID = OrchardID Int

type Orchards = EnumMapMap (OrchardID :& TreeID :& K AppleID) Apple

o = OrchardID 3
t = TreeID 34
a = AppleID 1
apple = EMM.lookup (o :& t :& K a) orchards

If it is being used as a single EnumMap then the Key must have type K.

type IntMap v = EnumMapMap (K Int) v

im :: IntMap String
im = EMM.singleton (K 3) "Three"
im' = EMM.insert (K 5) "Five" im

The code ensures that only the root EnumMap can be empty. There are lazy and strict variants. Both are strict in the keys, but the strict version is strict on values as well. The data structures are the same however the key types are different for the strict and lazy versions so strict operations can be performed on a lazy EnumMapMap.


Whole subtrees can be operated on at a time.

tree = EMM.lookup (o &: K t) $  orchards
apple = EMM.lookup (K a) tree
newOrchards = delete (K o) $ orchards
EMM.lookup (o :& t :& K a) newOrchards == Nothing


There is also 'EnumMapSet'. The terminating key type is S instead of K.


EnumMapMap has grown quite an unwieldy, or at least verbose, API. See the unit tests for the full horror. Run the benchmarking suite to see how EnumMapMap compares to IntMap for speed.

Because the keys are polymorphic you may have to specify types:

treeKey = (2 :: OrchardID) :& (K $ 3 :: TreeID)

EnumMapMap is a bit 'batteries included', so it has a number of dependencies, including Lens, SafeCopy, Semigroup and Data.Default. If this is an issue I could move them out to separate packages.