hoistWithKey

[int-index]

hoistWithKey :: (forall x. Typeable x => f x -> g x) -> TypeRepMap f -> TypeRepMap g

Adding this function will require adding another vector that will store TypeReps:

data TypeRepMap (f :: k -> Type) =
  TypeRepMap
    { fingerprintAs :: {-# UNPACK #-} !(PrimArray Word64) -- ^ first components of key fingerprints
    , fingerprintBs :: {-# UNPACK #-} !(PrimArray Word64) -- ^ second components of key fingerprints
    , anys          :: {-# UNPACK #-} !(Array Any)        -- ^ values stored in the map
    , keys          :: {-# UNPACK #-} !(Array Any)        -- ^ typerep keys
}

In hoistWithKey we can access a corresponding element in keys, cast it to TypeRep a and then apply https://hackage.haskell.org/package/base-4.11.1.0/docs/Type-Reflection.html#v:withTypeable to get the Typeable instance.

I'm hoping it won't affect the performance of lookups: the cachedBinarySearch function will stay unchanged.

Implementation plan:

1. add the keys field, check that the benchmarks show no change
2. implement hoistWithKey. it should be probably available only on 8.2+ because it relies on the new TypeRep for withTypeable

[int-index]

Having this function will also make it possible to write a useful map for TMap:

map :: (forall a. Typeable a => a -> a) -> TMap -> TMap

which is an instantiation of hoistWithKey to Identity