dependent-map

This library defines a dependently-typed finite map type. It is derived from Data.Map.Map in the containers package, but rather than (conceptually) storing pairs indexed by the first component, it stores DSums (from the dependent-sum package) indexed by tag. For example (using the types from the dependent-sum package's FooGADT example):

{-# LANGUAGE GADTs #-}
import FooGADT
import Data.Dependent.Map

x = fromList [Foo :=> pi, Baz :=> "hello there"]
y = singleton Bar 42
z = union y (read "fromList [Foo :=> (-1.1415926535897931)]")

addFoo :: Foo v -> v -> v -> v
addFoo Foo x y = x + y
addFoo _   x _ = x

main = mapM_ print
    [ x, y, z
    , unionWithKey addFoo x z
    ]

Which prints:

fromList [Foo :=> 3.141592653589793,Baz :=> "hello there"]
fromList [Bar :=> 42]
fromList [Foo :=> -1.1415926535897931,Bar :=> 42]
fromList [Foo :=> 2.0,Bar :=> 42,Baz :=> "hello there"]

This library can be found on Hackage: https://hackage.haskell.org/package/dependent-map