Awesome

This library provides type-level sets and finite maps to Haskell, with value level counterpart, and operations for taking the union, computing subsets/submaps, and splitting sets/maps.

This library was originally built based on "Embedding effect systems in Haskell" (Dominic Orchard, Tomas Petricek http://www.cs.kent.ac.uk/~dao7/publ/haskell14-effects.pdf) to embed effect sets.

The following shows an example:

{-# LANGUAGE DataKinds, TypeOperators, TypeFamilies, MultiParamTypeClasses #-}
    import Data.Type.Map

    -- Specifies how to combine duplicate key-value pairs for Int values
    type instance Combine Int Int = Int
    instance Combinable Int Int where
       combine x y = x + y

    foo :: Map '["x" :-> Int, "z" :-> Bool, "w" :-> Int]
    foo = Ext (Var :: (Var "x")) 2 
        $ Ext (Var :: (Var "z")) True 
        $ Ext (Var :: (Var "w")) 5
      Empty

    bar :: Map '["y" :-> Int, "w" :-> Int]
    bar = Ext (Var :: (Var "y")) 3
        $ Ext (Var :: (Var "w")) 1
        $ Empty 

    -- foobar :: Map '["w" :-> Int, "x" :-> Int, "y" :-> Int, "z" :-> Bool]
    foobar = foo `union` bar

The 'Map' type for 'foobar' here shows the normalised form (sorted with no duplicates). The type signatures is commented out as it can be infered. Running the example we get:

*Main> foobar	
{w :-> 6, x :-> 2, y :-> 3, z :-> True}

Thus, we see that values for 'w' are added.

import GHC.TypeLits
import Data.Type.Set
type instance Cmp (Natural n) (Natural m) = CmpNat n m

data Natural (a :: Nat) where
  Z :: Natural 0
  S :: Natural n -> Natural (n + 1)

-- foo :: Set '[Natural 0, Natural 1, Natural 3]
foo = asSet $ Ext (S Z) (Ext (S (S (S Z))) (Ext Z Empty))

-- bar :: Set '[Natural 1, Natural 2]
bar = asSet $ Ext (S (S Z)) (Ext (S Z) (Ext (S Z) Empty))

-- foobar :: Set '[Natural 0, Natural 1, Natural 2, Natural 3]
foobar = foo `union` bar