type-level-sets-0.5: Type-level sets (with value-level counterparts and various operations)

This package provides type-level sets (no duplicates, sorted to provide a nomral form) via Set, with value-level counterparts. Described in the paper "Embedding effect systems in Haskell" by Dominic Orchard and Tomas Petricek http://www.cl.cam.ac.uk/~dao29/publ/haskell14-effects.pdf (Haskell Symposium, 2014)

Here is a brief example:

import Data.Type.Set

foo :: Set '["x" :-> Int, "z" :-> Int, "w" :-> Int]
foo = Ext ((Var :: (Var "x")) :-> 2) $
        Ext ((Var :: (Var "z")) :-> 4) $
          Ext ((Var :: (Var "w")) :-> 5) $
            Empty

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

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

The Set 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:

>>> foobar
[(Var :-> 1), (Var :-> 2), (Var :-> 3), (Var :-> 4)]

Thus, we see that the first value paired with the "w" variable is dropped.