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@jyrimatti jyrimatti/Main.hs
Created Nov 22, 2018

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#! /usr/bin/env nix-shell
#! nix-shell -i runghc -p "haskellPackages.ghcWithPackages(p: with p; [type-level-sets])"
#! nix-shell -I nixpkgs=channel:nixos-18.03
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE TypeApplications #-}
module Main where
import Data.Type.Set (Set(..), Proxy(..))
class Get a s where
get :: Set s -> a
instance {-# OVERLAPS #-} Get a (a ': s) where
get (Ext a _) = a
instance {-# OVERLAPPABLE #-} Get a s => Get a (b ': s) where
get (Ext _ xs) = get xs
main :: IO ()
main = do
let lst = Ext "hello" $ Ext 10 $ Empty
putStrLn $ show $ get @String lst
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