module Data.HSet
       ( HSet(..)
       , HGet(..)
       , Contains
       , hask
       ) where

import Control.Monad.Reader
import Data.HSet.TypeLevel
#if !(MIN_VERSION_base(4, 8, 0))
import Control.Applicative
#endif

-- | Heterogeneous set (list) of elements with unique types. Usefull
-- with MonadReader
data HSet (elems :: [*]) where
  HSNil  :: HSet '[]
  HSCons :: ('False ~ (Elem elem elems)) => elem -> HSet elems -> HSet (elem ': elems)


class (i ~ (Index e els)) => HGet els e i where
  -- | Get any data from HSet for you
  hget :: HSet els -> e

instance HGet (e ': els) e 'Z where
  hget (HSCons e _) = e

instance (i ~ (Index e els), ('S i) ~ (Index e (e1 ': els)), HGet els e i) => HGet (e1 ': els) e ('S i) where
  hget (HSCons _ els) = hget els

type Contains els e = HGet els e (Index e els)

hask :: (MonadReader (HSet els) m, Contains els e) => m e
hask = do
  h <- ask
  return $ hget h


----------------------------------------------------------------
-- Tests: if this is compilable then tests done trollface.jpg --

fun :: HSet '[Int, Double, Integer, Float]
fun = HSCons 1
      $ HSCons 2
      $ HSCons 3
      $ HSCons 4
      $ HSNil

funInt :: Int
funInt = hget fun

funDoube :: Double
funDoube = hget fun

funInteger :: Integer
funInteger = hget fun

funFloat :: Float
funFloat = hget fun

this :: (Double, Float)
this =
  let h = (,) <$> hask <*> hask
  in runReader h fun