module Data.HSet.Get ( HGet(..) , HGettable , hgetTagged ) where import Data.HSet.Type import Data.HSet.TypeLevel import Data.Tagged #if !(MIN_VERSION_base(4, 8, 0)) import Control.Applicative #endif {- | Heterogeneous read arbitrarily element from hset >>> let x = HSCons (10 :: Int) $ HSCons (20 :: Double) HSNil >>> x HSCons (10) (HSCons (20.0) (HSNil)) >>> hget x :: Int 10 >>> hget x :: Double 20.0 Note that 'hget' takes specific element from list of uniquely typed elements depending on what type is required to be returned (return type polymorphism) -} class (i ~ (Index e els)) => HGet els e i | els i -> e where -- | Gets any data from HSet for you hget :: HSet els -> e instance HGet (e ': els) e 'Z where hget (HSCons e _) = e instance (('S i) ~ (Index e (e1 ': els)), HGet els e i) => HGet (e1 ': els) e ('S i) where hget (HSCons _ els) = hget els -- | Enables deriving of the fact that 'e' is contained within 'els' and it's -- safe to say that 'hget' can be performed on this particular pair. type HGettable els e = HGet els e (Index e els) {- | >>> let y = HSCons (Tagged 10 :: Tagged "x" Int) $ HSCons (Tagged 20 :: Tagged "y" Int) HSNil >>> y HSCons (Tagged 10) (HSCons (Tagged 20) (HSNil)) >>> hgetTagged (Proxy :: Proxy "x") y :: Int 10 >>> hgetTagged (Proxy :: Proxy "y") y :: Int 20 -} hgetTagged :: forall proxy label e els . (HGettable els (Tagged label e)) => proxy label -> HSet els -> e hgetTagged _ hset = let x = hget hset in unTagged (x :: Tagged label e)