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StateDemo.hs

{-# LANGUAGE DataKinds, FlexibleContexts, TypeOperators #-}
import Control.Monad.State
import Data.Vinyl

type OuterData = '("Outer", String)
type InnerData = '("Inner", String)

outerData :: SField OuterData
outerData = SField

innerData :: SField InnerData
innerData = SField

getOuterName :: (Functor m, MonadState (FieldRec s) m, OuterData ∈ s)
             => m String
getOuterName = fmap (getField . rget (SField::SField OuterData)) get

getInnerName :: (Functor m, MonadState (FieldRec s) m, InnerData ∈ s)
             => m String
getInnerName = fmap (getField . rget (SField::SField InnerData)) get

main :: IO ()
main = flip evalStateT (innerData =: "inny" <+> outerData =: "outy") $ do
         getInnerName >>= liftIO . putStrLn
         getOuterName >>= liftIO . putStrLn