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{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
module Main (main) where
import Data.Proxy (Proxy(..))
import GHC.Exts (withDict)
import GHC.TypeLits (Symbol)
import GHC.Exts
newtype SSymbol (s :: Symbol) = SSymbol String
class KnownSymbol (n :: Symbol) where
symbolSing :: SSymbol n
symbolVal :: forall n proxy . KnownSymbol n => proxy n -> String
symbolVal _ = case symbolSing :: SSymbol n of SSymbol x -> x
-- See Note [NOINLINE someNatVal] in GHC.TypeNats
{-# NOINLINE reifySymbol #-}
reifySymbol :: forall r. String -> (forall (n :: Symbol). KnownSymbol n => Proxy n -> r) -> r
reifySymbol n k = withDict @(SSymbol Any) @(KnownSymbol Any) (SSymbol n) (k @Any) (Proxy @(Any @Symbol))
main :: IO ()
main = print $ reifySymbol "Hello World" symbolVal
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