montis/src/Montis/Core/Extensions.hs


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module Montis.Core.Extensions where

import Data.Data
  ( Typeable,
    tyConModule,
    tyConName,
    tyConPackage,
  )
import Data.Kind (Constraint, Type)
import Text.Printf (printf)
import Type.Reflection (someTypeRep, someTypeRepTyCon)

-- | A key to key into the Extension maps.
data ExtensionKey where
  ExtensionKey :: {extensionKeyValue :: String} -> ExtensionKey
  deriving (Eq, Ord, Show)

data Extension (c :: Type -> Constraint) where
  Extension :: (Typeable a, c a) => a -> Extension c

class Nil a
instance Nil a

-- | Produces a string representation of a type used to key into the extensible
-- state map.
typeRepr :: forall proxy a. (Typeable a) => proxy a -> ExtensionKey
typeRepr proxy = ExtensionKey $ tyconToStr $ someTypeRepTyCon (someTypeRep proxy)
  where
    tyconToStr tc =
      printf "%s.%s.%s" (tyConPackage tc) (tyConModule tc) (tyConName tc)