QueryInterface.hs

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
module QueryInterface where

-- Maybe (GHC.Exts.DictBox a)
data QueryIntefaceResult a where
  HasInterface :: a => QueryIntefaceResult a
  NoInterface :: QueryIntefaceResult a

class QueryInterface a where
  queryInterface :: QueryIntefaceResult a

instance QueryInterface (Fractional Double) where
  queryInterface = HasInterface

instance QueryInterface (Integral Double) where
  queryInterface = NoInterface

instance QueryInterface (Fractional Int) where
  queryInterface = NoInterface

instance QueryInterface (Integral Int) where
  queryInterface = HasInterface

instance QueryInterface (Fractional ()) where
  queryInterface = NoInterface

instance QueryInterface (Integral ()) where
  queryInterface = NoInterface

f :: forall a. (QueryInterface (Fractional a), QueryInterface (Integral a)) => a -> a
f x =
  case queryInterface @ (Fractional a) of
    HasInterface -> x / 2
    NoInterface ->
      case queryInterface @ (Integral a) of
        HasInterface -> x `div` 2
        NoInterface -> x

test :: IO ()
test = do
  print $ f (100.0 :: Double)  -- 50.0
  print $ f (11 :: Int)  -- 5
  print $ f ()  -- ()