Puzzler.hs

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE CPP #-}
{-# LANGUAGE ExistentialQuantification #-}
module Main where

data Test = Test
  { testFoo :: Int
  , testBar :: Bool
  }

type family Key (a :: *) :: * where
  Key Test = Int

data Entity a =
  PersistField a => Entity
    { entityKey :: Key a
    , entityVal :: a
    }

class PersistField a

instance PersistField Test

main :: IO ()
main = do
  entries <- pure [ (Entity 1 (Test 1 True), Entity 2 (Test 2 False)) ]

  case entries of
    x:_ -> do
#ifdef BOOM
      --  Couldn't match expected type ‘Test’ with actual type ‘Test’
      let (Entity eId eVal, Entity eId' eVal') = x 
#else
      let (Entity eId eVal, Entity eId' (eVal' :: Test)) = x
#endif
      let y = testBar eVal'

      return ()