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{-# LANGUAGE RankNTypes, ScopedTypeVariables, StandaloneDeriving, GADTs, GeneralizedNewtypeDeriving, DeriveDataTypeable #-}
module Main where
import Data.Typeable
data Expr a where
Lit :: Typeable a => a -> Expr a
class A a where
mk :: a
class (Typeable a, A a) => B a where
mkExpr :: Expr a
mkExpr = Lit mk
-- dfunAE
instance B a => A (Expr a) where
mk = mkExpr
-- dfunAB
instance A Bool where
mk = True
newtype BOOL = BOOL Bool
deriving (Typeable, A)
instance B Bool
deriving instance B BOOL --dfunBB
showType :: forall a . Expr a -> String
showType (Lit _) = show (typeOf (undefined :: a))
test1 = showType (mk :: Expr BOOL) -- Prints "Bool" (wrong?)
test2 = showType (Lit mk :: Expr BOOL) -- Prints "Main.BOOL" (correct)
main = do { print test1; print test2 }
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