Hello! I attach a program which I suspect demonstrates a bug in GHC. The important lines are: 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) test1 and test2 give different results, even though showType shouldn't be able to tell them apart. It seems that the Typeable context packed with the Lit constructor is wrong in test2. I had to use two extra classes and newtype deriving to trigger this behavior. Note that if I change deriving instance B BOOL to instance B BOOL the result is correct. Tested with both 6.12.3 and 7.0.1. Should I report this as a bug? Thanks! / Emil
Yes, it's a bug all right. See http://hackage.haskell.org/trac/ghc/ticket/4846 Thanks Simon | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell- | users-bounces@haskell.org] On Behalf Of Emil Axelsson | Sent: 16 December 2010 13:26 | To: glasgow-haskell-users@haskell.org | Subject: Newtype deriving mixing up types | | Hello! | | I attach a program which I suspect demonstrates a bug in GHC. The important | lines are: | | 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) | | test1 and test2 give different results, even though showType shouldn't be | able to tell them apart. It seems that the Typeable context packed with the | Lit constructor is wrong in test2. | | I had to use two extra classes and newtype deriving to trigger this behavior. | Note that if I change | | deriving instance B BOOL | | to | | instance B BOOL | | the result is correct. | | Tested with both 6.12.3 and 7.0.1. | | Should I report this as a bug? | | Thanks! | | / Emil
participants (2)
-
Emil Axelsson -
Simon Peyton-Jones