#11251: isInstance does not work on Typeable with base-4.8 anymore -------------------------------------+------------------------------------- Reporter: songzh | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: Component: Template Haskell | Version: 7.10.1 Resolution: | Keywords: Typeable, | isInstance Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by songzh): I also find it is allowed to derive `Typeable` class several times for one data type. I am no with GHC 8.2 {{{ {-# LANGUAGE DeriveDataTypeable, StandaloneDeriving #-} import Data.Typeable data TT = TT deriving instance Typeable TT -- allowed to derive twice. deriving instance Typeable TT deriving instance Show TT -- Not allowed to derive twice -- deriving instance Show TT }}} Should this be allowed? Or they are the same problem with this ticket. I speculate these are the same problem since GHC does not know `TT` is already an instance of `Typeable` or not after the first time of deriving. Do I need to open another ticket for this problem? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11251#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler