#9255: Exception when DeriveFunctor fails -----------------------------------+--------------------------------------- Reporter: twanvl | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 7.8.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Compile-time crash Unknown/Multiple | Test Case: Difficulty: Unknown | Blocking: Blocked By: | Related Tickets: | -----------------------------------+--------------------------------------- The following program {{{ {-# LANGUAGE DeriveFunctor #-} data Foo a = Foo data Bar a = Bar [Foo a] deriving (Functor) }}} Gives the error message {{{ foo.hs:5:13:ghc: panic! (the 'impossible' happened) (GHC version 7.8.1 for x86_64-unknown-linux): Prelude.(!!): index too large }}} While it should give {{{ foo.hs:4:13:No instance for (Functor Foo) }}} The use of `[]` in `[Foo a]` is not essential, it can be any type constructor that is an instance of Functor. Without the extra type constructor an error is correctly reported. The cause seems to be in the error message. If it wraps to the next line, then ghc dies with: {{{ foo-with-a-longer-filename.hs:5:13: No instance for (Functorghc: panic! (the 'impossible' happened) }}} When using stand alone deriving ghc does not crash. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9255 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler