#7401: Can't derive instance for Eq when datatype has no constructor, while it is trivial do do so. ---------------------------------+------------------------------------------ Reporter: jpbernardy | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.1 Keywords: deriving | Os: Unknown/Multiple Architecture: Unknown/Multiple | Failure: GHC rejects valid program Difficulty: Unknown | Testcase: Blockedby: | Blocking: Related: | ---------------------------------+------------------------------------------ Changes (by igloo): * difficulty: => Unknown Old description:

On a "phantom datatype" D, one gets the message:

Can't make a derived instance of `Eq D': `D' must have at least one data constructor

However there is a trivial, correct instance of Eq D:

instance Eq D where (==) = undefined

New description: On a "phantom datatype" D, one gets the message: {{{ Can't make a derived instance of `Eq D': `D' must have at least one data constructor }}} However there is a trivial, correct instance of Eq D: {{{ instance Eq D where (==) = undefined }}} -- -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7401#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler