#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: infoneeded Priority: normal | Milestone: 7.8.1 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: | ---------------------------------+------------------------------------------ Comment(by jpbernardy): Basically what kosmikus said. This occurs often when you do generic programming. ie, when you encode the type A as A + 0 where 0 is a type with no constructor and + is an either-like type. So: yes, I have seen this in real programs. It's not too hard to write an instance manually, but the error message is confusing. I suspect that a proper fix (allowing to derive an instance) should be very easy, if it is not the case, a better error message (hinting at the user to write an instance with 'undefined') would also be an acceptable fix. -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7401#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler