#7401: Can't derive instance for Eq when datatype has no constructor, while it is trivial do do so. ----------------------------------------------+---------------------------- Reporter: jpbernardy | Owner: monoidal Type: feature request | Status: new Priority: normal | Milestone: 7.8.1 Component: Compiler | Version: 7.6.1 Resolution: | Keywords: deriving Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects valid program | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: ----------------------------------------------+---------------------------- Comment (by simonpj): OK, yes, I missed the `[ = constrs ]` part of the grammar. Yes, `EmptyDataDecls` is part of H2010. And yes, clause (6) in the [http://www.haskell.org/onlinereport/haskell2010/haskellch11.html#x18-1820001... deriving appendix] explicitly dis-allows `deriving( Eq )`, but here I think it would be OK for GHC to be a little more generous than the standard. That is, with `EmptyDataDecls` (or `Haskell2010`) we'd accept {{{ data T deriving( Eq ) }}} whereas H2010 would reject it. But failing to reject is very unlikely to break anyone's code. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7401#comment:18 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler