#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): Omer writes: I just started GHC development made a commit for ticket 7401: http://ghc.haskell.org/trac/ghc/ticket/7401 My patch is here: https://github.com/osa1/ghc/commit/3ec257ff48372de30df59cd8854ce28954c9db95 `make test` succeeds. My test case for this patch is something like this: {{{ data D deriving Eq main :: IO () main = print ((undefined :: D) == (undefined :: D)) }}} example: {{{ haskell$ ./ghc/inplace/bin/ghc-stage2 --interactive derive.hs GHCi, version 7.7.20130806: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( derive.hs, interpreted ) Ok, modules loaded: Main. ghci> main True it :: () }}} This behavior (returning True) is consistent with standalone deriving version: {{{ haskell$ cat derive_standalone.hs {-# LANGUAGE StandaloneDeriving #-} data D deriving instance Eq D main :: IO () main = print ((undefined :: D) == (undefined :: D)) haskell$ ./ghc/inplace/bin/ghc-stage2 --interactive derive_standalone.hs GHCi, version 7.7.20130806: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( derive_standalone.hs, interpreted ) Ok, modules loaded: Main. ghci> main True it :: () ghci> Leaving GHCi. }}} However, if you want (==) implementation in this case to be `error "Void (==)"`, I think I can also do that by first fixing the code generated by `StandaloneDeriving` extension and then fixing my current patch. Any comments and reviews would be appreciated! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7401#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler