#10447: DeriveFoldable rejects instances with constraints in last argument of data type -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #8678 | Differential Revisions: -------------------------------------+------------------------------------- Comment (by simonpj): OK, but can I ask: can you say precisely what `Foldable` code you would expect to get for these two data types? {{{ data T1 a where MkT1 :: (a~Int) => a -> T1 a data T2 a where MkT2 :: Int -> T2 Int data T3 a where MkT3 :: (a~Int) => a -> T3 Int }}} All three are equivalent. But I am guessing that you intend that they are '''not equivalent''' to the "deriving `Foldable`" algorithm? If so, that had better be clearly stated in the user manual. (Regrettably, the user manual is silent on how `Foldable`, `Traversable`, and `Functor` are generated. It'd be jolly good to have a wiki page that explained the deriving algorithm; [https://www.haskell.org/onlinereport/haskell2010/haskellch11.html#x18-182000... here's how it's done for Eq/Ord etc].) The documentation is probably the hard bit. The change to lift the condition is pretty easy: look at `TcDeriv.cond_functorOK`. (This same function actually handles `Traversable` and `Foldable`. A little refactoring would be needed to distinguish the `Foldable` case, perhaps by passing the class name instead of a boolean to the function.) I'd be happy to review a patch. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10447#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler