#12438: DeriveDataTypeable - deriving instance Data (Mu (Const ())) -------------------------------------+------------------------------------- Reporter: lspitzner | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: deriving Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): I had to make some changes to your code to get that exact error message. Here is what I tried: {{{#!hs {-# LANGUAGE DeriveDataTypeable, FlexibleInstances, StandaloneDeriving #-} module T12438 where import Control.Applicative (Const(..)) import qualified Data.Data data Mu f = Mu (f (Mu f)) deriving instance Data.Data.Data (Mu (Const ())) }}} Note the addition of `FlexibleInstances`. Also, `Const` wasn't in scope in your original program — I assume you are referring to the `Const` from `Control.Applicative`? If so, there's a simple explanation for why that program won't compile — `Const` doesn't have a `Data` instance. Sure enough, this program compiles just fine: {{{#!hs {-# LANGUAGE DeriveDataTypeable, FlexibleInstances, StandaloneDeriving #-} module T12438 where import Control.Applicative (Const(..)) import qualified Data.Data deriving instance (Data.Data.Data a, Data.Data.Data b) => Data.Data.Data (Const a b) data Mu f = Mu (f (Mu f)) deriving instance Data.Data.Data (Mu (Const ())) }}} In light of this, there's no bug in `DeriveDataTypeable` - it's just a Haskell library design question of whether `Const` should have a `Data` instance or not (I wouldn't be opposed to introducing one). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12438#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler