In the old days, DeriveDataTypeable enabled deriving both Data and Typeable. As of a fairly recent GHC version (7.10? 8.0?), Typeable instances are indeed derived automatically for all types that can get such instances, so DeriveDataTypeable is only used for deriving Data instances. I can't say whether it will ever be an error to write an explicit `deriving Typeable` clause, but I don't see much point in making it one.
Unrelatedly, 8.2 has a complete overhaul of Typeable that you should take a look at if you're interested in the class. The new Type.Reflection API is much more powerful than the old Data.Typeable one. On Mon, Jun 26, 2017 at 1:17 PM, Chris Martin <ch.mar...@gmail.com> wrote: > Exhibit A: > >> With -XDeriveDataTypeable, you can derive instances of the class Data, >> defined in Data.Data. See "Deriving Typeable instances" for deriving >> Typeable. > > Exhibit B: > >> -XDeriveDataTypeable >> Enable automatic deriving of instances for the Typeable typeclass > > Exhibit C: > >> Derived instances of Typeable are ignored, and may be reported as an error >> in a later version of the compiler. > > ---------------- > > A and B seem contradictory: Is this extension for deriving Data, or for > deriving Typeable? > > B and C seem... not technically contradictory, but why is there an extension > that enables the deriving of instances that will just be ignored? > > Is this extension meant to be deprecated? > > From experimentation, it seems like all types automatically get Typeable > instances whether you declare it or not. Is that accurate? > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users > _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
