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?