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 <
[hidden email]> 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?
>
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