Typeable instance for Keys?

[agrafix]

Would it be possible to generate a Typeable/Data instance for Database-Keys?

[snoyberg]

I just tried this out... looks like we can't derive Typeable for BackendKey SqlBackend. For the generated keys, you can currently use StandaloneDeriving to make this work I think. I'm hesitant to make this a default, as it can break existing programs that don't have DeriveDataTypeable turned on in their code.

[agrafix]

Maybe for the next "bigger" Version bump? :-) I'll try out StandaloneDeriving for now, I think it should work.

[parsonsmatt]

GHC automagically derives Typeable for everything now, so I'm tempted to close this out. But it also asks for Data. Unfortunately, I don't think we can do that:

persistent-template  > /home/matt/Projects/persistent/persistent-template/test/main.hs:44:1: e
rror:
persistent-template  >     • Couldn't match representation of type ‘c (BackendKey SqlBackend)’
persistent-template  >                                with that of ‘c (Key Address)’
persistent-template  >         arising from the coercion of the method ‘gfoldl’
persistent-template  >           from type ‘forall (c :: * -> *).
persistent-template  >                      (forall d b. Data d => c (d -> b) -> d -> c b)
persistent-template  >                      -> (forall g. g -> c g)
persistent-template  >                      -> BackendKey SqlBackend
persistent-template  >                      -> c (BackendKey SqlBackend)’
persistent-template  >             to type ‘forall (c :: * -> *).
persistent-template  >                      (forall d b. Data d => c (d -> b) -> d -> c b)
persistent-template  >                      -> (forall g. g -> c g) -> Key Address -> c (Key A
ddress)’
persistent-template  >       NB: We cannot know what roles the parameters to ‘c’ have;
persistent-template  >         we must assume that the role is nominal
persistent-template  >     • When deriving the instance for (Data (Key Address))

Perhaps we should drop the BackendKey business.