Hi Ryan,
I can't recall any particular reason to avoid including dataCast1 in the
Data instance for poly-kinded datatypes. Have you tried applying the
example in #4028 to a poly-kinded datatype? It might be that it was done
simply to avoid forcing the kind of the parameter to be *, and hence losing
the polymorphism (possibly at the price of losing generic function
extension).
I'm not aware of a way to define dataCast1 without the Data context. Then
again, I think it's only used for generic function extension (ext1Q and
friends); can you find a way to make that work without the Data constraint?
Cheers,
Pedro
On 23 February 2017 at 19:51, Ryan Scott
Hi Pedro,
I'm quite confused by a peculiarity of deriving Data (more info in Trac #13327 [1]). In particular, if you write this:
data T phantom = T deriving Data
Then the derived Data instance is NOT this:
instance Typeable phantom => Data (T phantom) where ...
But instead, it's this:
instance Data phantom => Data (T phantom) where ... dataCast1 f = gcast1 f
The gcast1 part is why it requires the stronger (Data phantom) context, as you noted in Trac #4028 [2].
What confuses me, however, is that is apparently does not carry over to poly-kinded datatypes. For instance, if you write this:
data T (phantom :: k) = T deriving Data
Then you do NOT get this instance:
instance Data (phantom :: *) => Data (T phantom) where ... dataCast1 f = gcast1 f
But instead, you get this instance!
instance (Typeable k, Typeable (phantom :: k)) => Data (T phantom) where ... -- No implementation for dataCast1
This is quite surprising to me. I'm not knowledgeable enough about Data to know for sure if this is an oversight, expected behavior, or something else, so I was hoping you (or someone else highly knowledgeable about SYB-style generic programming) could help me out here.
In particular:
1. Does emitting "dataCast1 f = gcast1 f" for datatypes of kind (k -> *) make sense? Or does it only make sense for types of kind (* -> *)? 2. Is there an alternate way to define dataCast1 that doesn't require the stronger Data context, but instead only requires the more general Typeable context?
Ryan S. ----- [1] https://ghc.haskell.org/trac/ghc/ticket/13327 [2] https://ghc.haskell.org/trac/ghc/ticket/4028#comment:5