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,
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 losin
Some thoughts on the topic: admittedly, probably not very useful.
A couple of obvious statements:
1)
gcast1 itself operationally makes sense regardless of the kind of the
argument you're skipping past.
gcast1 :: forall c t t' a. (Typeable t, Typeable t') => c (t a) -> Maybe (c
(t' a))
This i
For good or ill, Simon doesn't want RULES for datacons. T12689 has to be
removed (leaving T12689a, which is still fine). But I don't know enough about
what you're doing with T12689broken to know how to make it express the right
idea after this change. Can you please advise?
Thanks,
David Feuer
> Supplying the default shouldn't lock our data instance to the form T a. If
> for some reason adding this default would break the instance We can make a
> more interesting default that does something like look at the kind of the
> argument first to determine if it is kind * before proceeding af
That is a right mess.
I've now stepped a bit outside of what I think is a practical
recommendation, but let's just keep playing for the fun of it.
This is going to be hard to do without access to GHC at the moment but here
goes:
If phantom was of kind * then dataCast1 would need the (Data phanto
