I'm inclined to go with option 3 as `Functor (Proxy *)` is the only instance that would typecheck. Perhaps with a warning to let the user know the instance is less general than they might expect? I think this is the most user-friendly route: the user can suppress the warning if they don't care, or make it an error if they do care. On Mon, Mar 28, 2016, at 05:55, Richard Eisenberg wrote:
Hi devs,
Consider the following:
data Proxy k (a :: k) = P deriving Functor
What should happen when this is compiled? 1. Issue an error saying that `deriving` cannot instantiate visible parameters. 2. Type error: cannot match `k` with `*`. 3. Successfully produce `instance (Proxy *)`.
Currently, GHC does #3. But this ends up choosing a value for a visible (i.e. explicit) parameter to Proxy. Is this a good idea? I myself have flip-flopped on this issue; see https://ghc.haskell.org/trac/ghc/ticket/11732, comments 4 and 9.
I'd love to get feedback on this point.
Thanks! Richard _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs