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 _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
