28 Mar
2016
28 Mar
'16
12:55 p.m.
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