instantiating visible parameters in when deriving instances
eir at cis.upenn.edu
Mon Mar 28 12:55:16 UTC 2016
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.
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