On Mon, Jul 22, 2013 at 1:23 AM, José Pedro Magalhães
Thanks for bringing this up again. This was started in my data-proxy branch of base, but never really finished. We definitely want to have this in 7.8, and I think there's only some minor finishing work to do (check if we have all the instances we want, document, etc.). Perhaps you can look through what's there already, and what you think is missing? I'm more than happy to accept contributing patches too :-)
I'm looking at the current state of Data.Proxy in base (it looks like Richard merged data-proxy into master) and it looks pretty good instance-wise. Issues I'm aware of: * It looks like there's a SafeHaskell issue -- should this be marked Trustworthy? See https://github.com/ekmett/tagged/pull/13 * tagged's Data.Proxy defines some useful functions that aren't present in base. Two of them can move into tagged's Data.Tagged, but the other two should probably go in base's Data.Proxy. In particular asProxyTypeOf :: a -> proxy a -> a asProxyTypeOf = const reproxy :: proxy s -> Proxy t reproxy _ = Proxy When these are fixed, tagged can probably shuffle a couple of functions around and then use base's Data.Proxy rather than exporting its own module (for GHC ≥ 7.7). (By the way: Some instances are slightly different from what GHC would derive -- e.g. «_ == _ = True» is different from «Proxy == Proxy = True», which is ()'s Eq behavior. I think this is OK but I wanted to mention it.) Thanks, Shachaf