More general Generic and Data instances for Data.Proxy.Proxy?

[Edward Kmett]

See Scrap more boilerplate: reflection, zips, and generalised casts by Ralf
Lämmel and Simon Peyton Jones
<http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.114.6907&rep=rep1&type=pdf>
for
why gcast1 and dataCast1 exist as they do.

On Fri, Jul 17, 2015 at 8:58 PM, Ryan Scott <ryan.gl.scott at gmail.com> wrote:

> It does seem unnecessary. The Data instance for Proxy is manually written
> in Data.Data:
>
> proxyConstr :: ConstrproxyConstr = mkConstr proxyDataType "Proxy" [] PrefixproxyDataType :: DataTypeproxyDataType = mkDataType "Data.Proxy.Proxy" [proxyConstr]
>
>
> instance (Data t) => Data (Proxy t) where  gfoldl _ z Proxy  = z Proxy  toConstr Proxy  = proxyConstr  gunfold _ z c = case constrIndex c of                    1 -> z Proxy                    _ -> error "Data.Data.gunfold(Proxy)"  dataTypeOf _ = proxyDataType  dataCast1 f  = gcast1 f
>
> If dataCast1 weren't defined to be gcast1, then that instance would only require a Typeable t constraint, which would make it polykinded. To be honest, I'm not sure why gcast1 requires a Data t constraint (I don't use Data that often, so hopefully someone more knowledgeable than me can answer this).
>
>
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