I'm trying to add automagical json parsing to Data.Vinyl
Here is an instance for FromJSON for records with exactly one element.
It almost works, but I can't satisfy the KnownSymbol constraint, it seems to auto generate a new type variable on me.
instance (KnownSymbol sym, FromJSON a) => FromJSON (PlainRec '[ sym ::: a ]) where
parseJSON (Object v) = (field =:) <$> (v .: json_name)
where field = Field :: (sym ::: a)
json_name = T.pack $ show field
The error is
Could not deduce (KnownSymbol sym0) arising from a use of ‛show’
from the context (KnownSymbol sym, FromJSON a)
More context http://lpaste.net/101005
If I replace all instances of sym with "name", it works, and runs and it is wonderful. Now, I could use template Haskell to generate all the instances ahead of time, since I have a closed list of field names that I'll actually use, but that seems like such a shame. I know next to nothing about Data.Proxy, having just seen in used to define the show instance for the records of Data.Proxy.
You just have to enable ScopedTypeVariables.