2011/9/22 Bas van Dijk
I just discovered the predicate:
-- | Marks if this constructor is a record conIsRecord :: t c (f :: * -> *) a -> Bool
I think this can solve my problem.
I think I have solved the bug now using conIsRecord. This is the new implementation: https://github.com/basvandijk/aeson/blob/newGenerics/Data/Aeson/Types/Intern... However, I would still very much like to have the information, whether a constructor is a record or not, statically available. This has two advantages: * More efficient: programs can make a static instead of a dynamic choice. * No more ugly undefined instances: because the information is not statically available I need to add several "undefined" instances like: instance GFromRecord (a :+: b) where gParseRecord = undefined instance GFromRecord U1 where gParseRecord = undefined instance GFromRecord (K1 i c) where gParseRecord = undefined instance GFromRecord (M1 i c f) where gParseRecord = undefined These instances will never be evaluated at runtime. They only exist to satisfy the type-checker. So I propose making the following changes to GHC.Generics: Add a phantom type to C that specifies whether it's a record or not using the (empty) datatypes: data Record data Product Maybe it's also nice to have the type synonyms: type R1 = M1 (C Record) type P1 = M1 (C Product) I will make an official ticket for this. Regards, Bas