Certainly compact; as written, though, it has a few deficiencies compared to the Go version: 1. no specific error messages; these take up a good portion of the

Certainly compact; as written, though, it has a few deficiencies compared to the Go version: 1. no specific error messages; these take up a good portion of the Go code, although they could certainly be more automatic; 2. does not allow the same code to be used for encoding and decoding (i.e. you don't have to write a list of fields twice, and can specify which format to encode to/decode from once rather than separately specifying the encoding and decoding functions - note that the format may not be uniquely determined by the final type you want in the output, e.g. you just want a generic date value out, but you need to use a specific date format for the JSON conversion). The first is more of a nitpick (are those going to show up in 'e' anyway?), but the second is important. Do popular Haskell JSON libraries have a way to do that? Also, reflection is not really stepping outside the language.