Decoder for Maybe a

[karls]

Hi!

I've been playing around with this library to see if I can reduce the burden of defining decoders for my Hasql queries. So far, it's been promising! However, I can't seem to find a way to write an instance for Decodable (Maybe a) to signify nullable values returned by a query (using LEFT OUTER JOIN for example).

I don't know much about GHC Generics, so I've been fumbling in the dark for a while. Ideally, I'd like to define something like instance (Decodable a) => Decodable (Maybe a), but after trying various approaches, I'm stuck. Am I missing something obvious? Is there a way to define such an instance?

Thanks!