I might have touched upon one of the Haskell things that seem unnecessarily complex - apparently JSON and YAML decoding has to quite different type signatures.

For JSON parsing I am using aeson (https://hackage.haskell.org/package/aeson), more specifically this function:

eitherDecode :: FromJSON a => ByteString -> Either String a

For YAML parsing I am using yaml (https://hackage.haskell.org/package/yaml), more specifically this function:

decodeEither' :: FromJSON a => ByteString -> Either ParseException a

As it can be seen from the type signatures above, it is necessary to perform some handling of the Left case due to the difference in String vs ParseException. Furthermore, the ByteString also differ between the two. Aeson uses Data.ByteString.Lazy whereas yaml uses Data.ByteString.Lazy. This can easily be fixed though, by using Data.Aeson.eitherDecodeStrict instead.

Thus, I ended up with a wrapper:

yamlDecode :: FromJSON a => Data.ByteString.ByteString ->  Either String a
yamlDecode a = case Data.Yaml.decodeEither' a of  
  Left pe -> Left $ show pe
  Right a' -> Right a'

Cases such as this one are exactly the reason why I am creating these sort of templates - to familiarize myself with the ecosystem.