newtype instead of bare string in MimeUnrender / ResultStream?

[phadej]

Currently we have

mimeUnrender :: Proxy ctype -> ByteString -> Either String a

I think that we should have a wrapper over that String. The same error is also in ResultStream definition:

newtype ResultStream a = ResultStream (forall b. (IO (Maybe (Either String a)) -> IO b) -> IO b)

If newrype is too much at once, I'll be already happy with a type alias. I have no good candidate for the name though.

[alpmestan]

ContentTypeError? BodyDecodingError? 😞

I'd probably be happy with a type alias.

[jhmcstanton]

Just made two type synonyms, one per module, with the name BodyDecodingError (see d5f3cf1). Any other changes needed?

[asheshambasta]

So, we're using servant for our REST API, where our clients are ensured everything from the API is going to be JSON.

Now, we've got custom json validation on our side, and I've implemented :

data ReadJSON

-- | Describes the "Accept" header required to be able to process requests of this type.
instance Accept ReadJSON where
  contentType _ = "application" // "json" /: ("charset", "utf-8")

-- | /Unrender/ a value from incoming HTTP data (`ByteString`)
instance JSONRead a => MimeUnrender ReadJSON a where
  mimeUnrender _ bs = do
    v <- eitherDecode bs -- aeson
    decodeJSONEitherText v & _Left %~ T.unpack -- JSONRead

Now, what I'd like to do is be able to have an equivalent of mimeUnrender that outputs some custom type that can also be returned: Eg.

class IsServantAPIErr err where 
  displayErr :: err -> String 
  errJSON :: err -> Value 

class (IsServantAPIErr (UnrenderErr a), Accept a) => MyMimeUnrender a where 
  type UnrenderErr a -- assoc. type that indicates the error type 
  mimeUnrender :: ByteString -> Either (UnrenderErr a) a 

What would be the best way to achieve this? Is there something I can do with the current system to implement something to this effect?