JSON return type for Handlers?

[MaxGabriel]

So I was thinking about this blog post recently http://tech.frontrowed.com/2015/12/21/servant-style-handlers-for-yesod/, that uses UndecidableInstances to let you specify the return type of your Handlers that return JSON as Handler <RecordName> rather than Handler Value

Why not just have something like this?

data JSONResponse a = ToJSON a => JSONResponse a

instance ToContent (JSONResponse a) where
  toContent (JSONResponse a) = toContent $ toEncoding a

instance ToTypedContent (JSONResponse a) where
  toTypedContent = TypedContent typeJson . toContent

Then handlers are typed as:

postSignupR :: Handler (JSONResponse CreateUserResponse)

and the function returns:

return $ JSONResponse $ CreateUserResponse userId

Slightly more overhead than the blog post version. I don’t know if UndecidableInstances is actually worth avoiding though. Thoughts? I think endorsing one or the other of these options would be good for Yesod to do—it seems much better than returning Value.

[MaxGabriel]

cc @gregwebs since you authored that blog post

[gregwebs]

Looks good

[StevenXL]

Closed by #1592