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haskellstreamingservant

servant cannot stream JSON

I try to stream JSON objects to clients in order to have update notifications.

My API is declared as following:

type API = "poll" :> StreamGet NewlineFraming JSON (SerialT IO Notification)

And I have got the following error:

• Could not deduce (ToJSON chunk0) arising from a use of ‘serve’
      from the context: P.Persist p
        bound by the type signature for:
                   app :: forall (p :: * -> *).
                          P.Persist p =>
                          PollRoute p -> P.Configuration p -> Application
        at app/Main.hs:93:1-70
      The type variable ‘chunk0’ is ambiguous
      These potential instances exist:
        instance ToJSON DotNetTime
          -- Defined in ‘aeson-1.4.7.1:Data.Aeson.Types.ToJSON’
        instance ToJSON Value
          -- Defined in ‘aeson-1.4.7.1:Data.Aeson.Types.ToJSON’
        instance (ToJSON a, ToJSON b) => ToJSON (Either a b)
          -- Defined in ‘aeson-1.4.7.1:Data.Aeson.Types.ToJSON’
        ...plus 46 others
        ...plus 70 instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)

I am using Servant 0.19.2. (SerialT comes from streamly)

Solution

  • Since I have used a custom streaming support library it should (the instance definition module) be imported during the server definition.