`ThrowAll` and `EmptyServer`

[benperez]

I can't seem to use throwAll with an API type containing an EmptyServer anywhere in it. I have a MonadError ServantErr m instance for my monad stack (below) and throwAll works for other types of routes.

Here's a minimal reproduction:

newtype AppT m a
    = AppT
    { runApp :: ReaderT Environment (ExceptT ServantErr m) a
    } deriving ( Functor, Applicative, Monad, MonadReader Environment,
                 MonadError ServantErr, MonadIO)

data ApiUser = ApiUser
  { UserId :: Int64
  } deriving (Eq, Show, Generic, ToJSON, FromJSON)

instance ToJWT ApiUser
instance FromJWT ApiUser

type MyAPI auths = Auth auths ApiUser :> EmptyAPI

myPrivateServer :: MonadIO m => AuthResult ApiUser -> ServerT EmptyAPI (AppT m)
myPrivateServer (Authenticated _user) = emptyServer
myPrivateServer _ = throwAll err401

This unfortunately gives me the following error:

Could not deduce (Control.Monad.Error.Class.MonadError
                          ServantErr (Tagged (AppT m)))
        arising from a use of ‘throwAll’
      from the context: MonadIO m
        bound by the type signature for:
                   myPrivateServer :: MonadIO m =>
                                      AuthResult ApiUser -> ServerT EmptyAPI (AppT m)

[dbaynard]

Out of curiosity, what happens if you apply hoistServer to your throwAll err401 — so the throwAll is operating in Handler, say?

(Curious because I've been experimenting with that approach, but I don't know how well it actually works.)

[benperez]

what happens if you apply hoistServer to your throwAll err401

I'm not sure. Right now I am using the lts-9.21 snapshot in stack, which has servant version 0.11 and I believe hoistServer was added after that.

I may take a crack at upgrading to the new lts-11.4 snapshot, which looks like it has the latest servant-0.13 in it. I'll report back when I do that.