monad stack

[Corentin Dupont]

Hi guys,

I would like to run my API in this stack:

MyApp a = ReaderT AppConfig (ExceptT ServantErr IO) a

How do I need to configure the rest? I tried:

type ServerW api = ServerT api HandlerW

newtype HandlerW a = HandlerW { runHandlerW :: MyApp a }
  deriving
    ( Functor, Applicative, Monad, MonadIO, Generic
    , MonadError ServantErr
    , MonadThrow, MonadCatch
    )

type MyAPI = "api" :> "v1" :> Get '[JSON] [Perm]

server :: ServerW MyAPI
server = undefined

Is this correct?

With my attempt so far, I cannot combine different "ServerW" with the operator :<|> :

server :: ServerW MyAPI
server = Server1 :<|> Server2

       • Couldn't match type ‘ReaderT
                                 AppConfig (ExceptT ServantErr IO) ()’
                         with ‘HandlerW Token’
          Expected type: ServerW ()
            Actual type: (MyApp ())
                         :<|> (MyApp ())

[Corentin Dupont]

I found https://haskell-servant.readthedocs.io/en/stable/cookbook/using-custom-monad/UsingCustomMonad.html

which might give me some answers :)

[Alp Mestanogullari]

Yes, this page along with the tutorial section about it (https://haskell-servant.readthedocs.io/en/stable/tutorial/Server.html#using-another-monad-for-your-handlers) give you examples of using `hoistServer`, which will let you convert all the handlers from your monad to Handler, which is what servant-server knows how to serve.

Let us know here if you have any question though.

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Alp Mestanogullari