Unsure how to initialize a value using IOE before running reinterpret in my effect handler function.

[rmullin7286]

Basically I have an effect of the following form:

data ApiKey = ApiKey { accesskey :: String, secretKey :: String }

data Request

data Reponse

data Api :: Effect where
    CallEndpoint :: Request -> Api m Response

Before the effect can be run, I need to call /login with the ApiKey to initialize the session state. Something like

runApiIO :: (IOE :> es) => Eff (Api : es) a -> Eff es a
runApiIO = do
    session <- loginWithCredentials
    reinterpret (evalState session) $ \env -> \case
        CallEndpoint r -> do
            session <- get
            resp <- post session r
            return $ responseBodyToJson resp

This doesn't seem to work. I notice looking at the source for things like the State effect that they use unsafeEff_ to initialize mvars and other variables that require IO. I'd rather not have to lean on unsafe calls if I don't have to.

Is the recommended way to do this to create a function loginWithCredentials :: IOE :> es => ApiKey -> Eff es Session, call that beforehand, and then change runAPIIO to be IOE :> es => Session -> Eff (Api : es) a -> Eff es a?

[arybczak]

This doesn't seem to work

What exactly do you mean by that? runApiIO looks fine to me (well, what you wrote won't compile since you need to pass the argument of runApiIO to reinterpret, but the idea looks good).

[ocharles]

When I do things like this, I tend to prefer:

runApiIO :: (IOE :> es) => Eff (Api : es) a -> Eff es a
runApiIO = do
    reinterpret withSession $ \env -> \case
        CallEndpoint r -> do
            session <- get
            resp <- post session r
            return $ responseBodyToJson resp
  where
    withSession m = do
      session <- loginWithCredentials
      evalState session m

I think your problem is just that you forgot to bind the argument in runApiIO:

runApiIO :: (IOE :> es) => Eff (Api : es) a -> Eff es a
runApiIO m = do
    session <- loginWithCredentials
    flip (reinterpret (evalState session)) m $ \env -> \case
        CallEndpoint r -> do
            session <- get
            resp <- post session r
            return $ responseBodyToJson resp

Something like that