Offer HandlerM built on Aff (Asynchronous Eff)

[dvdsgl]

It would be really cool if HandlerM were a transformer on Aff (https://github.com/slamdata/purescript-aff), or to offer AsyncHandlerM so we could more easily write Handlers with lots of callbacks:

asyncHandler :: AsyncHandler
asyncHandler = do
  info <- liftAff $ JQuery.get "/info.json"
  users <- liftAff $ Database.query "SELECT * FROM USERS"
  sendJson { info: info, users: users }

app = do
  get "/foo" asyncHandler

Doing something like this with capture can get ugly.

[nkly]

Haven't heard of Aff, but it's definitely better than capture. Thanks!

[michaelficarra]

👍, I was just trying to use purescript-express with redis, and the node redis library is entirely asynchronous (and my wrapper for it is built on Aff), so I have no way to write a handler that can respond based on my interaction with redis. 😢

edit: I ended up doing something like this for now:

liftCallback :: forall a eff. ((a -> Eff eff Unit) -> Eff eff Unit) -> (a -> HandlerM Unit) -> HandlerM Unit
liftCallback f next = do
  cb <- capture next
  liftEff (f cb)

foreign import insert :: forall eff. String -> String -> Database -> (Boolean -> DBEff eff Unit) -> DBEff eff Unit

... do
  ...
  liftCallback (insert key value db) $ \success ->
    ...

but Aff support would be so much better.

[kika]

How should I handle errors in Aff? I have a code like this:

upstreamAuth (AppStateRecord state) token = do
  let opts = state.upstream 
           <> method := "POST"
           <> path := "/_session"
  reqdata <- liftEff $ Buffer.fromString token UTF8
  res     <- SR.simpleRequest opts reqdata
  pure res.body

checkAuth::forall e. AppState -> Token -> Aff (ref::REF, http::HTTP|e) Cookie
checkAuth stateref token = do
  state <- liftEff $ readRef stateref
  case getSession token state of
    Nothing      -> upstreamAuth state token
    Just session -> pure session.cookie

authHandler :: forall e.  AppState -> Handler (console::CONSOLE, http::HTTP, ref::REF | e)
authHandler statedata = do
  token <- getRequestHeader "Authorization"
  case token of
    Nothing   -> nextThrow $ error "NOAUTH"
    Just user -> case parseUser user of
      "" -> nextThrow $ error "NOAUTH"
      u  -> do
        liftEff $ log ("API token: " <> u)
        cookie <- liftAff $ checkAuth statedata u
        liftEff $ log ("Cookie: " <> cookie)
        next

And the errors which happen in upstreamAuth are propagated to runtime exceptions without any possibility to handle them. I suppose that's where I should use runAff, and put next and nextThrow as success and failure callbacks but it requires handlers which return Eff, not HandlerM, as runAff does. Obviously, I'm missing something.
How do you handle errors with Aff "subrequests" from Handlers?

[Risto-Stevcev]

@kika you can use attempt from the Aff lib

[kika]

@Risto-Stevcev right, that's what I've started with, but then it introduces another case ... of Left -> ... Right -> ... and I'm trying to join the Destroy All Ifs sect :-)