Elm Json request not working? - json

Question

In Elm, you can use the Json.decode and Http package to request json data. My attempt was to work out a periodic lookup for emails ( from this url ). The timer operation does work (i tried it with with a simple counter).
I have used this example and this SO question as reference.
Now the types:
type alias Email = { title: String, ... }
type Action =
NoAction
| TickCounter -- TODO rem
| AddEmails (List Email)
Then the main + state + actions ...
main: Signal Html
main = Signal.map (view actions.address) state
state: Signal Model
state = Signal.foldp update makeEmptyModel input
-- handle inputs (merging signals)
input : Signal Action
input =
Signal.mergeMany
[ actions.signal
-- , other actions
, Signal.map checkForNewMails (Time.every (Time.minute / 6.0) ) -- TODO precise timer (quick test)
]
actions: Signal.Mailbox Action
actions = Signal.mailbox NoAction
update: Action -> Model -> Model
update action model =
case action of
NoAction -> model
TickCounter -> { model | count = model.count + 1 }
AddEmails newMails -> { model | emails = newMails }
checkForNewMails: Time -> Action
checkForNewMails t =
let mails = startGettingEmailData
in TickCounter -- TODO replace with AddEmails using mails
The TickCounter is an Action, which i have used to test my timing operation. But the problem is startGettingEmailData. It uses the next snippet, but it doesn't fire any JSON request (i have checked it through the console). Once that has been resolved, i can convert mails to an action so that i can add the emails in the model.
-- url to json
jsonUrl = "https://dl.dropboxusercontent.com/u/14070433/temp.json"
-- get emails
startGettingEmailData: Task Http.Error (List Email)
startGettingEmailData = Http.get emailJsonDecoder jsonUrl
emailJsonDecoder: Json.Decoder (List Email)
emailJsonDecoder =
let makeEmail = Json.object4
(\ti fr da bo -> makeNewEmail -1 ti fr da bo )
("title" := Json.string)
("from" := Json.string)
("date" := Json.string)
("body" := Json.string)
in
"emails" := Json.list makeEmail
Is there a problem with my code ? If not, then is there a way to check the Http.Error content ? (Maybe the problem lies not in my code, but in the network, but i can access the dropbox file by browser...)

Answer 1

Your code for checkForNewMails doesn't actually do anything with mails, so it never gets invoked. The let statement doesn't make any calls, it only lets you define one-off functions within the body of a larger function. Since the in portion merely returns TickCounter, then it means this function only ever returns TickCounter and does nothing else.
Furthermore, startGettingEmailData is returning a Task, which means it only gets invoked when in a port. You can't use it in a function that only returns an Action, because it would never get run.
You'll instead want to write a port that triggers on a timer, then creates a Task which polls your url, then maps the response of that GET request to an Action, calling the actions mailbox. You can use Task.onError to write a simple error handler, which could forward an error message to your view by creating an Error String constructor on your Action type.
Here's an example:
getEmailData _ =
let
request =
Http.get emailJsonDecoder jsonUrl
|> Task.map AddEmails
in
request
`Task.onError` (\err -> Task.succeed (Error (toString err)))
`Task.andThen` (\action -> Signal.send actions.address action)
port getEmails : Signal (Task a ())
port getEmails =
Signal.map getEmailData (Time.every (Time.minute / 6.0) )
The above code will cause the URL to be retrieved, parsed, then, on success, it will trigger you actions mailbox and cause an update of the view. If there is an error, it will send the new action of Error with a message, which you should handle in the view. (You'll have to add Error String to the Action union type and handle it in the model, update, and view functions).