Continued from part one, Recursive HTTP Requests with Elm.

In my last post, I described my first pass at building a library to fetch data from a paginated JSON REST API. It worked, but it wasn’t too clean. In particular, the handling of the multiple pages and concatenation of results was left up to the calling code. Ideally, both of these concerns should be handled by the library, letting the application focus on working with a full result set. Using Elm’s Tasks, we can achieve exactly that!

What’s a Task?

A Task is a data structure in Elm which represents an asynchronous operation that may fail, which can be mapped and chained. What this means is, we can create an action, transform it, and chain it with additional actions, building up a complex series of things to do into a single Task , which we can then package up into a Cmd and hand to the Elm runtime to perform. You can think of it like building up a Future or Promise, setting up a sort of callback chain of mutations and follow-up actions to be taken. The Elm runtime will work its way through the chain and hand your application back the result in the form of a Msg .

So, tasks sound great!

Moving to Tasks

Just to get things rolling, let’s quit using Http.send , and instead prepare a simple toTask function leveraging the very handy Http.toTask . This’ll give us a place to start building up some more complex behavior.

send : ( Result Http . Error ( Response a) -> msg) -> Request a -> Cmd msg send resultToMessage request = toTask request |> Task . attempt resultToMessage toTask : Request a -> Task Http . Error ( Response a) toTask = httpRequest >> Http . toTask

Shifting the recursion

Now, for the fun bit. We want, when a request completes, to inspect the result. If the task failed, we do nothing. If it succeeded, we move on to checking the response. If we have a Complete response, we’re done. If we do not, we want to build another task for the next request, and start a new iteration on that.

All that needs to be done here is to chain our response handling using Task.andThen , and either recurse to continue the chain with the next Task , or wrap up the final results with Task.succeed !

recurse : Task Http . Error ( Response a) -> Task Http . Error ( Response a) recurse = Task . andThen ( \ response -> case response of Partial request _ -> httpRequest request |> Http . toTask |> recurse Complete _ -> Task . succeed response )

That wasn’t so bad. The function recursion almost seems like cheating: I’m able to build up a whole chain of requests based on the results without actually having the results yet! The Task lets us define a complete plan for what to do with the results, using what we know about the data structures flowing through to make decisions and tack on additional things to do.

Accumulating results

There’s just one thing left to do: we’re not accumulating results yet. We’re just handing off the results of the final request, which isn’t too helpful to the caller. We’re also still returning our Response structure, which is no longer necessary, since we’re not bothering with returning incomplete requests anymore.

Cleaning up the types is pretty easy. It’s just a matter of switching out some instances of Response a with List a in our type declarations…

send : ( Result Http . Error ( List a) -> msg) -> Request a -> Cmd msg toTask : Request a -> Task Http . Error ( List a) recurse : Task Http . Error ( Response a) -> Task Http . Error ( List a)

…then changing our Complete case to return the actual items:

Complete xs -> Task . succeed xs

The final step, then, is to accumulate the results. Turns out this is super easy. We already have an update function that combines two responses, so we can map that over our next request task so that it incorporates the previous request’s results!

Partial request _ -> httpRequest request |> Http . toTask |> Task . map (update response) |> recurse

Tidying up

Things are tied up pretty neatly, now! Calling code no longer needs to care whether the JSON endpoints its calling paginate their results, they’ll receive everything they asked for as though it were a single request. Implementation details like the Response structure, update method, and httpRequest no longer need to be exposed. toTask can be exposed now as a convenience to anyone who wants to perform further chaining on their calls.

Now that there’s a cleaner interface to the module, the example app is looking a lot cleaner now, too:

module Example exposing (..) import Html exposing ( Html ) import Http import Json.Decode exposing (field, string) import Paginated type alias Model = { repositories : Maybe ( List String ) } type Msg = GotRepositories ( Result Http . Error ( List String )) main : Program Never Model Msg main = Html . program { init = init , update = update , view = view , subscriptions = \ _ -> Sub . none } init : ( Model , Cmd Msg ) init = ( { repositories = Nothing } , getRepositories ) update : Msg -> Model -> ( Model , Cmd Msg ) update msg model = case msg of GotRepositories result -> ( { model | repositories = Result . toMaybe result } , Cmd . none ) view : Model -> Html Msg view model = case model . repositories of Nothing -> Html . div [] [ Html . text "Loading" ] Just repos -> Html . ul [] <| List . map ( \ x -> Html . li [] [ Html . text x ]) repos getRepositories : Cmd Msg getRepositories = Paginated . send GotRepositories <| Paginated . get "http://git.phoenixinquis.net/api/v4/projects?per_page=5" (field "name" string)