Sequence Http.get in Elm - elm

Question

Below I have a button that attempts to load remote content ...
import Post exposing (Post)
import Html exposing (..)
import Html.Events exposing (..)
import Http
import Json.Decode as Decode
type alias Model =
{ posts : List Post }
type Msg
= Search String
| PostsReceived (Result Http.Error (List Post))
update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
case msg of
Search s ->
let
cmd =
(Decode.list Post.decode)
|> Http.get ("/posts?author=" ++ s)
|> Http.send PostsReceived
in
( model, cmd )
PostsReceived (Ok posts) ->
{ model | posts = posts }
! []
PostsReceived (Err error) ->
( model, Cmd.none )
view : Model -> Html Msg
view model =
button
[ onClick (Search "amelia") ]
[ text "Read posts by Amelia" ]
This is a valid Elm program, only there's one little problem: The API doesn't allow me to search by string. This is not allowed
/posts?author=amelia => Malformed Request Error
However, this is allowed
/posts?author=2 => [ {...}, {...}, ... ]
So I must first fetch an author to get his/her id, and then I can fetch posts using the author's id...
/author?name=amelia => { id: 2, name: "amelia", ... }
/posts?author=2
How can I sequence one request after the next? Ideally I'd like to cache the authors somewhere in the model so we're only requesting ones that we haven't seen before.

Answer 1

You can use Task.andThen to chain two tasks together. Assuming that the /posts response includes the author ID, you can then add that author ID into you model when you handle the response.
Search s ->
let
getAuthor =
Author.decode
|> Http.get ("/author?name=" ++ s)
|> Http.toTask
getPosts author =
(Decode.list Post.decode)
|> Http.get ("/posts?author=" ++ author.id)
|> Http.toTask
cmd =
getAuthor
|> Task.andThen getPosts
|> Task.attempt PostsReceived
in
( model, cmd )
I've got this compiling at https://ellie-app.com/DBJc6Kn3G6a1 if that helps

Answer 2

You can chain together tasks using Task.andThen. You'll first have to convert the web requests to tasks using Http.toTask:
postsByAuthorName : String -> Cmd Msg
postsByAuthorName name =
Http.get ("/author?name=" ++ name) (Decode.field "id" Decode.int)
|> Http.toTask
|> Task.andThen (\id ->
Http.get ("/posts?author=" ++ toString id) (Decode.list decodePost)
|> Http.toTask)
|> Task.attempt PostsReceived

Answer 3

A a dictionary and a couple more Msg options should do it.
You'll have to write the decoder for the Author response, but other than that this should work
type alias Model =
{ posts : List Post
, authors : Dict String Int }
type Msg
= Search String
| SearchAuthor String
| AuthorReceived (Result Http.Error Int String)
| PostsReceived (Result Http.Error (List Post))
update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
case msg of
Search author ->
case (Dict.get author model.authors) of
Nothing ->
let
cmd =
(Decode.list Post.decode)
|> Http.get ("/author?name=" ++ author)
|> Http.send AuthorReceived
in
(model,cmd)
Just num ->
let
cmd =
(Decode.list Author.decode)
|> Http.get ("/posts?author=" ++ num)
|> Http.send PostsReceived
in
( model, cmd )
AuthorReceived (Ok number name) ->
let
updatedAuthors = Dict.inster name number model.authors
cmd =
(Decode.list Post.decode)
|> Http.get ("/posts?author=" ++ number)
|> Http.send PostsReceived
in
{model | authors = updatedAuthors } ! [cmd]
AuthorReceived (Err error) ->
(mode, Cmd.none )
PostsReceived (Ok posts) ->
{ model | posts = posts }
! []
PostsReceived (Err error) ->
( model, Cmd.none )
view : Model -> Html Msg
view model =
button
[ onClick (Search "amelia") ]
[ text "Read posts by Amelia" ]