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Simple JSON REST API library for making requests and handling responses
version 1.0.2
license BSD3
native-modules False
elm-version 0.18.0 <= v < 0.19.0
Tag 2.0.0
Committed At 2018-02-04 21:17:50 UTC
krisajenkins/remotedata 4.3.3 <= v < 5.0.0 4.5.0
elm-lang/http 1.0.0 <= v < 2.0.0 1.0.0
elm-lang/html 2.0.0 <= v < 3.0.0 2.0.0
elm-lang/core 5.1.1 <= v < 6.0.0 5.1.1
elm-community/list-extra 7.0.1 <= v < 8.0.0 7.1.0
NoRedInk/elm-decode-pipeline 3.0.0 <= v < 4.0.0 3.0.1



An Elm application often needs to implement CRUD operations through a JSON REST API. The HTTP requests and list manipulation involved in this communication can tend to be very similar, and writing the resulting boilerplate code, while not overly difficult, can still be time-consuming.

This package makes the most common cases easier to implement by providing two modules of simple helper functions:

  • Request - for constructing URLs, encoding resources as JSON, and sending HTTP requests.
  • Response - for using a response to update a List of resources.


This package makes several assumptions about communication with your REST API:

  • The JSON body representing a resource, sent in the request and received in the response, are always structurally the same, and can be successfully decoded with the same Decoder.
  • A collection of resources in your client application is represented as a RemoteData.WebData (List resource) (read more about WebData here).
  • Resources are managed in the following way:
    • A list of resources is fetched with a GET to the base URL, and it replaces the current list if successfully returned.
    • A resource is created with a POST to the base URL, and it is added to the list if successfully returned.
    • A resource is updated with either a PUT or PATCH to a resource-specific URL, and it replaces an existing resource in the list if successfully returned.
    • A resource is deleted with a DELETE to a resource-specific URL, and it removes an existing resource in the List if successfully returned.
  • Error responses should not modify the list of resources.
  • The Http.Error type is sufficient to represent error states.


Define a collection of resources in your Model as a WebData (List resource):

import RemoteData exposing (WebData)

type alias Model =
    { articles : WebData (List Article)

Define a Config resource urlBaseData urlSuffixData for the API you're going to use, including:

  • A Decoder for your resource type
  • An encode function for your resource type
  • A toUrlBase for creating the URL to be requested, using your urlBaseData type
  • A toUrlSuffix for creating the URL suffix (used for PUT/PATCH and DELETE requests for specific resources) from your urlSuffixData type (usually just the id type of your resource)
  • A List of options—currently this package supports options for:
    • Adding request headers
    • Using the PATCH verb instead of the default PUT for updating a resource
import JsonRestApi.Request as Request
import JsonRestApi.Response as Response

articleApi : Request.Config Article () String
articleApi =
        { decoder = articleDecoder
        , encoder = encodeArticle
        , toUrlBase = (\_ -> "")
        , toUrlSuffix = (\id -> "/" ++ id)
        , options = []

Make HTTP requests by calling the Request helper functions in your application's update, passing:

  • The Config
  • The urlBaseData, which can simply be a () if your base URL is static
  • When necessary, a resource and urlSuffixData (data for creating the URL suffix)
  • A Msg to be sent when the response arrives
update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        GetAllRequest ->
            ( model, Request.getAll articleApi () GetAllResponse )

        CreateRequest article ->
            ( model, Request.create articleApi article () CreateResponse )

        UpdateRequest article ->
            ( model, Request.update articleApi article () UpdateResponse )

        DeleteRequest article ->
            ( model, Request.delete articleApi article () DeleteResponse )

Update the List of resources in your Model by calling the Response helper functions in the response Msgs, passing:

  • The Result
  • The collection of resources
  • When updating or deleting, an equality test function for comparing two resources (i.e. by id)
        GetAllResponse result ->
            ( { model | articles = Response.handleGetIndexResponse result model.articles }, Cmd.none )

        CreateResponse result ->
            ( { model | articles = Response.handleCreateResponse result model.articles }, Cmd.none )

        UpdateResponse result ->
            ( { model | articles = Response.handleUpdateResponse result isSameArticle model.articles }, Cmd.none )

        DeleteResponse result ->
            ( { model | articles = Response.handleDeleteResponse result isSameArticle model.articles }, Cmd.none )