In this tutorial, we will walk through building a simple Elm application that fetches random quotes from an API. We will break down the code into logical sections and explain each part in detail. By the end of this tutorial, you will have a good understanding of how to make HTTP requests and handle the responses in Elm.

Introduction

Elm is a functional language for building reliable web applications. This tutorial will guide you through creating an application that fetches random quotes and displays them. You can find more information about handling JSON and HTTP in Elm in the official guide.

The Complete Code

Here’s the full code for the application. We’ll break it down into sections and explain each part.

  -- Press a button to send a GET request for random quotes.
--
-- Read how it works:
--   https://guide.elm-lang.org/effects/json.html
--

import Browser
import Html exposing (..)
import Html.Attributes exposing (style)
import Html.Events exposing (..)
import Http
import Json.Decode exposing (Decoder, map4, field, int, string)

Main Function

The main function is the entry point of our Elm application. It initializes the application with Browser.element and sets up the init, update, subscriptions, and view functions.

  main =
  Browser.element
    { init = init
    , update = update
    , subscriptions = subscriptions
    , view = view
    }

Model

The Model represents the state of our application. We define a custom type Model with three possible states: Failure, Loading, and Success with a Quote.

  type Model
  = Failure
  | Loading
  | Success Quote

type alias Quote =
  { quote : String
  , source : String
  , author : String
  , year : Int
  }

Initialization

The init function initializes the application state. We start with the Loading state and immediately fetch a random quote.

  init : () -> (Model, Cmd Msg)
init _ =
  (Loading, getRandomQuote)

Update

The update function handles messages and updates the model accordingly. We define two message types: MorePlease and GotQuote.

  type Msg
  = MorePlease
  | GotQuote (Result Http.Error Quote)

update : Msg -> Model -> (Model, Cmd Msg)
update msg model =
  case msg of
    MorePlease ->
      (Loading, getRandomQuote)

    GotQuote result ->
      case result of
        Ok quote ->
          (Success quote, Cmd.none)

        Err _ ->
          (Failure, Cmd.none)

Subscriptions

The subscriptions function manages subscriptions to external events. In this application, we don’t have any subscriptions.

  subscriptions : Model -> Sub Msg
subscriptions model =
  Sub.none

View

The view function renders the HTML based on the current model state. It uses a helper function viewQuote to display the quote or relevant messages.

  view : Model -> Html Msg
view model =
  div []
    [ h2 [] [ text "Random Quotes" ]
    , viewQuote model
    ]

viewQuote : Model -> Html Msg
viewQuote model =
  case model of
    Failure ->
      div []
        [ text "I could not load a random quote for some reason. "
        , button [ onClick MorePlease ] [ text "Try Again!" ]
        ]

    Loading ->
      text "Loading..."

    Success quote ->
      div []
        [ button [ onClick MorePlease, style "display" "block" ] [ text "More Please!" ]
        , blockquote [] [ text quote.quote ]
        , p [ style "text-align" "right" ]
            [ text "— "
            , cite [] [ text quote.source ]
            , text (" by " ++ quote.author ++ " (" ++ String.fromInt quote.year ++ ")")
            ]
        ]

HTTP Request

The getRandomQuote function sends an HTTP GET request to fetch a random quote. It uses a decoder to parse the JSON response into a Quote.

  getRandomQuote : Cmd Msg
getRandomQuote =
  Http.get
    { url = "https://elm-lang.org/api/random-quotes"
    , expect = Http.expectJson GotQuote quoteDecoder
    }

quoteDecoder : Decoder Quote
quoteDecoder =
  map4 Quote
    (field "quote" string)
    (field "source" string)
    (field "author" string)
    (field "year" int)

Conclusion

This tutorial covered the essential parts of an Elm application that fetches and displays random quotes. You learned about the main structure of an Elm app, how to define a model, handle updates, manage subscriptions, and make HTTP requests.

Feel free to modify the code and experiment with different features to deepen your understanding of Elm. Happy coding!

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Last updated 17 Aug 2024, 12:31 +0200 . history