Part 4: Making http requests from Elm to IHP

Communication between Elm and IHP through HTTP JSON requests.

This is part 4 of the series IHP with Elm

We have Elm set up in IHP, initialized values from flags and we are just going through the final part of making our Elm widgets fully interoperable with IHP: HTTP requests.

The architecture is pretty much in place now, so this part should be easy 🙂

Install elm/http

This tutorial only needs one additional package, the official elm/http, so let's just install that right away.

elm-json install elm/http

Support json response from /Books

The /Books endpoint currently delivers HTML by default, but we can easily make it return JSON as well.

Add this import in the top of Application/View/Books/Index.hs

module Web.View.Books.Index where
import Web.View.Prelude
import Web.JsonTypes ( bookToJSON )

Then all we have to do is to add the line json IndexView {..} = toJSON (books |> map bookToJSON) to the bottom of this instance:

instance View IndexView where
html IndexView { .. } = [hsx|
<nav>
<ol class="breadcrumb">
<li class="breadcrumb-item active">
<a href={BooksAction}>Books</a>
</li>
</ol>
</nav>
<h1>
Index
<a href={pathTo NewBookAction}
class="btn btn-primary ml-4">
+ New
</a>
</h1>
<div class="table-responsive">
<table class="table">
<thead>
<tr>
<th>Book</th>
<th></th>
<th></th>
<th></th>
</tr>
</thead>
<tbody>{forEach books renderBook}</tbody>
</table>
</div>
|]

json IndexView {..} = toJSON (books |> map bookToJSON)

It's nice that we can use the same type for the JSON API that we defined for the Elm flags.

A list of Book now maps into a list of BookJSON. In Elm, it can be decoded by the same bookDecoder we generated earlier.

The /Books endpoint will still serve you HTML by default as before. But if you set the header Accept: application/json, it will display the JSON version instead. You can test it with curl:

curl -H "Accept: application/json" http://localhost:8000/Books
# You Should get a payload like this listing all your books
# [{"publishedAt":"1858-11-17T00:00:00Z","review":null,"id":"27f0f16b-2186-4d63-9fb4-4182e63304e9","title":"Don Quixote","pageCount":864,"hasRead":false}]%

Add http action in Elm for fetching books in IHP

Let's first create the file elm/Api/Http.elm for a place to make http requests.

touch elm/Api/Http.elm

To make sure we remember to set the Accept: application/json header, let's define a wrapper around Elm's Http.request and call it ihpRequest.

The getBooksAction function will take in a string and a generic msg type taking in a Result. You could use RemoteData of course, but we'll skip it for this tutorial.


module Api.Http exposing (..)

import Api.Generated exposing (Book, bookDecoder)
import Http
import Json.Decode as D


getBooksAction :
    String
    -> (Result Http.Error (List Book) -> msg)
    -> Cmd msg
getBooksAction searchTerm msg =
    ihpRequest
        { method = "GET"
        , headers = []
        , url = "/Books?searchTerm=" ++ searchTerm
        , body = Http.emptyBody
        , expect = Http.expectJson msg (D.list bookDecoder)
        }


ihpRequest :
    { method : String
    , headers : List Http.Header
    , url : String
    , body : Http.Body
    , expect : Http.Expect msg
    }
    -> Cmd msg
ihpRequest { method, headers, url, body, expect } =
    Http.request
        { method = method
        , headers =
            [ Http.header "Accept" "application/json" ] ++ headers
        , url = url
        , body = body
        , expect = expect
        , timeout = Nothing
        , tracker = Nothing
        }

Make an ErrorView module

It can be nice to display some Http errors of various sorts in a standardised way. I like to make a view function for this.

Let's create a new Elm module at elm/ErrorView.elm

touch elm/ErrorView.elm

Let write this little snippet:


module ErrorView exposing (..)

import Http
import Html exposing (Html, pre, text)

httpErrorView : Http.Error -> Html msg
httpErrorView error =
    case error of
        Http.BadUrl info ->
            pre [] [ text "BadUrl: ", text info ]

        Http.NetworkError ->
            pre [] [ text "Network Error" ]

        Http.Timeout ->
            pre [] [ text "Timeout" ]

        Http.BadStatus code ->
            pre [] [ text ("BadStatus: " ++ String.fromInt code) ]

        Http.BadBody info ->
            pre [] [ text info ]

Add interactivity to BookSearch

Let's import the stuff we need at the top of elm/Widget/BookSearch.elm.


import Api.Generated exposing (Book)
import Api.Http exposing (getBooksAction)
import ErrorView exposing (httpErrorView)
import Html exposing (..)
import Html.Attributes exposing (href, type_)
import Html.Events exposing (onInput)
import Http

We should then make the Model inside a bit more complex, so we can turn it into a record to track both the search-term and search-result.


type alias Model =
    { searchResult : Result Http.Error (List Book)
    , searchTerm : String
    }


initialModel : Model
initialModel =
    { searchResult = Ok []
    , searchTerm = ""
    }

To update the model, we are making two messages and some update logic.

Whenever the search input changes, we will update the model and at the same time make a query to IHP through the getBooksAction function.


type Msg
    = SearchInputChanged String
    | GotSearchResult (Result Http.Error (List Book))


update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        SearchInputChanged text ->
            ( { model | searchTerm = text }, getBooksAction text GotSearchResult )

        GotSearchResult result ->
            ( { model | searchResult = result }, Cmd.none )

And the view functionality is getting some more logic.


view : Model -> Html Msg
view model =
    div []
        [ h2 [] [ text "📚 Search Books 📚" ]
        , input
            [ type_ "search"
            , onInput SearchInputChanged
            ]
            []
        , searchResultView model.searchResult
        ]


searchResultView : Result Http.Error (List Book) -> Html msg
searchResultView searchResult =
    case searchResult of
        Err error ->
            httpErrorView error

        Ok books ->
            ul [] (List.map bookItem books)


bookItem : Book -> Html msg
bookItem book =
    let
        bookLink =
            "/ShowBook?bookId=" ++ book.id
    in
    li [] [ a [ href bookLink ] [ text book.title ] ]

Make Books searchable

Only thing left it to adjust the controller to support the searchTerm parameter.

This will be done in the BooksAction on Web/Controller/Books.hs:

    action BooksAction = do
books <- query @Book |> fetch
let searchTerm :: Text = paramOrDefault "" "searchTerm"
books <-
query @Book
|> filterWhereILike (#title, "%" <> searchTerm <> "%")
|> fetch
render IndexView{..}

Wrapping up

That should be it. You now have a highly interactive book search functionality without leaving the page.

This widget has an advantage by not demanding IHP data directly from flags.

You can put it anywhere and it will just query the correct IHP endpoint.

This should be a fine starting point for making an IHP app with all the Elm widgets you will need. Good luck 😊

See the complete code on Github.


This article belongs to a 4 part series

  1. Part 1: How to setup IHP with Elm

    Get Elm with hot reloading on top of IHP, the new framework that makes Haskell a cool kid in web dev.

  2. Part 2: Initialize data from IHP to Elm through flags

    Generate types, encoders and decoders for Elm automatically in IHP.

  3. Part 3: Structure Elm into a multi-widget app for IHP

    Making the widget-equivalent of Richard Feldmans's RealWorld SPA.

  4. Part 4: Making http requests from Elm to IHP

    Communication between Elm and IHP through HTTP JSON requests.