Elm: User Input Part 1

Lets spike some user input in Elm

So came back to Elm. Enjoying it, something clicked. Probably the moment i started writing currying functions in c#.

For those not in the know. c# doesn’t support currying, i ended up returning lambdas that closed over the parameters of the function called CreateSomeFunction. It felt really broken …

So too the hint, clearly a good time to go back to something functional and so back to Elm. Once again playing with the kool aid kids.

So Started to write a game (because a game is always a really smooth learning curve for a new language - I also suck at web dev …)

So this will be fun.

Lets set up the project

git init

Then head over to github and grab an elm .gitignore file

Next install the base elm package

elm package install

Then update the sources folder

{
  "source-directories": [
        "./src"
    ],
}

Now we are going to need a keyboard library. If you google ‘elm language’ you will end up at this page. Install it like this:

elm-package install elm-lang/keyboard

The code

Below is a big blurt of code:

Highlights

• import Set: Need a de duplicated non ordered collection of keys
• Subscription: This is what kicked my ass. A subscription is of type Sub Msg yet we need to sign up to both key up and key down events turns out you can create a batch.

The code for this is: (and no i dont have the foggiest how it works)

{-| When you need to subscribe to multiple things, you can create a `batch` of
subscriptions.
**Note:** `Sub.none` and `Sub.batch [ Sub.none, Sub.none ]` and
`Sub.batch []` all do the same thing.
-}
batch : List (Sub msg) -> Sub msg
batch =
Elm.Kernel.Platform.batch

The code for handling user input is below. Put it in ./src/ and elm-reactor

import Html exposing (..)
import Keyboard exposing (KeyCode)
import Set exposing (..)

main : Program Never Model Msg
main =
    Html.program
        { init = init
        , view = view
        , update = update
        , subscriptions = subscriptions
    }

type alias Model =
    { pressedKeys : Set KeyCode
    }

type Msg
    = KeyChange Bool KeyCode

update : Msg -> Model -> (Model, Cmd Msg)
update msg model =
    case msg of
        KeyChange pressed keyCode  ->
            (handleKeyChange pressed keyCode model, Cmd.none)

handleKeyChange : Bool -> KeyCode -> Model -> Model
handleKeyChange pressed keyCode model =
  let
    fn = if pressed then Set.insert else Set.remove
    pressedKeys = fn keyCode model.pressedKeys
  in
    { model | pressedKeys = pressedKeys }


view : Model -> Html Msg
view model =
    div []
        [ (Set.isEmpty model.pressedKeys)
          |> toString
          |> text
        ]

init : (Model, Cmd Msg)
init =
    (Model Set.empty  , Cmd.none)

subscriptions : Model -> Sub Msg
subscriptions model =
    let
        keys = [ Keyboard.downs (KeyChange True)
               , Keyboard.ups (KeyChange False)
               ]
    in
    keys |> Sub.batch