Random


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So far we have only seen commands to make HTTP requests, but we can command other things as well, like generating random values! So we are going to make an app that rolls dice, producing a random number between 1 and 6.

We need the elm/random package for this. The Random module in particular. Let’s start by just looking at all the code:

import Browser
import Html exposing (..)
import Html.Events exposing (..)
import Random



-- MAIN


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



-- MODEL


type alias Model =
  { dieFace : Int
  }


init : () -> (Model, Cmd Msg)
init _ =
  ( Model 1
  , Cmd.none
  )



-- UPDATE


type Msg
  = Roll
  | NewFace Int


update : Msg -> Model -> (Model, Cmd Msg)
update msg model =
  case msg of
    Roll ->
      ( model
      , Random.generate NewFace (Random.int 1 6)
      )

    NewFace newFace ->
      ( Model newFace
      , Cmd.none
      )



-- SUBSCRIPTIONS


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



-- VIEW


view : Model -> Html Msg
view model =
  div []
    [ h1 [] [ text (String.fromInt model.dieFace) ]
    , button [ onClick Roll ] [ text "Roll" ]
    ]

The new thing here is command issued in the update function:

Random.generate NewFace (Random.int 1 6)

Generating random values works a bit different than in languages like JavaScript, Python, Java, etc. So let’s see how it works in Elm!

Random Generators

The core idea is that we have random Generator that describes how to generate a random value. For example:

import Random

probability : Random.Generator Float
probability =
  Random.float 0 1

roll : Random.Generator Int
roll =
  Random.int 1 6

usuallyTrue : Random.Generator Bool
usuallyTrue =
  Random.weighted (80, True) [ (20, False) ]

So here we have three random generators. The roll generator is saying it will produce an Int, and more specifically, it will produce an integer between 1 and 6 inclusive. Likewise, the usuallyTrue generator is saying it will produce a Bool, and more specifically, it will be true 80% of the time.

The point is that we are not actually generating the values yet. We are just describing how to generate them. From there you use the Random.generate to turn it into a command:

generate : (a -> msg) -> Generator a -> Cmd msg

When the command is performed, the Generator produces some value, and then that gets turned into a message for your update function. So in our example, the Generator produces a value between 1 and 6, and then it gets turned into a message like NewFace 1 or NewFace 4. That is all we need to know to get our random dice rolls, but generators can do quite a bit more!

Combining Generators

Once we have some simple generators like probability and usuallyTrue, we can start snapping them together with functions like map3. Imagine we want to make a simple slot machine. We could create a generator like this:

import Random

type Symbol = Cherry | Seven | Bar | Grapes

symbol : Random.Generator Symbol
symbol =
  Random.uniform Cherry [ Seven, Bar, Grapes ]

type alias Spin =
  { one : Symbol
  , two : Symbol
  , three : Symbol
  }

spin : Random.Generator Spin
spin =
  Random.map3 Spin symbol symbol symbol

We first create Symbol to describe the pictures that can appear on the slot machine. We then create a random generator that generates each symbol with equal probability.

From there we use map3 to combine them into a new spin generator. It says to generate three symbols and then put them together into a Spin.

The point here is that from small building blocks, we can create a Generator that describes pretty complex behavior. And then from our application, we just have to say something like Random.generate NewSpin spin to get the next random value.

Exercises: Here are a few ideas to make the example code on this page a bit more interesting!

  • Instead of showing a number, show the die face as an image.
  • Instead of showing an image of a die face, use elm/svg to draw it yourself.
  • Create a weighted die with Random.weighted.
  • Add a second die and have them both roll at the same time.
  • Have the dice flip around randomly before they settle on a final value.

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