Elm:
Generating random numbers

How to:

Elm’s pure functional nature means that you can’t generate random numbers directly as you might in imperative languages. Instead, you use the Random module in conjunction with commands. Here’s a basic example that generates a random integer between 1 and 100.

First, install the Random module with elm install elm/random. Then import it into your Elm file, along with the necessary HTML and event modules, like so:

src/Main.elm

module Main exposing (..)

import Browser
import Html exposing (Html, button, text, div)
import Html.Events exposing (onClick)
import Random

For this to be a self-contained example, you can add this boilerplate:

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

init : () -> (Model, Cmd Msg)
init _ =
  (Model 0, Cmd.none)

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

Next, define a command to generate a random number. This involves setting up a Msg type to handle the random number once it’s generated, a Model to store it, and an update function to tie it all together.

type Msg
    = Generate
    | NewRandom Int

type alias Model = { randomNumber : Int }

update : Msg -> Model -> (Model, Cmd Msg)
update msg model =
    case msg of
        Generate ->
            ( model, Random.generate NewRandom (Random.int 1 100) )

        NewRandom number ->
            ( { model | randomNumber = number }, Cmd.none )

To trigger a number generation, you would send a Generate message, for instance, through a button in your view:

view : Model -> Html Msg
view model =
    div []
        [ div [] [ text ("Random Number: " ++ String.fromInt model.randomNumber) ]
        , button [ onClick Generate ] [ text "Generate" ]
        ]

When you click the “Generate” button, a random number between 1 and 100 will be displayed.

This simplistic approach can be adapted and expanded, leveraging other functions in the Random module to produce random floats, lists, or even complex data structures based on custom types, providing a vast playground for adding unpredictability to your Elm applications.

The Elm Guide goes into much more detail. It also has an example of rolling a six-sided die.