Reading Note: Randomness in Elm


In JavaScript, we use Math.random to produce random numbers. It does not expect a seed.

Elm is compiled to JavaScript. However, elm does not use the native implementation for random number generation.


The generators are behind all the randomness in Elm.

Let’s create a generator that describes how to produce a random integer from 0 to 3.

> 0 3
Generator <function> : Random.Generator Int

-- Let try to wrap it in a function
> zeroToThreeGenerator : Random.Generator Int
| zeroToThreeGenerator =
| 0 3
Generator <function> : Random.Generator Int

Now, imagine that I would like to randomly select an item from a list. For example, you are creating a game where your hero has to select a random weapon in his weapon list. (Do you know Kite in Hunter x Hunter?)

type Weapon
    = Sword
    | Cannon
    | Scythe
    | Knife

numToWeapon : Int -> Weapon
numToWeapon num =
    case num of
        0 ->
        1 ->
        2 ->
        _ ->

weaponGenerator : Random.Generator Weapon
weaponGenerator = numToWeapon ( 0 3)

-- type of weaponGenerator
Generator <function> : Random.Generator Weapon

When we typed these codes in elm repl because they are the generators, they cannot produce the values directly.

Produce the values

There are two main approaches to generating random numbers:

TRNGs take a longer time to generate random numbers because they generate the numbers from truly random physical phenomena (atmospheric noise picked up by radio).

PRNGs take an initial value (called seed) and apply an algorithm to generate a seemingly random number.

Generating random numbers without seed

We could not produce the random values with side effects directly in elm repl. Let’s look at this random weapon application I wrote: RandomWeaponWithoutSeed

In this example, I use Random.generate to produce the values.

type Msg
    = GenerateRandomWeapon
    | NewRandomWeapon Weapon

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        GenerateRandomWeapon ->
            ( model, Random.generate NewRandomWeapon weaponGenerator )

        NewRandomWeapon weapon ->
            ( weapon, Cmd.none )
-- END:update

It relies on real-time clock behind the scenes. So, it will not be efficient randomness if you run this command multiple times consecutively. It is a risk of getting the same value from running Random.generate multiple times consecutively.

Generating random numbers with seed

Let’s create a seed:

> Random.initialSeed
<function> : Int -> Random.Seed

> seed0 = Random.initialSeed 132132
Seed 2090120966 1013904223 : Random.Seed

elm/random provides us Random.step. Each time we call Random.step we need to provide a generator and a seed. This will produce a tuple containing a random value and a new seed to use if we want to run other generators later.

> Random.step
<function> : Random.Generator a -> Random.Seed -> ( a, Random.Seed )

Now, we could use the Random.step function to generate a random value.

> Random.step weaponGenerator seed0
(Knife,Seed 2961089197 1013904223)
    : ( Weapon, Random.Seed )

Let’s add a seedGenerator to our previous application:

seedGenerator : Random.Generator Random.Seed
seedGenerator = Random.minInt Random.maxInt
        |> (Random.initialSeed)

Then, we add seed into the model. In the beginning, there is no seed, so I use the Maybe Random.Seed type for this one.

type alias Model =
    { seed : Maybe Random.Seed
    , weapon : Weapon

init : () -> ( Model, Cmd Msg )
init _ =
    ( { seed = Nothing
    , weapon = Sword
    , Random.generate UpdateSeed seedGenerator

Because we define the seed type as Maybe Random.Seed. So, we need to use and Maybe.withDefault in the update function to step the value.

type Msg
    = UpdateSeed Random.Seed
    | PutRandomWeapon

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        UpdateSeed seed ->
            ( { model | seed = Just seed }
            , Cmd.none
        PutRandomWeapon ->
                newModel : Model
                newModel =
                        |> (Random.step weaponGenerator)
                            (\(randWeapon, seed) ->
                                { model | seed = Just seed, weapon = randWeapon }
                        |> Maybe.withDefault model
            (newModel, Cmd.none)

Let’s check this new approach:

This will resolve the problem when using Random.generate to produce the random values. For the full source code please check this: RandomWeaponWithSeed

In short