Random is not pure in Elm

Elm requires that functions be pure, that is, the same arguments should produce the same outputs every time. Random necessarily injects some uncertainty into what the outputs that way and Elm has decided to handle random differently than in other languages.

First, install the random package:

elm install elm/random

The Random package allows you to create numbers in a couple of different ways, but the most idiomatic is to create a generator:

generator = (Random.int 1 10)

And then create a message that will let the Elm runtime know to produce a random number with the parameters defined by the generator.

type Msg = ConsumeRandomValue

msg = Random.generate ConsumeRandomValue generator

This message can then be placed into the (model, msg) tuple that is returned from the update function. The update function is then called to respond to the message, using the message type to wrap the random value that has been produced.

import Random

type Msg = ProduceRandomValue | ConsumeRandomValue Int

update msg model =
    case msg of
        ProduceRandomValue -> 
            (model, Random.generate ConsumeRandomValue (Random.int 1 10))
        ConsumeRandomValue randomValue ->
            ({model | rValue = randomValue}, Cmd.none)

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