Generating unique random number

Question

I have an application that I want to generate a list of 5 unique numbers on init three times after reading a json. So basically I want to get something like [31, 59, 62, 72, 2, 16, 2, 38, 94, 15, 55, 46, 83, 2, 10]. My challenge is that I am new to functional programming and elm and I am a bit lost. So I understand that Random.generate takes a msg and a generator and returns a cmd message and it is mostly used in the update function but this is not where I need as it is a helper function and doesn't need to communicate with the client. I think it can be used in init but I don't know how. Again my function is recursive and I don't know how to apply this logic with Random.generate recursively.

I understand my code will not work and I have tried it because Random.int does not generate a random number but a type of generate but still I don't know how to apply this to get what I want.

recursion : Int -> List a -> List number
recursion a b =
  if List.length b > 5
      then b
  else 
      let 
          rand = Random.int 0 a
      in
          if(List.member rand b)
              then recursion a b
          else
              recursion a (rand :: b)

can be called with:

recursion 50 []

I want to generate a list/array of 5 unique random 3 times.

Answer 1

Great question. There are two parts here: - generating random numbers, and - wiring this all up

You will need the Random library for the former, and a little inspection will lead you to something like

get15Randoms = Random.generate OnRandomNumbers <| Random.list 5 (int 0 100)

This has type Cmd Msg - it's an async operation that will return on a Msg.

Wiring it up will be a series of stages. You refer to doing it in 'init' but that's not how Elm is going to work for you here. In the init function you can start off your json request. Then you'll need something like

init = ( initModel
       , Http.get {url =..., expect = Http.expectJson OnJson yourDecoder} 
       )

update msg model = 
    case msg of 
        OnJson (Ok data) ->
            -- attach json
            ( {model | data = data }, get15Randoms )
        OnRandomNumbers ints ->
            ( { model | ints = ints }, Cmd.none )

In other words, once the json comes back you can attach that, and use that iteration of the update to launch your random number request, and then catch the result in a subsequent iteration