Convert Random.int to String for use in Url.Builder

[thesamuraiwho]

Hello, I’m trying to build in a randomize button where each time a user clicks on the button, the user gets a new image from an api. The api works as https://example.com/resource/123, where I can change 123 to some number in a range. I was trying to use Url.Builder to create the url for Http.get and Random.int in a “roll” function where the function just returns a Random.Generator Int and the body is just Random.int 1 10. The problem I have is that the second argument of Url.Builder's crossOrigin is a list of String but “roll” returns a Random.Generator Int. I tried to use String.fromInt but String.fromInt needs an Int not a Random.Generator Int. The Http.get { url = crossOrigin "https://example.com/resource/" [roll] [], expect = ...}. Any help would be appreciated. Thanks!

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

gotRandomResource : Cmd Msg
gotRandomResource =
  Http.get
    { url = crossOrigin "https://example.com/resource/" [roll] []
    , expect = Http.expectJson GotImg dataDecoder
    }

[Augustin82]

Hi!
To convert “wrapped” values (such as Maybe a or Random a), you usually want to use that module’s map function. In your case, you just need to use Random.map String.fromInt roll.

This could be a function:

randomIntToString : Random Int -> Random String
randomIntToString randomInt =
    Random.map String.fromInt randomInt

[thesamuraiwho]

Thanks for the reply Augustin82. I seem to misunderstand Random. The following changes I made are as follows:

randomIntToString : Random.Generator Int -> Random.Generator String
randomIntToString randomInt =
    Random.map String.fromInt randomInt

gotRandomResource : Cmd Msg
gotRandomResource =
  Http.get
    { url = crossOrigin "https://example.com/resource/" [randomIntToString roll] []
    , expect = Http.expectJson GotImg dataDecoder
    }

However, result in a Type Mismatch where I have List (Random.Generator String) but crossOrigin needs List String. With random, we are supposed to call Random.generate on the random function in update right?

update : Msg -> Model -> (Model, Cmd Msg)
update msg model =
  case msg of
    MorePlease ->
      (Loading, gotRandomResource)  -- Should I call `Random.generate` here?

    GotImg result ->
      case result of
        Ok url ->
          (Success url, Cmd.none)

        Err _ ->
          (Failure, Cmd.none)

I’m very new to Elm and wanted to try something I thought was relatively simple. It appears I don’t quite understand how Random works. Thank you again for the help!

[wolfadex]

You can either use Random.generate in update like you mentioned, or you can use Random.step to do it all in 1 update branch like so

let
    ( randStr, nextSeed ) =
        Random.step (randomIntToString roll) model.seed
in
( { model | seed = nextSeed }
, Http.get
    { url = crossOrigin "https://example.com/resource/" [randStr] []
    , expect = Http.expectJson GotImg dataDecoder
    }
)

This would require updating you Model and init to include a Random.Seed.

[Augustin82]

Apologies, my answer was partial and not really helpful! Thankfully, @wolfadex did a better job than I did!

[thesamuraiwho]

@Augustin82 and @wolfadex Thank you both for the help! I really appreciate it because I’m new to functional programming and Elm so it’s great to hear the community is so responsive, helpful, and nice.