Elm create-elm-app Error: Problem with the flags given to your Elm program on initialization. Json.Decode.oneOf failed in the following 2 ways

Question

I am working on an elm app, and when I try to change the Model to anything other than {} I get the error

Error: Problem with the flags given to your Elm program on initialization. Json.Decode.oneOf failed in the following 2 ways: (1) Problem with the given value: undefined Expecting null (2) Problem with the given value: undefined Expecting an INT

The error is reproduced here

Answer 1

The error is because init is expecting Maybe Model but in HTML section of Ellie you’re not passing in a flags.

There are two options, you can either handle the flags, or remove the flags.

To keep parsing flags: You’ll want to change Elm.Main.init({ node: document.querySelector('main') }) to be

var app = Elm.Main.init({
  node: document.querySelector('main'),
  flags: <some value>
});

It’s also usually recommended to change your flags to be Json.Decode.Value and manually decode them So your init would become

init : Value -> ( Model, Cmd Msg )
init flags =
  case Json.Decode.decodeValue flagDecoder flags of
    Ok decodedFlags -> ...
    Err err -> ...

that way you can handle the flags being invalid or missing.

Remove Flag parsing

The common way is to us the unit type (). So your main becomes

main : Program () Model Msg

and

init becomes

init : () -> ( Model, Cmd Msg )