how to get the value from (Task String Value)?

Question

I am trying to load a json from localstorage with this code:

let 
    val = Storage.getItem "exercises" decodeExerciseList
in 
    ({ model | exercises = val }, Cmd.none)

but I am getting this error: The 5th and 6th branches of this case produce different types of values. - The 5th branch has this type:

( Model, Cmd Msg )

But the 6th is:

( { exercise : Maybe Model.Exercise
, exercises : Task String (List Model.Exercise)
}
, Cmd msg
)

I thought maybe this could help:

case val of
    succeed -> ({ model | exercises = val }, Cmd.none)
    fail -> ({model | exercises = []}, Cmd.none)

but no luck. in this case I got this other error:

The 1st and 2nd branches of this case produce different types of values. - The 1st branch has this type:

( { ..., exercises : Task String (List Model.Exercise) }, Cmd msg )

But the 2nd is:

( { ..., exercises : List a }, Cmd msg )

---

so basically I am still having the issue with the Task String X instead of just X.

any idea of what to do here?

Answer 1

Although localstorage is access synchronously in javascript, such access is still impure and is thus handled in Elm using tasks as you have seen.

So you code needs to work in 2 stages - initiating the Task - using the returned result

Something like the following

type Msg 
    = LoadFromStorage 
    | OnLocalStorage (Result String (List Model.Exercise))

update message model =
    case message of 
        LoadLocalStorage -> 
            ( model
            , Storage.getItem "exercises" decodeExerciseList
                |> Task.attempt OnLocalStorage
            )
        OnLocalStorage res -> 
            case res of 
                Ok val -> 
                    ({ model | exercises = val }, Cmd.none)
                Err err -> 
                    handle error