Panic in Elm
import Html exposing (Html, div, text)
import Platform.Cmd as Cmd
main : Program () Model Msg
main =
Html.program
{ init = init
, view = view
, update = update
, subscriptions = \_ -> Sub.none
}
type alias Model =
{ error : Maybe String
}
type Msg
= SimulatePanic
init : () -> ( Model, Cmd Msg )
init _ =
( { error = Nothing }, Cmd.none )
update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
case msg of
SimulatePanic ->
( { model | error = Just "a problem" }, Cmd.none )
view : Model -> Html Msg
view model =
div []
[ text (Maybe.withDefault "No error" model.error)
]
In Elm, there’s no direct equivalent to the panic
function. Instead, Elm uses a more structured approach to error handling. This example demonstrates how you might simulate a “panic-like” situation in Elm.
The Model
type includes an error
field which is a Maybe String
. This allows us to represent the presence or absence of an error.
The SimulatePanic
message is used to trigger our “panic” situation. When this message is received, the update
function sets the error
field to Just "a problem"
.
In the view
function, we display the error message if it exists, or “No error” if it doesn’t.
It’s important to note that Elm’s type system and architecture are designed to prevent many of the situations that might cause panics in other languages. Elm encourages handling all possible cases explicitly, which often eliminates the need for panic-like behavior.
For example, instead of panicking when creating a file fails, in Elm you would typically model this as a possible outcome in your types:
type FileCreationResult
= Success
| Failure String
createFile : String -> Task FileCreationResult
createFile path =
-- Implementation would depend on how you're interfacing with the file system
This approach forces you to handle both the success and failure cases wherever you use createFile
, making your program more robust and eliminating the need for panics in many situations.
Remember, in Elm, you’re encouraged to model your domain in a way that makes impossible states impossible, reducing the need for runtime panics and increasing the reliability of your application.