Elm:
Reading command line arguments

How to:

Elm runs in the browser, so it doesn’t have direct access to command line arguments like a traditional server-side or desktop language does. However, for illustration, let’s assume you’re using Elm with a server-side framework like Node.js through elm server or a similar setup that allows passing arguments. Your code won’t handle the arguments directly, but we’ll mimic the pattern:

-- Assume incoming arguments from server-side
type alias Flags = 
    { arg1 : String
    , arg2 : Int
    }

-- Sample Elm `init` function using Flags
init : Flags -> ( Model, Cmd Msg )
init flags =
    ( { defaultModel | passedArg1 = flags.arg1, passedArg2 = flags.arg2 }
    , Cmd.none
    )

Sample output (structured as if passed by the server):

{ "arg1": "Hello", "arg2": 42 }

Deep Dive

Since Elm is a frontend language, it traditionally doesn’t handle command line arguments. Elm operates in the browser’s controlled environment. The command line is a remnant from early computing days, serving as a window into the system.

In Node.js or similar environments, you’d typically use process.argv to get arguments. With Elm, the closest you get is flags when initializing your Elm app from JavaScript, allowing injection of external data. You indirectly accept command line arguments via the server-side language, then pass them to Elm as flags.

For deep integration, Elm apps are bundled with server-side code, providing a seamless experience to users. This pattern of starting an Elm program with specific flags is powerful; it allows for flexible, dynamic initialization that adapts to different environments and use cases.

See Also

• Elm’s official guide on flags: https://guide.elm-lang.org/interop/flags.html
• Node.js documentation on command line arguments: https://nodejs.org/docs/latest/api/process.html#process_process_argv
• An example of Elm with Node.js: https://medium.com/@_rchaves_/using-elm-with-node-elm-server-side-rendering-via-http-nodejs-and-elm-0-19-6c97f062f7eb