Configuring Conditional Debug Logging in Elm with Runtime Flags

Answer

Yes – you can create a small helper that forwards Debug.log only when a runtime Boolean flag is true, and the helper will be stripped entirely in optimized builds.

Confirmed facts

• Elm’s Debug.log calls are removed by the optimizer when you compile with elm make --optimize (or --release).
• Elm flags are decoded once at program start; a mismatch causes a startup error, making the problem visible early.
• Because the flag is read‑only after initialization, changing it requires a full page reload.

Likely explanation & steps

1. Declare a flag in your flags record, e.g. debug : Bool.
2. In JavaScript, pass the flag when initializing the Elm app:

   const app = Elm.Main.init({
     node: document.getElementById('elm'),
     flags: { debug: true }   // set false for production
   });
   

3. Decode the flag as a Boolean in Elm:

   type alias Flags = { debug : Bool }
   flagsDecoder : Json.Decode.Decoder Flags
   flagsDecoder = Json.Decode.map2 Flags
     (Json.Decode.field "debug" Json.Decode.bool)
   

4. Create the wrapper (place it in a module, e.g. DebugHelper.elm):

   module DebugHelper exposing (debugLog)
   
   debugLog : String -> a -> a
   debugLog label value flags =
       if flags.debug then
           Debug.log label value
       else
           value
   

5. Thread the flag through your update/init so the wrapper can see it. A simple way is to store it in the model:

   type alias Model = { debug : Bool, ... }
   init : Flags -> ( Model, Cmd Msg )
   init flags =
       ( { debug = flags.debug, ... } , Cmd.none )
   

6. Use the helper wherever you would have called Debug.log:

   update msg model =
       case msg of
           SomeEvent ->
               let
                   !_ = DebugHelper.debugLog "SomeEvent" someValue model.debug
               in
                   ( { model | ... } , Cmd.none )
           _ -> ...
   

Verification

• Development build: elm make --debug src/Main.elm – toggle the flag between true and false and observe console output.
• Production build: elm make --optimize src/Main.elm – run the compiled JavaScript; no Debug.log output should appear regardless of the flag.
• Type safety test: pass a non‑boolean value for debug in JavaScript; the Elm runtime should fail to start with a decoding error.

Open point – need one diagnostic detail

Are you compiling your production bundle with --optimize (or --release)? If not, the Debug.log calls will remain in the output and the wrapper will not suppress them.