Basic debugging in Elm uses two methods. Debug.log and Debug.crash. Use log to print to the web console, and crash when you want to halt execution of your app. Usage of either method should be removed before deploying your code to production.
Samples
Array.toList (Debug.log "shuffled" (shuffle fullDeck 0 seed))
case Array.get i deck of
Nothing -> Debug.crash "index out of bounds"
Just c -> c
Links