Log a tagged value on the developer console, and then return the value.

1 + log "number" 1        -- equals 2, logs "number: 1"
length (log "start" [])   -- equals 0, logs "start: []"

Notice that log is not a pure function! It should only be used for investigating bugs or performance problems.

Crash the program with an error message. This is an uncatchable error, intended for code that is soon-to-be-implemented. For example, if you are working with a large ADT and have partially completed a case expression, it may make sense to do this:

type Entity = Ship | Fish | Captain | Seagull


drawEntity entity =
  case entity of
    Ship ->
      ...


    Fish ->
      ...


    _ ->
      Debug.crash "TODO"

The Elm compiler recognizes each Debug.crash and when you run into it at runtime, the error will point to the corresponding module name and line number. For case expressions that ends with a wildcard pattern and a crash, it will also show the value that snuck through. In our example, that'd be Captain or Seagull.

Use this if you want to do some testing while you are partway through writing a function.

Do not use this if you want to do some typical try-catch exception handling. Use the Maybe or Result libraries instead.