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:

data 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.

Watch a particular value in the debugger. Say we want to know the value of a variable called velocity because it may not be updated correctly. Adding Debug.watch allows us to name the value and show it with the debugger. The result of evaluating such an expression is unchanged.

  Debug.watch "velocity" velocity == velocity

That means it's easy to add Debug.watch to any value.

Note that calling Debug.watch on a signal is not useful. Instead, it needs to be mapped into the signal (to act on the contained value). So if you want to watch a timer signal, instead of Debug.watch "time" (Time.every 1000) you need Signal.map (Debug.watch "time") (Time.every 1000).

Watch a summary of a particular value in the debugger. This function is pretty much the same as watch but it lets you specify a way to summarize the value you are interested in. For example, maybe you only want to see part of a record:

  Debug.watchSummary "velocity" .velocity object

This is the same as just writing object, but it creates a watch that only looks at the value of object.velocity. You can also show summary statistics like length of a list:

  Debug.watchSummary "Number of clicks" length clicks

Again, this evaluates to clicks but we get to see how long that list is in the debugger.

Trace all past positions of a Form in the debugger. Add this to a Form and you will see a line tracing its entire history.