[zeveb]

Same way that you'd document exceptions in a library.

> If the user is not handles the error(forgets about a type or a new type is added in an update) will the user notice the error happened?

In Lisp, ERROR invokes the debugger if the condition is not handled. So an unhandled error condition pauses execution. One cool thing is that the user can use the debugger to invoke restarts. It's extremely nice.

There's also CERROR, which establishes a restart to return from itself — so some errors can be continued on from — WARN, which offers automatically, mufflable warning output, and SIGNAL, which offers non-erroneous condition signalling (i.e., while ERROR never returns, SIGNAL can).

It's really, really full-featured but also easy to use.