10.5.3 Errors

When Emacs Lisp attempts to evaluate a form that, for some reason, cannot be evaluated, it signals an error.

When an error is signaled, Emacs's default reaction is to print an error message and terminate execution of the current command. This is the right thing to do in most cases, such as if you type C-f at the end of the buffer.

In complicated programs, simple termination may not be what you want. For example, the program may have made temporary changes in data structures, or created temporary buffers that should be deleted before the program is finished. In such cases, you would use unwind-protect to establish cleanup expressions to be evaluated in case of error. (See section 10.5.4 Cleaning Up from Nonlocal Exits.) Occasionally, you may wish the program to continue execution despite an error in a subroutine. In these cases, you would use condition-case to establish error handlers to recover control in case of error.

Resist the temptation to use error handling to transfer control from one part of the program to another; use catch and throw instead. See section 10.5.1 Explicit Nonlocal Exits: catch and throw.

10.5.3.1 How to Signal an Error		How to report an error.
10.5.3.2 How Emacs Processes Errors		What Emacs does when you report an error.
10.5.3.3 Writing Code to Handle Errors		How you can trap errors and continue execution.
10.5.3.4 Error Symbols and Condition Names		How errors are classified for trapping them.