5.11.7 Exceptions

A common requirement in applications is to want to jump non-locally from the depths of a computation back to, say, the application's main processing loop. Usually, the place that is the target of the jump is somewhere in the calling stack of procedures that called the procedure that wants to jump back. For example, typical logic for a key press driven application might look something like this:

     main-loop:
       read the next key press and call dispatch-key
     
     dispatch-key:
       lookup the key in a keymap and call an appropriate procedure,
       say find-file
     
     find-file:
       interactively read the required file name, then call
       find-specified-file
     
     find-specified-file:
       check whether file exists; if not, jump back to main-loop
       ...

The jump back to main-loop could be achieved by returning through the stack one procedure at a time, using the return value of each procedure to indicate the error condition, but Guile (like most modern programming languages) provides an additional mechanism called exception handling that can be used to implement such jumps much more conveniently.

• Exception Terminology: Different ways to say the same thing.
• Catch: Setting up to catch exceptions.
• Throw Handlers: Adding extra handling to a throw.
• Lazy Catch: Catch without unwinding the stack.
• Throw: Throwing an exception.
• Exception Implementation: How Guile implements exceptions.