Guile Reference Manual: Exceptions

Warning: This is the manual of the legacy Guile 2.0 series. You may want to read the manual of the current stable series instead.

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6.13.8 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: Handling exceptions before unwinding the stack.

• Throw: Throwing an exception.

• Exception Implementation: How Guile implements exceptions.