Warning: This is the manual of the legacy Guile 2.2 series. You may want to read the manual of the current stable series instead.
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The throw primitive is used to throw an exception. One argument,
the key, is mandatory, and must be a symbol; it indicates the type
of exception that is being thrown. Following the key,
throw accepts any number of additional arguments, whose meaning
depends on the exception type. The documentation for each possible type
of exception should specify the additional arguments that are expected
for that kind of exception.
Invoke the catch form matching key, passing arg … to the handler.
key is a symbol. It will match catches of the same symbol or of
#t.
If there is no handler at all, Guile prints an error and then exits.
When an exception is thrown, it will be caught by the innermost
catch or throw handler that applies to the type of the thrown
exception; in other words, whose key is either #t or the
same symbol as that used in the throw expression. Once Guile has
identified the appropriate catch or throw handler, it handles the
exception by applying the relevant handler procedure(s) to the arguments
of the throw.
If there is no appropriate catch or throw handler for a thrown
exception, Guile prints an error to the current error port indicating an
uncaught exception, and then exits. In practice, it is quite difficult
to observe this behaviour, because Guile when used interactively
installs a top level catch handler that will catch all exceptions
and print an appropriate error message without exiting. For
example, this is what happens if you try to throw an unhandled exception
in the standard Guile REPL; note that Guile’s command loop continues
after the error message:
guile> (throw 'badex) <unnamed port>:3:1: In procedure gsubr-apply … <unnamed port>:3:1: unhandled-exception: badex ABORT: (misc-error) guile>
The default uncaught exception behaviour can be observed by evaluating a
throw expression from the shell command line:
$ guile -c "(begin (throw 'badex) (display \"here\\n\"))" guile: uncaught throw to badex: () $
That Guile exits immediately following the uncaught exception
is shown by the absence of any output from the display
expression, because Guile never gets to the point of evaluating that
expression.
Next: Exception Implementation, Previous: Throw Handlers, Up: Exceptions [Contents][Index]