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26.7.3 Throwing Exceptions
--------------------------
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.
-- Scheme Procedure: throw key . args
-- C Function: scm_throw (key, args)
Invoke the catch form matching KEY, passing ARGS 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' expression that applies to the type of the thrown exception; in
other words, the innermost `catch' whose KEY is `#t' or is the same
symbol as that used in the `throw' expression. Once Guile has
identified the appropriate `catch', it handles the exception by
applying that `catch' expression's handler procedure to the arguments
of the `throw'.
If there is no appropriate `catch' 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.
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