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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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