( Dynamic Roots

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 32.3 Dynamic Roots
 A "dynamic root" is a root frame of Scheme evaluation.  The top-level
 repl, for example, is an instance of a dynamic root.
    Each dynamic root has its own chain of dynamic-wind information.
 Each has its own set of continuations, jump-buffers, and pending CATCH
 statements which are inaccessible from the dynamic scope of any other
 dynamic root.
    In a thread-based system, each thread has its own dynamic root.
 Therefore, continuations created by one thread may not be invoked by
    Even in a single-threaded system, it is sometimes useful to create a
 new dynamic root.  For example, if you want to apply a procedure, but to
 not allow that procedure to capture the current continuation, calling
 the procedure under a new dynamic root will do the job.
  -- Scheme Procedure: call-with-dynamic-root thunk handler
  -- C Function: scm_call_with_dynamic_root (thunk, handler)
      Evaluate `(thunk)' in a new dynamic context, returning its value.
      If an error occurs during evaluation, apply HANDLER to the
      arguments to the throw, just as `throw' would.  If this happens,
      HANDLER is called outside the scope of the new root - it is called
      in the same dynamic context in which `call-with-dynamic-root' was
      If THUNK captures a continuation, the continuation is rooted at
      the call to THUNK.  In particular, the call to
      `call-with-dynamic-root' is not captured.  Therefore,
      `call-with-dynamic-root' always returns at most one time.
      Before calling THUNK, the dynamic-wind chain is un-wound back to
      the root and a new chain started for THUNK.  Therefore, this call
      may not do what you expect:
           ;; Almost certainly a bug:
            (lambda ()
               (lambda ()
                 (display 'fnord)
               (lambda (errcode) errcode))))
      The problem is, on what port will `fnord' be displayed?  You might
      expect that because of the `with-output-to-port' that it will be
      displayed on the port bound to `some-port'.  But it probably won't
      - before evaluating the thunk, dynamic winds are unwound,
      including those created by `with-output-to-port'.  So, the
      standard output port will have been re-set to its default value
      before `display' is evaluated.
      (This function was added to Guile mostly to help calls to
      functions in C libraries that can not tolerate non-local exits or
      calls that return multiple times.  If such functions call back to
      the interpreter, it should be under a new dynamic root.)
  -- Scheme Procedure: dynamic-root
  -- C Function: scm_dynamic_root ()
      Return an object representing the current dynamic root.
      These objects are only useful for comparison using `eq?'.  They
      are currently represented as numbers, but your code should in no
      way depend on this.
  -- Scheme Procedure: quit [exit_val]
      Throw back to the error handler of the current dynamic root.
      If integer EXIT_VAL is specified and if Guile is being used
      stand-alone and if quit is called from the initial dynamic-root,
      EXIT_VAL becomes the exit status of the Guile process and the
      process exits.
    When Guile is run interactively, errors are caught from within the
 read-eval-print loop.  An error message will be printed and `abort'
 called.  A default set of signal handlers is installed, e.g., to allow
 user interrupt of the interpreter.
    It is possible to switch to a "batch mode", in which the interpreter
 will terminate after an error and in which all signals cause their
 default actions.  Switching to batch mode causes any handlers installed
 from Scheme code to be removed.  An example of where this is useful is
 after forking a new process intended to run non-interactively.
  -- Scheme Procedure: batch-mode?
      Returns a boolean indicating whether the interpreter is in batch
  -- Scheme Procedure: set-batch-mode?! arg
      If ARG is true, switches the interpreter to batch mode.  The `#f'
      case has not been implemented.
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