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(guile.info)Dynamic Roots

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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
another.

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.

 - primitive: 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
     evaluated.

     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:
          (with-output-to-port
           some-port
          
           (lambda ()
             (call-with-dynamic-root
              (lambda ()
                (display 'fnord)
                (newline))
              (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.)

 - primitive: 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.

 - 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.

 - procedure: batch-mode?
     Returns a boolean indicating whether the interpreter is in batch
     mode.

 - procedure: set-batch-mode?! arg
     If ARG is true, switches the interpreter to batch mode.  The `#f'
     case has not been implemented.