Ok, I think I better understand your attitude about non-local exits
and error signalling.

To test my understanding, and before replying to the proposal further,
let me just say back to you two things that I think you would agree
with (and think are obviously true):

1) Non-local exits to upward continuations currently afford no
   general mechanism for cleanups in FFI-using C.

   In particular, the "GUI example":

      call chain:

	scheme code captures continuation K
        scheme code enters foreign binding for GUI library
        GUI library invokes C callback function
        C callback function calls out to scheme
        scheme code invokes K

   is not supported by SRFI-50.   Later SRFIs will have to add
   additional mechanisms to the FFI to handle such situations.

   (Of course, for a limited case of using just a fixed set of
    FFI-using C libraries, problems like the GUI example can perhaps
    be solved at the Scheme level, mostly likely by redefining
    call-with-current-continuation.  The point here is just that there
    is no mechanism for such problems that all FFI-using libraries can
    be counted on to use -- two particular FFI-using libraries that
    independently solve the problem at the Scheme level may be
    difficult to combine in a single application as a result.)

2) The specification of error signalling could be made clearer:

   Currently it says:

        The following macros explicitly signal certain errors, and
        immediately return to Scheme. Signalling an error performs all
        necessary clean-up actions to properly return to Scheme,
        including adjusting the stack of protected variables. Besides
        that, the actual effect of signalling an error is
        undefined. (It is expected that a future SRFI will deal with
        the issue of handling error situations resulting from bugs in
        the program.

   but is could say something more like:

        The following macros explicitly signal certain errors.
        If an error is signalled, either:

          1) the computation must be terminated

          2) the computation may be continued in part by invoking
             a continuation which is upwards relative to the
             C call that triggered the error.  (see "Calling Scheme
             procedures from C".)

        Other details of error signalling are unspecified by this
        SRFI but are expected to be further specified in a later
        SRFI.

Is that about right?

-t