[Xsb-development] Error handling and backtraces.

[Terrance S. <ts...@cs...>]

All:

I'm "pretty sure" the following changes will be transparent to people,
but I figured I should notify everyone.

I've made some changes to the system that include a backtrace list in
the error ball thrown by throw.  Here is how it works:

If you call throw/1 directly, it throws the exact argument you give
it.

If you call any of the ISO throws, such as domain_error,
instantiation_error, permission_error, etc., a term of the form
error(Template,Message,Backtrace) is thrown.

The system error handlers catch balls of the form error/2, error/3,
'_$abort_ball'/1, and '_$abort_ball'/2.  The default system error
handler ignores the Backtrace argument in any of these balls, if
present.

Thus, for future flexibility, I think its best to use the prepackaged
error routines (domain_error, instantiation_error, etc) rather than
performing an explicit throw if you want the default system or user
error handler to catch it.

A couple of issues:

	1) I think we should probably get rid of the abort_balls and
	treat them simply as miscellaneous error terms.

	2) Right now I'm not printing out backtraces a la java.  We
	should probably have an xsb_flag that makes the default
	handler print backtraces (default off).

If I dont hear any comments in a couple of days, I'll go ahead and
make the changes.

Terry