Re: Controlled types and exception safety

[tmoran@acm.org]

> it's *my* responsibility to judge the tradeoffs for each case separately.
  If you want "X := Y;" to leave X unchanged if there are problems during
the assignment, consider how you might handle a similar problem if you were
copying an important file on disk.  You would probably do something like
  rename X -> X.bak
  copy Y -> X
  delete X.bak
so that if their was a failure during copy you could still retrieve the
file's data.  How about doing something similar for the stack object?
  type stack is ...
    backup_copy : access stack;
Finalize then makes a backup copy (perhaps just by swapping pointers
around), which Adjust then deletes when it successfuly finishes, or which
Adjust's exception handler uses to restore X if there's a problem.
Finalize also puts the backup_copy, with a "delete after time T" time
marker, on a queue for later garbage collection if this is a "terminal"
Finalize - analogous to a nightly "delete *.bak".