When a problem situation is detected either by the implementation or by a user program, a representation of that situation called a condition (which can be any Scheme object) is constructed, and the situation represented by the condition is announced by an action called raising the condition. This action allows a dynamically established handler an opportunity to resolve the problem; the current handler is the value of the parameter condition-handler. At any given time, only one handler is active. The current handler is stored in a SRFI 39 parameter.
When a condition is raised, the active handler is called with one argument, a condition which represents the situation. The handler function will execute in the dynamic environment of the call to raise, except that the value of condition-handler is rebound to match the dynamic handler state that was current at the point that the handler function was established as the active handler. This means that handlers are not expected to handle errors in themselves. In order to have a handler handle its own errors, the handler may re-establish itself as a handler within its own body by rebinding the current handler to itself.
If a handler returns normally, the dynamically enclosing handler is invoked in its place, and so on until the root handler is executed. Exiting to non-handler control is performed by calling a captured continuation. For example, a handler may return to the point of raise by invoking a continuation stored inside the condition. Alternatively, it may return to the point where it was made active by invoking a continuation bound to a variable lexically visible to the handler. In any case, the appropriate current handler will be restored as part of the dynamic environment.
There is a system-specific procedure known as the root condition handler that is the initial condition handler. It generally invokes a top-level exit continuation for the whole program, or the current thread (if some concept of threading exists), possibly displaying useful debugging information to some interested parties in some implementation-specific way. Implementations are permitted to provide an interactive debugger that lets the programmer perform actions other than invoking the top-level exit continuation, perhaps invoking retries or other arbitrary continuations.
This raises the condition <object>. It never returns normally.
This constructs a condition using string and the optional objects and raises it. It is compatible with SRFI 23.
This returns the root condition handler.
This is a parameter representing the current condition handler. Its initial value is the root condition handler. Invoking it with no arguments returns the handler; invoking it with one argument sets the handler. Passing it to the parameterize syntax form causes it to be dynamically rebound to a new handler.
Thanks to Taylor Campbell, Alaric Snell-Pym as the author of ErrorsSnellPym, and Kent Pitman as the author of the ISLisp specification, from all of whom I have derived inspiration. They bear no responsibility for infelicities in this proposal.