[r6rs-discuss] [Formal] Recursive exception handling considered harmful

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Submitter: John Cowan
Email address: cowan_at_ccil.org
Issue type: Defect
Priority: Major
Component: Exceptions
Report version: 5.92
Summary:  Recursive exception handling considered harmful
Note:  This comment is in part based on suggestions by Taylor Campbell,
but he bears no responsibility for its content.
In R5.92RS, a condition handler is invoked in a dynamic environment
identical to that of the signaler, but with the condition handler
stack popped by one frame.  Failing to do so will cause the handler
to be re-entered whenever an exception is raised within it, which is
obviously disastrous.  (Scheme48 behaves this way.)  As a consequence,
"raise" cannot tail-call the current handler, because it must pop and
later restore the handler stack.
Unfortunately, R5.92RS prescribes that a handler must reraise an exception
in order to get it processed by the next handler in the dynamic chain.
This loads the call stack with alternating calls to "raise" and to each
handler in turn.  This is a Bad Thing in an environment where there are
many handlers and most only handle specific kinds of conditions.
I propose, therefore, that the protocol for calling handlers be changed:
1) To decline to handle a condition and propagate it up to the next
enclosing handler, a condition handler simply returns the condition
that is to be propagated to the next enclosing handler.  Only a constant
amount of stack space, then, will delimit the handler and the signaler,
regardless of how many handlers there are.  No debugger traces will be
cluttered by unnecessary frames caused by recursive signaling, except in
the case where a procedure called by a raises a genuinely new exception.
2) In order to deal with continuable exceptions, "raise" is given a
second argument, which is a continuation to be invoked if all handlers
decline to handle the exception.  The default value of this argument
is system-dependent but does not return to the caller of "raise".
The "raise-continuably" procedure is removed from R6RS.
-- 
John Cowan   http://ccil.org/~cowan  cowan_at_ccil.org
[P]olice in many lands are now complaining that local arrestees are insisting
on having their Miranda rights read to them, just like perps in American TV
cop shows.  When it's explained to them that they are in a different country,
where those rights do not exist, they become outraged.  --Neal Stephenson