[gambit-list] Your feedback would be much appreciated re: Proposal for enabling IO errors to be reported through returning of a custom value instead of by throwing exception, through DSL with exports: ##io-error-behavior param, ##default-io-error-behavior unique value, ##last-io-error param.

Mikael mikael.rcv at gmail.com
Wed Mar 20 15:46:25 EDT 2013

Hi Marc!

2013/3/18 Marc Feeley <feeley at iro.umontreal.ca>

> The problem I see with the approach you propose (whether it uses a
> parameter or a port specific flag) is that the semantics of an "io-error"
> is vague.  What is an IO error?  The definition is important because
> exceptions that are IO errors are going to be processed using this new
> mechanism, and non-IO exceptions will use a different mechanism (normal
> exception handling).
> The approach I propose does not have this problem because the programmer
> has complete control over the definition of an IO error.  The exception
> object can be inspected to see if it qualifies as an IO error and an
> appropriate action can be taken.  The definition of IO error can depend on
> the type of port, the type of primitive which caused the exception
> (read-u8, read-char, read), etc.
> Marc
Aha. To really get this, there are two quite fundamental things about the
applicability of the with-exception-handler possibility that I maybe don't
get yet or at least would benefit of clarification, can we have a look at

Also last a question re semantics of io-error.

So, for the with-exception-handler mechanism to work out, it needs to go
together with other use of exception handling use that's being done in the
same scope.

In a setup where there's another exception handler *outside* the IO
with-exception-handler, there would be no issue as the IO w.e.h. 's handler
would be invoked first for any exception that occurs, and do its matching
and handling.

In a setup where the other exception handler is made *inside* the IO w.e.h.
though, any IO exception that arises within that exception handler will be
picked up by it first, and for the IO w.e.h. thing to work out, that
handler needs to be able to pass on the exception to the parent exception
handler (which is the IO w.e.h.) completely in its original shape, in such
a way that if the IO w.e.h. handler returns a value, that will be passed as
return value to the original |raise| call.

An example of this would be a web-based PI calculator that uses exception
handling locally in its PI calculate request thunk to pick up invalid user

So let's ask how this could be done.

Let's say below that we have a procedure (install-io-w.e.h. port thunk) that
installs the IO w.e.h. that makes IO primitives return #f on exception, so
that thunk is invoked with that w.e.h. installed.

Then, we have application logics (logics) that, aside from using IO,
internally uses exception handling.

So a setup something like,

(define (logics)
    (lambda (e) "Logics failed due to invalid user input!")
    (lambda ()
      (pp (read-u8 broken-port))
      (/ 0 0) ; This is to simulate an exception due to invalid user input.
(install-io-w.e.h. broken-port logics)

The desired behavior here is to print #f to the console and for logics to
then return "Logics failed due to invalid user input!".

The issue now becomes, how would this local exception handler need to be
implemented as for this to work out.

The local exception handler needs to be specific about what kind of error
it looks for as to know which to handle locally and which to re-raise. This
might be a complete PITA in some situations as you're looking for a
catch-all behavior, as in the example above!

Perhaps the order of exception handlers could be tweaked somehow, so that
the IO w.e.h. would get highest priority or something, though how could
that be made as a 'clean' abstraction? I mean, who's in a place to claim at
a general level that one exception is of higher prio than another? -
different classes could be introduced, like, "user exceptions" and "io
exceptions" or "system exception" (this would lead to an at least almost
functional equivalent of the port specific flag solution, just with a more
indirect code path!), or, the exception matching procedure could be
exported to a separate mechanism, and the exception handler claiming the
highest specificity in the matching would get to handle it e.g.
exception-match exception-handler thunk) where exception-match takes an
exception argument e and returns how well it matches the exception, i duno
as a boolean or 0-10 or a symbol.

Or, a global parameter object |is-nonlocal-exception?| could be introduced
that any exception handler is free to invoke as to check if it should
re-raise the exception, and to overlap.. hmm, a more general solution would
be better.

Do you see any general solution to this specificity problem?

Now to get to the next thing, let's just presume there's a solution to this
and we represent it in this example as a (if (is-nonlocal-exception? e)
(raise e) condition in the local exception handler, again not because this
would necessarily represent a general solution but just to get on to the
next thing in the reasoning; so now we have

(define (logics)
    (lambda (e) (if (is-nonlocal-exception? e) (raise e) "Logics failed due
to invalid user input!"))
    (lambda ()
      (pp (read-u8 broken-port))
      (/ 0 0) ; This is to simulate an exception due to invalid user input.
(install-io-w.e.h. broken-port logics)

Now, how do you make this *parent* exception handler (the IO w.e.h.) that
got the exception passed to it, able to pass a return value to the original
|raise| call?

This is required for the exception handling-based IO error handling
behavior we're looking for to work.

The problem reduces to

(define (logics) (raise "Please return 'properly-handled!"))

(define (proxy-exception-handler/catcher2 thunk) (lambda ()
(with-exception-catcher (lambda (e) (raise e)) thunk)))

(define (proxy-exception-handler/catcher1 thunk) (lambda ()
(with-exception-handler (lambda (e) (raise e)) thunk)))

(define (parent-exception-handler thunk) (continuation-capture (lambda
(cont) (with-exception-handler (lambda (e) 'properly-handled!) thunk))))

where proxy-exception-handler/catcher 1 & 2 are to represent an arbitrary
chain of exception handlers that logics code may come up with. The intended
behavior is

(parent-exception-handler (proxy-exception-handler/catcher1
logics))) => 'properly-handled!

(parent-exception-handler (proxy-exception-handler/catcher1 logics)) =>

(parent-exception-handler (proxy-exception-handler/catcher2 logics)) =>

Actually evaluating these three tests showed:

(parent-exception-handler (proxy-exception-handler/catcher1
logics))) => infinite loop (!)

(parent-exception-handler (proxy-exception-handler/catcher1 logics))
=> infinite
loop (!)

(parent-exception-handler (proxy-exception-handler/catcher2 logics)) =>

To start with, great that we see that with-exception-catcher delivers out
of the box for this usecase!

Thus we have with-exception-handler left. I guess the best way would be if
with-exception-handler inherently somehow would deliver for this, so that
support for this kind of use would be transparent and not require possible
updates of user code (e.g. any typical user code such as a PI calculator
etc. could without needing code review just be run within a web server that
uses this special IO error handling).

Is there any way to make with-exception-handler deliver for this usecase?

Last, what about that for introducing a port specific flag there would be
the issue that the semantics of an IO error is vague -

Maybe you see something here I didn't get. As IO error for a port would
count any error reported from the OS about the port, as well as timeouts
within Gambit's IO system.

So this would correspond to any OS IO primitive invocation that gives an
error return value (other than one that asks for a reiteration of the
procedure, which some of them come with).

Another way to relate to it would be that such a port specific flag would
serve to protect from requirement of programmer or admin intervention for
any IO error that could possibly come up regarding the addressed ports.

What do you see here?

Best regards,
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