[gambit-list] Proposal: Addition of a parameter option for signalling IO error as #f/#!eof result value instead of as exception

Mikael mikael.rcv at gmail.com
Sat Jan 19 13:13:19 EST 2013


Dear Marc and Gambit community,

Here's a reflection on Gambit's IO routines' use of exceptions and a
proposition for a tweak. For context, please check the previous email. Here
we go:



I've come to relate to exceptions as a suitable medium pretty much solely
for conveying an error state that requires manual administrator/programmer
intervention.

Exceptions to the "solely" would be if there's some rare invocation of some
routine, where a catch-all "it failed" kind of result would be satisfactory
for error handling.


Thus, for instance if you make a laboratory project of connecting one
computer to another and suddenly a read-u8 operation fails, that's a good
reason to throw an exception. But, if you do read-u8:s all the time in a
context where some of them are expected to fail, I suggest the view that
making that result in an exception is a terrible idea because of the
reasons below.


So to sum up, Gambit's IO routines currently throw a quite wide variety of
exceptions. At least a bunch of them essentially convey the same thing as a
#f or #!eof result would have done.



*Lots of cases when exception on IO error is a bad idea*
A typical example of when this makes a difference, is when you want to make
some IO work in Gambit, where some form of eof-like IO failure is an
expected outcome. Examples:

 * read-u8 from a TCP port that may or may not be closed now

 * write-substring to a TCP port that may or may not be closed now

 * A HTTP request, in particular if it's to a host that's not under your
control


All these calls cannot be wrapped individually in
with-exception-catcher blocks,
you'd need a 1Thz core for per-byte reading of large amounts of data with
each read-u8 wrapped in a with-exception-catcher to not be too expensive,
so my best understanding today is that the locations in Gambit's runtime
that currently always produce exceptions in these cases, would benefit of a
tweak:


*What about: Introduce a global parameter object that introduces the option
of #f/#!eof result instead of exception on IO error*
As to keep code simple and effective, I would propose the introduction of a
global parameter object io-prefer-eof-result-to-exception to control this
behavior, and perhaps one or two more global parameters to be assigned
detailed error info in the event of io error.

Whenever a user needs to use IO with performance and in a might-fail
context, he/she just switches this parameter to #t.

What's important is that IO can be done at high performance when it works.
Unlike individual invocations wrapped in with-exception-catcher, the
presence of a parameter value has no overhead on individual procedure
invocations. IO operation failures are extremely rare compared to IO
operation successes (maybe 1:10,000), so the error handling code will be
executed quite rarely and thus unwrapping of a parameter object is a no
brainer expense in the context.


As to get to this suggestion, I experimented over a long period with
with-exception-catcher and with extracting Gambit* *IO routine code as to
make custom exception-safe variants of them, this lead to the understanding
that that cannot be reliably done, and involves copying of so much code
that it's by far better to make a Gambit patch for this in the first place
anyhow.


*#f preferable over #!eof as IO operation return value in case of EOF due
to its more central position in the type system?*
Last, there is some #!eof returning in case of EOF already in Gambit's IO
API. I have the impression that it's not completely consistent.

It's struck me that #!eof not is so universal as EOF signal afterall: In a
great many cases, #f would be better due to its role in the type system -
to do meaningful stuff with #!eof you need to typecast it to boolean (i.e. (eq?
result-value #!eof)) to do anything meaningful with it anyhow, and all the
regarded procedures never throw #f within their current definition anyhow.

Because of this at most or all of

     read-subu8vector        read-substring           read-u8
read-char  peek-char read-line
     write-subu8vector       write-substring          write-u8
 write-char
     force-output            close-input-port         close-output-port
close-port
     input-port-timeout-set! output-port-timeout-set!

would do better with #f than #!eof as return value to signal EOF.


*Preliminary thought about next step: Make a patch and when it's mature,
include it in the Gambit repo*
Preliminarily I think this would be a good idea to pursue by making a
Gambit patch that incorporates this functionality, and when it's mature,
incorporating that patch into Gambit's master branch.

The value of having it in the master branch is that this way, Gambit-based
libraries such as a HTTP client, can be written in the coding style that
follows from this, which would clean up the code *alot* and calibrate
design decisions in an advantageous direction:

Basically, error handling is done more explicitly but is much more specific
- which is completely OK for IO-dense code for real world use, as this is
what that kind of code does effectively anyhow - and there's some kind of
performance increase.



Please feel free to share any thoughts you have on this topic, what do you
say about the idea?



Best regards,
Mikael
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