Cleaning up after yourself, prologue.

June 22, 2011 at 3:32 pm (errors in error handling, thirty misfeature pileup)

Errors that happen during onCleanup are transformed into warnings? Really?

It doesn’t help that cleanup functions also can’t be closures — they can’t actually respond to data about a resource that was gathered during a program. But first things first.

Hey: if an error happens in my code, that is an error. My program should not continue unless it specifically handles that error. If an error happens while my program is trying to clean up after itself, that means something is wrong and MATLAB should not force my program to blithely continue and wreak further havoc.

I’ve restrained myself from picking too hard on The Mathworks’ decision to promise deterministic destruction for closures and objects, even though it has unacceptable performance penalties. The reason for the restraint is that I can see the argument for object lifecycle management: when your objects correspond to exclusive resources you hold, you do want to have control and guarantees over when they get released.

Well, I just looked into onCleanup and as usual, the Mathworks fucked it up: You cannot write robust programs using onCleanup, because exceptions during cleanup are swallowed.

So I’m going to have to start kicking at the thirty misfeature pileup where MATLAB’s memory management meets its error handling, after all.

There are a number of languages that offer both automatic memory management and exceptions. A few of them are Python, R, Java, and MATLAB. All of these except MATLAB deal with resource cleanup easily with a try/finally statement, which MATLAB lacks, and most also offer some extra sugar in the form of a try-with-resource, which MATLAB tries to do with deterministic destructors, and fails.

One of these things is not like the others:

Python try/finally

“If finally is present, it specifies a ‘cleanup’ handler…. If there is a saved exception, it is re-raised at the end of the finally clause. If the finally clause raises another exception or executes a return or break statement, the saved exception is lost.”

Python with

“That way, if the caller needs to tell whether the __exit__() invocation *failed* (as opposed to successfully cleaning up before
propagating the original error), it can do so.”

Java try/finally

“If a finally clause is executed because of abrupt completion of a try block and the finally clause itself completes abruptly, then the reason for the abrupt completion of the try block is discarded and the new reason for abrupt completion is propagated from there.”

Java try-with-resources

“If exceptions are thrown from both the try block and the try-with-resources statement, then the method readFirstLineFromFile throws the exception thrown from the try block; the exception thrown from the try-with-resources block is suppressed. In Java SE 7 and later, you can retrieve suppressed exceptions”

R tryCatch

“The finally expression is then evaluated in the context in which tryCatch was called; that is, the handlers supplied to the current tryCatch call are not active when the finally expression is evaluated.”

MATLAB delete

“A delete method should not generate errors”

One of these things is not like the others, and the one that fucked up is of course MATLAB.

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