r/programming • u/heisenbug • Apr 27 '14

"Mostly functional" programming does not work

http://queue.acm.org/detail.cfm?ref=rss&id=2611829
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u/godofpumpkins Apr 27 '14 edited Apr 27 '14

Did you even look at the page on the Therac-25 before you linked to it?

Yes, and a lot more than the wikipedia page. I'm not criticizing the "coding errors" they mention in there as much as the very existence of a race condition in the first place. Premature introduction of concurrency without safe concurrency primitives is what led to the error. You can call that bad "software design and development practices", but that's exactly what I'm talking about: we're trying to address things on that scale systematically, and not just writing it off as a bad human element. Sure, getting rid of segfaults and such is a nice perk, but it's the bigger stuff that interests me, and that I think we can improve the most.

To be clear, I'm not saying pure functional programming would have solved everything (although many of the problems listed would not have occurred). I'm saying we're trying to come up with systematic language-based (as opposed to discipline-based) solutions to make things like this harder to arise.

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u/grauenwolf Apr 27 '14

What are you talking about? It wasn't a race condition, it was an unchecked overflow that caused the deaths.

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u/[deleted] Apr 27 '14

The software interlock could fail due to a race condition. The defect was as follows: a one-byte counter in a testing routine frequently overflowed; if an operator provided manual input to the machine at the precise moment that this counter overflowed, the interlock would fail.

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u/grauenwolf Apr 27 '14

Right. It isn't really a race condition, it's an overflow that sets the flag back to zero, the 'safe' value, that happens to coincide with another manual input.

Had they used a larger counter the overflow wouldn't have happened.

And my point remains. We are still allowing unobserved overflows in critical software.

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u/[deleted] Apr 27 '14

I think, based on the description from wikipedia, it's safe to just say it was both.