5 ms·

What you're saying is just "trusting any system is insane", which sure, it might be (depending on the criticalness of said system). But that's not very insight

by ique 9y ago

What you're saying is just "trusting any system is insane", which sure, it might be (depending on the criticalness of said system). But that's not very insightful or informative, how does that drive any sort of decision-making or improvement process? Do you have any suggestion on how to improve the correctness of systems that work better than formal verification?

edanm 9y ago
QA testing on various levels, and real-world usage without incident? This is what most software does and it seems to work fine in the real world, in most cases at least.
- hateduser2 9y ago
  >it seems to work fine in the real world I thought the whole premise of this conversation was that this is a total fabrication?
  - abritinthebay 9y ago
    I don’t believe so - I think it was it isn’t formally showing it is correct. Which is true.
oldgradstudent 9y ago
Formal verification techniques can be very useful, especially for finding deep, often insanely complex bugs. The process of developing the proof can produce a lot of insight about the system and expose hidden assumptions that were never considered while designing the system. It is the final proof itself which is the problem. It does not contribute anything except for terminating the debugging process. Because the specification can be just complex as the system, the proof can only provide a false sense of security. A correctness theatre.
- zzzcpan 9y ago
  This is what many people are missing in their understanding of formal verification. Things that reduce bugs are not proofs, but a different thought process that sort of double checks everything from a different perspective.
- seanwilson 9y ago
  > Because the specification can be just complex as the system Can you give an example of this? I don't find this true in general at all e.g. writing the specification of a sorting algorithm is much easier than implementing most practical sorting algorithms. > the proof can only provide a false sense of security. A correctness theatre. You could use this argument to say that unit tests, type systems, code review etc. are correctness theatre. All these things prevent mistakes.
  - mafribe 9y ago
    > Can you give an example of this? It's pretty clear that a fully formal specification of a program must be (roughly) as complex as the program itself, for otherwise you could construct an infinitely descending chain of ever simpler specifications which leads to a contradiction. The reason that you found the specification of a sorting algorithm simpler than the sorting algorithm is that you probably have not fully specified the algorithm. Consider the sorting algorithm in C's standard library, which allows the user to specify a comparison function. Now imagine you'd pass something like the following comparison function: def compare (x, y): printf ( "I'm comparing %i with %i\n", x, y ) if ( x < y ) return 1 if ( x == y ) return 0 else return -1 It almost completely exposes the inner workings of sorting. Hence a full spec needs to account for this.
    - seanwilson 9y ago
      > It's pretty clear that a fully formal specification of a program must be (roughly) as complex as the program itself, for otherwise you could construct an infinitely descending chain of ever simpler specifications which leads to a contradiction. That's like saying a theorem statement must be roughly as complex as the proof. Also consider that it's much harder to reason (and prove) that heavily optimised versions of algorithms are correct compared to naive implementations. For example, a proof of correctness for heapsort is more involving than one for insertion sort.
abritinthebay 9y ago
Trusting any sufficiently complex system to be correct is insane. It’s virtually impossible to do and be confident at all levels your proof is correct once you move past much more than trivial systems. This is why modern software testing takes a more... I guess you could call it Bayesian... approach. QA, unit testing, integration testing, etc - these are all ways of reducing probabilities of incorrectness. Never formally prove it of course, but make your confidence higher with each. So suggestions? Proper testing and QA.
- lmm 9y ago
  > QA, unit testing, integration testing, etc - these are all ways of reducing probabilities of incorrectness. Never formally prove it of course, but make your confidence higher with each. And formal proofs don't make your confidence higher? IME they're the most efficient tool in the toolbox if you need to get to really low defect rates. (Of course, the sad economic reality is that for most consumer software even a 5% defect rate is perfectly acceptable).
  - zzzcpan 9y ago
    It's not that simple. Defects don't necessarily have to cause problems, depending on how you design your software (think OTP-style architecture). And avoiding problems is in fact what you always need, not low defect rates. It's just so happens that sometimes you can only achieve decent reliability by pursuing very low defect rates. But for a lot of software it cannot economically beat other approaches.
  - abritinthebay 9y ago
    Given a formal proof of a complex system that doesn’t contain any assumptions or mis-modeling is - practically speaking - unachievable... ... then in comparison the time is better spent on something that will produce demonstratable results, even if imperfectly applied. And that is QA & testing.
    - lmm 9y ago
      > the time is better spent on something that will produce demonstratable results, even if imperfectly applied. How is formal proof not such a thing?

by ique 9y ago

edanm 9y ago

QA testing on various levels, and real-world usage without incident? This is what most software does and it seems to work fine in the real world, in most cases at least.

hateduser2 9y ago
>it seems to work fine in the real world I thought the whole premise of this conversation was that this is a total fabrication?
- abritinthebay 9y ago
  I don’t believe so - I think it was it isn’t formally showing it is correct. Which is true.

oldgradstudent 9y ago

Formal verification techniques can be very useful, especially for finding deep, often insanely complex bugs. The process of developing the proof can produce a lot of insight about the system and expose hidden assumptions that were never considered while designing the system. It is the final proof itself which is the problem. It does not contribute anything except for terminating the debugging process. Because the specification can be just complex as the system, the proof can only provide a false sense of security. A correctness theatre.

zzzcpan 9y ago
This is what many people are missing in their understanding of formal verification. Things that reduce bugs are not proofs, but a different thought process that sort of double checks everything from a different perspective.
seanwilson 9y ago
> Because the specification can be just complex as the system Can you give an example of this? I don't find this true in general at all e.g. writing the specification of a sorting algorithm is much easier than implementing most practical sorting algorithms. > the proof can only provide a false sense of security. A correctness theatre. You could use this argument to say that unit tests, type systems, code review etc. are correctness theatre. All these things prevent mistakes.
- mafribe 9y ago
  > Can you give an example of this? It's pretty clear that a fully formal specification of a program must be (roughly) as complex as the program itself, for otherwise you could construct an infinitely descending chain of ever simpler specifications which leads to a contradiction. The reason that you found the specification of a sorting algorithm simpler than the sorting algorithm is that you probably have not fully specified the algorithm. Consider the sorting algorithm in C's standard library, which allows the user to specify a comparison function. Now imagine you'd pass something like the following comparison function: def compare (x, y): printf ( "I'm comparing %i with %i\n", x, y ) if ( x < y ) return 1 if ( x == y ) return 0 else return -1 It almost completely exposes the inner workings of sorting. Hence a full spec needs to account for this.
  - seanwilson 9y ago
    > It's pretty clear that a fully formal specification of a program must be (roughly) as complex as the program itself, for otherwise you could construct an infinitely descending chain of ever simpler specifications which leads to a contradiction. That's like saying a theorem statement must be roughly as complex as the proof. Also consider that it's much harder to reason (and prove) that heavily optimised versions of algorithms are correct compared to naive implementations. For example, a proof of correctness for heapsort is more involving than one for insertion sort.

abritinthebay 9y ago

Trusting any sufficiently complex system to be correct is insane. It’s virtually impossible to do and be confident at all levels your proof is correct once you move past much more than trivial systems. This is why modern software testing takes a more... I guess you could call it Bayesian... approach. QA, unit testing, integration testing, etc - these are all ways of reducing probabilities of incorrectness. Never formally prove it of course, but make your confidence higher with each. So suggestions? Proper testing and QA.

lmm 9y ago
> QA, unit testing, integration testing, etc - these are all ways of reducing probabilities of incorrectness. Never formally prove it of course, but make your confidence higher with each. And formal proofs don't make your confidence higher? IME they're the most efficient tool in the toolbox if you need to get to really low defect rates. (Of course, the sad economic reality is that for most consumer software even a 5% defect rate is perfectly acceptable).
- zzzcpan 9y ago
  It's not that simple. Defects don't necessarily have to cause problems, depending on how you design your software (think OTP-style architecture). And avoiding problems is in fact what you always need, not low defect rates. It's just so happens that sometimes you can only achieve decent reliability by pursuing very low defect rates. But for a lot of software it cannot economically beat other approaches.
- abritinthebay 9y ago
  Given a formal proof of a complex system that doesn’t contain any assumptions or mis-modeling is - practically speaking - unachievable... ... then in comparison the time is better spent on something that will produce demonstratable results, even if imperfectly applied. And that is QA & testing.
  - lmm 9y ago
    > the time is better spent on something that will produce demonstratable results, even if imperfectly applied. How is formal proof not such a thing?