|Subject:||Re: [Axiom-developer] Proving Axiom Correct|
|Date:||Wed, 4 Apr 2018 22:03:49 +0000|
Thanks for your reply. I acknowledge my ignorance (and bias) on most of the advances in program proving.
it to mostly work" is not mathematics.
That is true, but "getting it to work always" must be mathematics and yet, even mathematics itself is not perfect: there are gaps in mathematical proofs that authors sometimes miss but can be fixed, and there are ones that cannot be fixed because the argument
is simply wrong as shown by counter-examples. Proving a program correct, however, is more than mathematics alone, and however careful the theory of program proving and self-applied to its implementation
to create a proven kernel, it is done by humans and humans make errors. Coq falls into this category:
In computer science, Coq is an interactive theorem prover. It allows the _expression_ of mathematicalassertions, mechanically checks proofs of these assertions, helps to find formal proofs, and extracts a certified program from the constructive proof of its formal specification. Coq works within the theory of the calculus of inductive constructions, a derivative of the calculus of constructions. Coq is not an automated theorem prover but includes automatic theorem proving tactics and various decision procedures.
Seen as a programming language, Coq implements a dependently typed functional programming language,while seen as a logical system, it implements a higher-order type theory.
Coq appears to be suitable to prove formally specified algorithms (I could be wrong) in a strongly typed language like Axiom. So any Axiom function may be given a formal specification in order to prove a particular implementation is correct. Can you point me to documentation on how much of Axiom library has passed through Coq this way? I am particularly concerned about the bootstrap portion (where the dependency of types might be circular---you have worked hard on that, and how Coq helps). I suppose that is what Coq on Coq does, getting a proven kernel.
Proven hardware, proven operating systems, proven compilers, and proven languages exist now.
That depends on what the objectives and meaning of "proven": for what set of specifications was attached to an implementation (hardware or software) that was "proven"---not merely "verified". While the theory behind CC may be formal mathematics, in practice, it seems to be a rewriting system with inference rules (for example the beta-reductions).
My point is simply to distinguish the task for proving correctness of an implementation from the correctness of an algorithm or the (mathematical) theory behind the algorithm. The experts of course realize (or should have realized) that.
>>My website contains 6 years of work I did on defining the hardware semantics of the Intel instruction set.
I admire your dogged tenacity. You have rightly limited your scope to the "Intel instruction set". Of course that means someone else should work (and I assume have worked) on other instruction sets (including those that emulate the Intel instruction set).
In my local version of Axiom, during the build phase, I run ACL2 and Coq to "re-prove" the code that has been proven. Proof checking is considerably easier than proof discovery. So, yes, the "program proof >>is repeated from scratch" during build.
What do you mean by "proof checking"? If I understand Coq, it (discovers and) produces a step by step logical sequence proving the specifications (including any alleged proof of theorems) are correct (and correctly implemented?). Are you referring to checking
that sequence? That would be valid if the program under re-certification has not changed, but any change (even if just by renaming one variable, say due to a typo). would require a de novo program proof (throwing away the previously discovered sequence of
proof that established correctness).
I didn't say and I am not saying that working towards a more perfect and error-free program proving system is not important. Keep up the good work.
I better stop until I learn more on the topic.
Department of Mathematics
The City College of The City University of New York
New York, NY 10031
From: Tim Daly <address@hidden>
Sent: Tuesday, April 3, 2018 5:45 PM
To: William Sit; axiom-dev; Tim Daly
Subject: Re: [Axiom-developer] Proving Axiom Correct
William,That's an interesting reply. In general, I agree with the points you make.
Some overall comments are in order. For the last 18 months I've been working
with people at Carnegie Mellon in Computer Science. One thing that really
stands out is that virtually all the courses have a strong emphasis, if not
a complete focus, on type theory and program proof. The "next generation"
is being taught this from day 1. People like me are WAY behind the curve.
Another comment is there is a HUGE literature on program proof which I
was mostly unaware of. There is great progress being made, both in the
theory and in the practice.
>One of the main goals of program proof projects is to prove correctness
>of a piece of computer code. Given the myriad of computer languages,
>subject matters and their theories, algorithms, and implementation (which
>is not necessarily the same as the algorithm itself, since implementation
>requires choices in data representation whereas algorithms are more
>abstract), a proof system must limit its scope to be manageable, say
>for a particular language, a narrow subject matter, one of the theories,
>basic algorithms. It must then allow mappings of data representation to
>theoretical concepts/objects and verify compatibility of these mappings on
>inputs before proceeding to prove a certain implementation is correct. In
>certain situations (such as erratic inputs), error handling needs to be included.
>In addition, there are hardware-software interfaces (assemblers, compilers,
>instruction sets, computer architecture, microcodes, etc) that must have
>their own (program) proof systems.
>So any claim that a certain program is "correct" can only be one small
>step in a long chain of program proofs. And if any change is made,
>whether to hardware or software in this chain, the whole program
>proof must be repeated from scratch (including the proof for the
>program prover itself).
In my local version of Axiom, during the build phase, I run ACL2 and
Coq to "re-prove" the code that has been proven. Proof checking is
considerably easier than proof discovery. So, yes, the "program proof
is repeated from scratch" during build.
>Of course, proving any step in this long chain to be "correct" will be
>progress and will give more confidence to the computed results.
>Nonetheless, in the end, it is still always: "well-it-mostly-
>If a computation is mission critical, a real-world practitioner should
>always perform the same computation using at least two independent
>systems. But even when the two (or more) results agree, no one
>should claim that the programs used are "correct" in the absolute
>sense: they just appear to give consistent results for the problem
>and inputs at hand. For most purposes, particularly in academic
>circles, that is "good enough". If assurance of correctness is paramount,
>the results should be verified by manual computations or with
>assistance from other proven programs (verification is in general
>different from re-computation and often a lot simpler)
Clearly you're unaware of the notion of a proven kernel. See
Barras and Werner "Coq in Coq" which goes into much greater
detail than is appropriate here.
Verifying algorithms, e.g. the GCD, has been done since around 1968.
The world has gotten much better in the last 50 years. At the bleeding
edge, Homotopy Type Theory has things like a theory of coercion which
may end up being part of the Axiom proof pile.
>As a more concrete example, consider an implementation of an
>algorithm which claims to solve always correctly a quadratic equation
>in one variable: $a x^2 + b x + c = 0$ over any field (given $a, b, c$ in
>the field). The quadratic formula (in case $a \ne 0$) provides one
>algorithm, which can easily be proved mathematically, albeit less so
>by a computer program. However, the proof of the correctness of any
>implementation based on this formula is a totally different challenge.
>This is due to at least two problems. First is the problem of representation
>involving field elements and implementation of field operations. The
>second is that the roots may not lie in the field (error handling).
You're well aware of the Category structure of Axiom. Consider that each
proposition is attached at the appropriate place in the Category hierarchy.
Propositions are also attached to Domains.
Proving the GCD in the Domain NNI is different from proving the GCD in
any of the other Domains. You inherit different propositions and have local
propositions appropriate to the chosen domain. The representation of
elements of these domains differ and they are available during proofs.
As for the issue of error handling, what you identify is an error that is
generic to the problem domain, not an error in the implementation of the
algorithm. These are different classes of errors.
>A correct implementation may need to distinguish whether the field is
>the rational numbers, the real numbers, complex numbers, ...
This is the beauty of Axiom's structure. There are different algorithms
for different domains, each having different propositions that carry the
assumptions relevant to that Domain and the particular algorithm, as
well as any that are Categorically available.
>The point is, a general algorithm that can be proved (usually
>mathematically, or through pseudo-code using loop invariant methods
>if loops are involved) may still need to distinguish certain inputs (in the
>quadratic equation example, can the coefficients be 0? what type of
>field?) or else its implementation (also proved to correspond correctly
>to the quadratic formula) could still fail miserably (see Pat M. Sterbenz,
>Floating-point Computation, Prentice Hall, 1974, Section 9.3).
See Gustafson, "The End of Error" and his online debate with Kahan.
>Questions that should be asked of any claim of proven implementation
>include: what are the specifications of the algorithm? is the algorithm
>(data) representation independent? how accurate are the specifications?
All good questions. What is the specification of a sort? It not only must
state that the results are ordered but also that the result is a permutation
of the input. Axiom has the advanage that a portion of its algorithms have
mathematical specifications (e.g. Sine, GCD, etc.). Furthermore, additional
properties can be proven (gcd(x,y)=gcd(y,x)) and used by the algebra at
a later time.
>How complete are they? A claim like "Groebner basis, is proven" (or
>Buchberger's algorithm is proven) probably has little to with proof of an
>implementation if it is done with "unreadable, consisting mostly of
>'judgements' written in greek letters" (which I guess meant pure logical
>deductions based on mathematical assertions and computations
>involving some form of lambda-calculus).
Start with Martin-Lof and Gentzen. I can't begin to generate a response
to this because it's taken me 18 months to get up to speed. Without an
intense study of the subject we have no common ground for understanding.
I'm not trying to dismiss your claim. It is perfectly valid. But there has been
50 years of work on the program proof side of mathematics and without a
deeper understanding of that work it is hard to communicate precisely.
It would be like trying to explain Computer Algebra's ability to do Integration
without mentioning Risch.
>Of course, for implementations, "well-they-
>will be with us for a very long time. While we should continue to work
>towards ideals (kudos to Tim), we must recognize that "good enough"
>is good enough for most users.
Yeah, they "mostly work", except when they don't. Distinguish hacking
from mathematics. "Getting it to mostly work" is not mathematics. Which
is fine, if that's all that matters to you. I agreed that hacking is "good enough
for most users".
That said, this effort is research. It is an attempt to unite two areas of
computational mathematics, both with 50 years of rich history. Most users
don't care. In fact, no one cares but me. Which is fine. I fully expect to
publish a series of minor papers no one will ever read. And, given the
magnitude of the task, I am certain it cannot be completed. In fact, I
devote a whole chapter of Volume 13 to "Why this effort will not succeed".
To quote Richard Hamming:
"If what you are working on is not important,
why are you working on it?"
I think this research is important.
On Tue, Apr 3, 2018 at 1:11 PM, William Sit <address@hidden> wrote:
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