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Maxixe: Design
Design Goals
Maxixe's design goals are:

Simplicity: the implementation should be simple. Reimplementing Maxixe in a different host language should be a conceivable weekend project.

Familiarity: proofs should look more or less like "classical" proofs which consist of a list of steps and the justification for each step.

Explicitness: the supplied proof must be written out completely explicitly; the implementation must not be made to search for any information that is not stated.

Generality: the proof language itself should make as few assumptions about the object language as possible. It shouldn't be tied to any particular system of logic — as far as possible, it should be purely a proof calculus.

Predicate Logic: on the other hand, the proof language should, when given the rules of predicate logic, be powerful enough to allow one to check proofs using predicate logic (a.k.a. firstorder logic).

Correctness: the proofchecking capability of the language must not claim that an incorrect proof is correct for the given rules. It would also be nice if it does not claim that a correct proof is incorrect.
Here are some things which are not goals of Maxixe:

Theorem proving. Maxixe need only check that a proof supplied to it is a valid proof. It need not search for a proof given a theorem.

Efficiency.
How well are these goals met?

Simplicity: as of this writing, the reference implementation of Maxixe, in Python, is less than 700 lines of code. About 250 lines of that is the parser and scanner. The AST AST and term (matching, replacing) infrastructure occupy around another 250 lines.

Familiarity: aside from the term language (meaning, you have to say
impl(A,B)
instead ofA→B
), the layout of a proof is fairly classical. A proof consists of a set of rules of inference (which includes axioms and premises), the goal to be proved, and a list of steps. Each step includes a phrase "by [rule] with [previous steps]" as its justification. The last step must equal the goal to be proved. 
Explicitness: virtually everything must be spelled out. Maxixe will not even search for what previous proof steps were used in a step.

Generality: instead of coding the rules for Existential Instantiation (etc) in the proof language, we have coded constraints on hypotheses and conclusions which allow rules like EI to be written. It's basically a natural deductionlike system in which propositional logic, predicate logic, and other logics (ones that don't rely too heavily on side conditions for their rules) can be captured.

Predicate Logic: see the Generality point above. I believe the examples show that a significant portion, but perhaps not yet all, of firstorder logic, can be coded in Maxixe.

Correctness: see the Disclaimer in the README. It's probably very close, but I wouldn't put money on it quite yet. Bugs are always possible.
Related Work
The most proximal influence on Maxixe is certainly The Incredible Proof Machine, which displays proof steps graphically as a network of nodes, and lets you hook them up to form a valid proof. I learned a lot from it and have had a lot of fun with it. You should check it out too.
But I've been interested in writing a proof checker since reading (I think) an example in one of Emil Post's early papers on Post canonical systems. I don't remember the exact example, but this was part of the research on effective computation, and he was showing that what a mathematician or logicial does when checking a proof is essentially a stringrewriting process. Symbol manipulation. Replacing equals with equals.
My interest was further piqued by a few mentions of this in the book Advanced Topics in Term Rewriting.
And there is a proof in ML for the Working Programmer that I've always wanted to mechanize (forthcoming in Maxixe, hopefully.)
I looked into Agda and while the HowardCurry Isomorphism is really cool, I have to say it also makes it possible to conflate two concepts, computing and proving, which are in many respects different. And for whatever reason, proofs written as computations look just terribly pained to me. I wanted something with a more classical approach.
And Coq is probably nice for what it does, but I really don't care much right now about finding proofs, I just want to check ones I've already found.
And both of these tools are really quite heavyweight; I wanted something lighter.
A modern but somewhat older proofchecker that is based on symbol manipulation is MetaMath. It fits the bill in many ways, but its syntax is quite ugly. It has a huge body of work rendered to web pages though, so you should check it out too.