Propositional logic proofs are awful

In my last post, I gave an implementation for a program that searches through every proof in a particular proof notation for propositional logic, and collects all the theorems that it finds. The proof formalism used was Hindley–Milner, with types being the formulas of the theory and well-typed lambda terms the valid proofs (per the Curry–Howard correspondence).

The program’s output looked like this: \a. DoubleNegElim (\b. a) : forall a. False -> a. On the right side of the colon, we have our type/theorem: for every proposition \(a\), \(\bot \rightarrow a\). The notation there is about as good as you’re going to get in ASCII text. But then we have the proof, on the left side of the colon. It’s an inscrutable lambda term encoding a natural deduction proof. The type inference engine is filling in the steps in the proof based only on which deductive rules were used, with the result that, even if you know how to read the notation, you really have to stare at it for a while to figure out why the proof works. While this admittedly lends an aura of elegance and mystery to the proceedings, I think I’d prefer a proof that is comprehensible by a human without significant head-scratching.
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The shortest proof for every theorem

Suppose we want to show that there is a number \(n\) such that no upper bound on the busy beaver number \(\Sigma(n)\) is provable from, say, the axioms of ZFC. One possible proof is as follows: Assume that no such \(n\) exists. That is, assume that for every \(n\), there is a proof in first-order logic that uses only the axioms of ZFC and derives a sentence \(\Sigma(n) \leq m\) for some \(m\). Then we could compute an upper bound for \(\Sigma(n)\) by exhaustively searching all of the possible valid ZFC proofs until we found one that deduced a theorem of the form \(\Sigma(n) \leq m\); by assumption, we must eventually find such a proof. But we know that we cannot compute an upper bound on \(\Sigma(n)\) for arbitrary \(n\); thus, we have derived a contradiction, and the original assumption must be false.

Enumerating proofs until you find one with the desired properties is one of those grand CS traditions that, like calling an \(O(n^{12})\) algorithm “efficient” while dismissing an \(O(1.001^n)\) algorithm as “intractable”, is handy for theoretical purposes but has essentially no bearing on the real world whatsoever. After all, no one would really write a program that loops through all possible proofs and tests each one.

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Google Guava makes Java… usable

Hello World! I’m planning to use this blog to collect the various discoveries I make about software development in my day-to-day job as a software engineer at CERTON, Inc. The perhaps-vain hope is that I will help some other poor soul who stumbles across my posts and finds the solution to the exact problem they were encountering at that instant. I’ll also probably post mathematical curiosities and software engineering commentary. The ultimate goal is to keep the quality level high enough that I feel less guilty of time-wasting writing here than if I were participating in the standard social grooming networks.

But, of course, you’re not here for me, you’re here for you. So let’s talk about Google Guava. For the unfamiliar, Guava is a collection of utilities for performing common tasks in Java, similar in spirit to Apache Commons or C++’s Boost. I was introduced to Guava by a fellow CERTON developer for the purpose of performing some set operations more easily in the selection logic in CertSAFE Modeler. Since then, the use of Guava in our codebase has expanded greatly, making it far and away the library we get the most use out of.

Even if you’re already using Guava, there’s most likely parts of it that you’re not using currently only because you don’t know they exist. I know I find myself saying “Wow! I didn’t know Guava had that!” on a regular basis. Here’s a quick list of the most common uses of Guava in the code I write. Undoubtedly there’s things I haven’t listed here because I haven’t discovered them yet—and you might just find something in this list that you wish you had known about a year ago.
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