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tl;dr: online TLA+ manual/advanced techniques/examples here. TLA+ is a tool for testing abstract software designs. I first stumbled on it in 2016 and found it so useful I wrote a free online guide to help others learn it. Then I decided the guide wasn’t good enough and wrote Practical TLA+ in 2018. Then I decided the book wasn’t good enough and needed a second edition. Just kidding! Book’s never g

“Don’t write clever code.” Why not? “Because it’s hard to understand.” People who say this think of clever code such as Duff’s Device: send(to, from, count) register short *to, *from; register count; { register n = (count + 7) / 8; switch (count % 8) { case 0: do { *to = *from++; case 7: *to = *from++; case 6: *to = *from++; case 5: *to = *from++; case 4: *to = *from++; case 3: *to = *from++; case

It’s implementation-defined what happens if you decrement a 0 cell or move the pointer past the last cell. Every compiler and runtime would make its own decisions. Every other symbol is a noop, which 1) makes brainfuck relatively easy to comment, and 2) means you can secretly embed brainfuck programs in other language programs (multicoding). Brainfuck is in the class of “Turing tarpits”, languages

New: My new book, Logic for Programmers, is now in early access! This is part one of the Crossover Project. Part two is here and part three is here. A conference talk based on this work is now available here. I sat in front of Mat, idly chatting about tech and cuisine. Before now, I had known him mostly for his cooking pictures on Twitter, the kind that made me envious of suburbanites and their 75

My job these days is teaching TLA+ and formal methods: specifying designs to find bugs in them. But just knowing the syntax isn’t enough to write specs, and it helps to have examples to draw from. I recently read Chris Siebenmann’s Even in Go, concurrency is still not easy and thought it would make a good case study for writing a spec. In it, he gives an example of Go code which deadlocks: /*1 */

The other day I read 20 most significant programming languages in history, a “preposterous table I just made up.” He certainly got preposterous right: he lists Go as “most significant” but not ALGOL, Smalltalk, or ML. He also leaves off Pascal because it’s “mostly dead”. Preposterous! That defeats the whole point of what “significant in history” means. So let’s talk about some “mostly dead” langua

New: My new book, Logic for Programmers, is now in early access! I’ve been using Vim for eight years and am still discovering new things. This is usually seen as a Good Thing About Vim. In my head, though, it’s a failing of discoverability: I keep discovering new things because Vim makes it so hard to know what’s available. While people often talk about the beauty of modal editing or text objects,

New: My new book, Logic for Programmers, is now in early access! I saw this question on the Software Engineering Stack Exchange: What are the barriers that prevent widespread adoption of formal methods? The question was closed as opinion-based, and most of the answers were things like “its too expensive!!!” or “website isn’t airplane!!!” These are sorta kinda true but don’t explain very much. I wr

ALGOL didn’t have a special operator for initialization. Rather, you created the variable with the type, and then later used an operator to assign to it. You could do integer x; x := 5;, but you couldn’t do integer x := 5;. This makes FORTRAN the only one that used = for any kind of assignment, so would seem the right candidate for modern usage. But we know that C descends from ALGOL, which means

New: My new book, Logic for Programmers, is now in early access! Update 01/07/19 Greetings from 2019! The good news is that Chicago isn’t yet a radioactive crater. The bad news is almost everything I said about refinement in this article is wrong. I’m working on writing a more in-depth, rigorous treatment of refinement as its own article. But this one is currently explaining something that definit