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Agora.HigherLevelThinkingr1.1 - 25 Mar 2019 - 17:02 - GregorioIvanoff

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This is why he was so intrigued when, in the appendix of a paper he’d been reading, he came across a strange mixture of math and code—or what looked like code—that described an algorithm in something called “TLA+.” The surprising part was that this description was said to be mathematically precise: An algorithm written in TLA+ could in principle be proven correct. In practice, it allowed you to create a realistic model of your problem and test it not just thoroughly, but exhaustively. This was exactly what he’d been looking for: a language for writing perfect algorithms.

TLA+, which stands for “Temporal Logic of Actions,” is similar in spirit to model-based design: It’s a language for writing down the requirements—TLA+ calls them “specifications”—of computer programs. These specifications can then be completely verified by a computer. That is, before you write any code, you write a concise outline of your program’s logic, along with the constraints you need it to satisfy (say, if you were programming an ATM, a constraint might be that you can never withdraw the same money twice from your checking account). TLA+ then exhaustively checks that your logic does, in fact, satisfy those constraints. If not, it will show you exactly how they could be violated.

The language was invented by Leslie Lamport, a Turing Award–winning computer scientist. With a big white beard and scruffy white hair, and kind eyes behind large glasses, Lamport looks like he might be one of the friendlier professors at the American Hogwarts. Now at Microsoft Research, he is known as one of the pioneers of the theory of “distributed systems,” which describes any computer system made of multiple parts that communicate with each other. Lamport’s work laid the foundation for many of the systems that power the modern web.

For Lamport, a major reason today’s software is so full of bugs is that programmers jump straight into writing code. “Architects draw detailed plans before a brick is laid or a nail is hammered,” he wrote in an article. “But few programmers write even a rough sketch of what their programs will do before they start coding.” Programmers are drawn to the nitty-gritty of coding because code is what makes programs go; spending time on anything else can seem like a distraction. And there is a patient joy, a meditative kind of satisfaction, to be had from puzzling out the micro-mechanics of code. But code, Lamport argues, was never meant to be a medium for thought. “It really does constrain your ability to think when you’re thinking in terms of a programming language,” he says. Code makes you miss the forest for the trees: It draws your attention to the working of individual pieces, rather than to the bigger picture of how your program fits together, or what it’s supposed to do—and whether it actually does what you think. This is why Lamport created TLA+. As with model-based design, TLA+ draws your focus to the high-level structure of a system, its essential logic, rather than to the code that implements it.

Newcombe and his colleagues at Amazon would go on to use TLA+ to find subtle, critical bugs in major systems, including bugs in the core algorithms behind S3, regarded as perhaps the most reliable storage engine in the world. It is now used widely at the company. In the tiny universe of people who had ever used TLA+, their success was not so unusual. An intern at Microsoft used TLA+ to catch a bug that could have caused every Xbox in the world to crash after four hours of use. Engineers at the European Space Agency used it to rewrite, with 10 times less code, the operating system of a probe that was the first to ever land softly on a comet. Intel uses it regularly to verify its chips.

But TLA+ occupies just a small, far corner of the mainstream, if it can be said to take up any space there at all. Even to a seasoned engineer like Newcombe, the language read at first as bizarre and esoteric—a zoo of symbols. For Lamport, this is a failure of education. Though programming was born in mathematics, it has since largely been divorced from it. Most programmers aren’t very fluent in the kind of math—logic and set theory, mostly—that you need to work with TLA+. “Very few programmers—and including very few teachers of programming—understand the very basic concepts and how they’re applied in practice. And they seem to think that all they need is code,” Lamport says. “The idea that there’s some higher level than the code in which you need to be able to think precisely, and that mathematics actually allows you to think precisely about it, is just completely foreign. Because they never learned it.”

Lamport sees this failure to think mathematically about what they’re doing as the problem of modern software development in a nutshell: The stakes keep rising, but programmers aren’t stepping up—they haven’t developed the chops required to handle increasingly complex problems. “In the 15th century,” he said, “people used to build cathedrals without knowing calculus, and nowadays I don’t think you’d allow anyone to build a cathedral without knowing calculus. And I would hope that after some suitably long period of time, people won’t be allowed to write programs if they don’t understand these simple things.”

Newcombe isn’t so sure that it’s the programmer who is to blame. “I’ve heard from Leslie that he thinks programmers are afraid of math. I’ve found that programmers aren’t aware—or don’t believe—that math can help them handle complexity. Complexity is the biggest challenge for programmers.” The real problem in getting people to use TLA+, he said, was convincing them it wouldn’t be a waste of their time. Programmers, as a species, are relentlessly pragmatic. Tools like TLA+ reek of the ivory tower. When programmers encounter “formal methods” (so called because they involve mathematical, “formally” precise descriptions of programs), their deep-seated instinct is to recoil.

Most programmers who took computer science in college have briefly encountered formal methods. Usually they’re demonstrated on something trivial, like a program that counts up from zero; the student’s job is to mathematically prove that the program does, in fact, count up from zero.

“I needed to change people’s perceptions on what formal methods were,” Newcombe told me. Even Lamport himself didn’t seem to fully grasp this point: Formal methods had an image problem. And the way to fix it wasn’t to implore programmers to change—it was to change yourself. Newcombe realized that to bring tools like TLA+ to the programming mainstream, you had to start speaking their language.

For one thing, he said that when he was introducing colleagues at Amazon to TLA+ he would avoid telling them what it stood for, because he was afraid the name made it seem unnecessarily forbidding: “Temporal Logic of Actions” has exactly the kind of highfalutin ring to it that plays well in academia, but puts off most practicing programmers. He tried also not to use the terms “formal,” “verification,” or “proof,” which reminded programmers of tedious classroom exercises. Instead, he presented TLA+ as a new kind of “pseudocode,” a stepping-stone to real code that allowed you to exhaustively test your algorithms—and that got you thinking precisely early on in the design process. “Engineers think in terms of debugging rather than ‘verification,’” he wrote, so he titled his internal talk on the subject to fellow Amazon engineers “Debugging Designs.” Rather than bemoan the fact that programmers see the world in code, Newcombe embraced it. He knew he’d lose them otherwise. “I’ve had a bunch of people say, ‘Now I get it,’” Newcombe says.

He has since left Amazon for Oracle, where he’s been able to convince his new colleagues to give TLA+ a try. For him, using these tools is now a matter of responsibility. “We need to get better at this,” he said.

“I’m self-taught, been coding since I was nine, so my instincts were to start coding. That was my only—that was my way of thinking: You’d sketch something, try something, you’d organically evolve it.” In his view, this is what many programmers today still do. “They google, and they look on Stack Overflow” (a popular website where programmers answer each other’s technical questions) “and they get snippets of code to solve their tactical concern in this little function, and they glue it together, and iterate.”

“And that’s completely fine until you run smack into a real problem.”

[...]


Keywords: modeling-based view, failure of education, mathematical thinking

-- GregorioIvanoff - 25 Mar 2019
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