Programming, Fast and Slow

programming while being chased by a lion

Note: This post was also published on Atlas’s blog.

Kahneman famously distinguished between two different modes of thinking:

  • type-1, or “fast” thinking
  • type-2, or “slow” thinking

The former is the kind of thinking that’s automatic, low-effort, instinctual. It can be done while driving a car. When you decide what to do about a lion crouching nearby, you’re exhibiting type-1 thought.

Type-2 thinking, by contrast, is intentional, high-effort, and cannot be done while multitasking. When you try to multiply 17 * 23 in your head, you’re exhibiting type-2 thought.

I think there are two analogous [0] modes of programming:

  • type-1, or “fast” programming
  • type-2, or “slow” programming

Type-1 programming, like type-1 thinking, is for situations that demand fast responses, where the risk of responding too slowly outweighs the risk of responding sub-optimally. Stated in terms of the Pareto Principle, type-1 programming seeks to exert 20% of the effort while producing 80% of the value. [1]

Type-2 programming, like type-2 thinking, is for situations that demand accuracy, precision, and correctness; where the cost of error is higher than the cost of time. This kind of programming seeks to provide the solution to problems: to do things “the right way”.

I often find myself having to jump between these two modes. When I do, it’s kind of like throwing a switch in my brain. All my habits change.

If I’m doing type-1 programming, I tackle a problem by immediately writing code. I’m not sketching out a design, I’m following my instincts. I’m probably only writing a few tests, and even then only after things already work, and probably only for the most important edge/use cases. If there are open-source libraries that could speed up the work, it’s very likely that I’m going to use one. And if I try a library and it doesn’t work right away, I’m probably going to just move on to another. Little point spending the time to figure things out if I can find something that just works.

If I’m doing type-2 programming, I’m definitely starting by writing things down. I’m investigating different solutions, I’m circulating my designs for feedback, I’m checking for blindspots and biases. It’s a given that I’m writing tests, maybe even first. Coverage is going to be high. Maybe I use a library, maybe I don’t. Libraries make trade-offs and I might not like them. If I’m going to be using a library I’m probably going to make a heatmap of the different options and pick what’s optimal. If the best library isn’t working, I’m reading the source code to find out why.

For example, suppose the feature is “users can query data”. Imagine this feature is table-stakes: you’re losing deals right and left because you don’t have it. So you switch into “fast” mode and push out some functionality that lets users download CSVs. The code isn’t ideal. Maybe you’re doing a lot of stuff in memory. If someone has a lot of data they’re going to get a 500. Etc. The feature isn’t ideal either. In a perfect world, the users would be able to just query within your interface. Regardless – now you have the feature and sales is closing deals again. At this point, you switch to “slow” mode. Now you’re sitting down with design and figuring out a proper query interface. You’re sketching the architecture that would support it. You’re deleting a lot of what you wrote before, improving abstractions, handling edge cases, adding test cases, etc.

If you’ve been an engineer for a while, and worked in a bunch of different business contexts, you probably know what I’m talking about. Your habits might be different than mine, but you probably know how it feels to shift gears between the two modes. It’s especially obvious when you’re switching from a job that requires little/no type-1 programming to one that does.

When I worked at Stripe, nearly all of the programming I did was type-2. That’s because nothing I did was existential for the business. The company wasn’t going to go under if I didn’t ship my features. The company could afford for me to go slow, to do the right thing the right way. It could afford to let me research the heck out of a problem, get tons of feedback on design/architecture, and go incredibly deep. It also needed to afford this, because Stripe has a reputation to protect. The cost of not doing things right could have been massive – much higher than the cost of doing nothing at all. The only times I was ever in type-1 mode was during incidents: when something was wrong in production and we had to stop the bleed asap. It was a relatively rare occurrence.

I currently work at an early-stage startup. And a lot (though not all) of what I’m doing is type-1 programming. That’s because tons of problems are existential for us. If we don’t ship a feature fast enough, the company might just be dead. There usually isn’t time to research what we’re doing for 2 weeks, write out a few design docs, circulate them, incorporate feedback, and then write immaculate code. We’ll never keep up with our competitors or keep our customers happy if we do that.

I think it’s worth distinguishing these types of programming because some engineers struggle to make the switch between them, and this can meaningfully determine the kind of company and role at which they can be successful.

I’ve worked with engineers who seemed constitutionally incapable of type-1 programming. They just couldn’t get past their architectural patterns, their design docs, their need to go really deep. It made them visibly uncomfortable to stop at 20%.

I’ve also met engineers who only seem capable of type-1 programming. It hurts them to go really deep, it’s painful for them to try to find the solution to a problem. They want to think about stuff, but only so much. They move fast because they only want to go fast. Anything else is tedious, boring.

Some people can do both. And I think that’s what you really need at a startup. A lot of what you do at a startup doesn’t differentiate you. Your design, your hosting, your docs, your splash page. That can probably all be done with type-1 programming.

But the really important stuff – your differentiator, your technical moat – is going to take type-2 programming. It’s going to need to be good. You’re going to have to go deep. You’re going to need to solve your core problem better than anyone else.

Each mode has its place.


[0] These modes of programming are analogous, though not reducible, to Kahneman’s modes of thinking. Programming – of any sort – is quintessentially type-2 thinking. You can’t do it while driving a car, it takes considerable effort, it requires intense focus, etc.

[1] Type-1 programming should not be confused with sloppy or bad programming, or programming that produces technical debt. Like anything, it can be done well or poorly, with better or worse abstraction, etc. This could easily be it’s own blog post.