Reasonable Language Choice

Mar 08, 2020 · ☕☕ 8 min read

Epistemic Effort: I talked about it a few times and wanted to formalize my notes.

Epistemic Status: Exploratory

I believe that language choice isn’t the most crucial in software development. Significant problems translate fairly well between the languages, and many developers I respect are polyglots.

However, let me ponder the choice between ReasonML and TypeScript. I hope you’ll find it interesting.

I find ML family languages pleasing. Despite this, I couldn’t recommend ReasonML to many teams.

TypeScript is a nonromantic, and possibly unpleasant, but reasonable language choice.

Let me explain.

I work in the web ecosystem — The JavaScript ecosystem. I build apps and libraries, which solve real but fairly ordinary problems. I won’t call it simple, but it’s not really rocket science. I strive to do it well, aim for a short feedback loop with correct, maintainable, and cheap to produce code.


I need correctness, so I reach for a static type system. Tests are simply not enough. I want to describe types passing through my system. The person who reads my code can then read them, and see the bird’s-eye view.

You might be familiar with the famous quote “TypeError: undefined is not a function.”

I claim that writing strict functional TypeScript provides the same level of correctness and control over your codebase as ReasonML.

I’d like to avoid expanding on it now, but what I mean by strict functional TypeScript is shortly:

  • compilerOptions.strict: true. Avoid any and !
  • Prefers head(array) over array[0] (#11122)
  • Generous use of discriminated unions
  • Prefers libraries designed for TypeScript over JavaScript typed in DT (fp-ts over lodash-fp or ramda)


Let’s assume that it is easier to have maintainable code in a “good” language with which our developers are familiar. “Good” is highly subjective and not easy to define. Take the following with a grain of salt.

Language “Quality”

TypeScript fixes a lot of JavaScript problems, but it still inherits a big chunk of them.

null >= 0 is a build-time error, not true, but adding methods to the right-hand side of the assignment is still possible. What’s more, it is a thing that really smart people seriously consider.

ReasonML certainly wins this competition by not being JavaScript. Pattern matching and Hindley-Milner type inference of ReasonML are beautiful things, but we don’t even have to mention them. Not being JavaScript is enough.

One could respond: “Git gud. Just learn JavaScript!” The thing is, I have no intention to remember the quirks of JavaScript! I want to focus on delivering business value, and this is just accidental complexity! I don’t care that {} + [] equals 0. If I wrote this, it was an accident; I didn’t have my morning coffee. I want my tools to protect me and tell me “Yo dude, that’s weird” with bright red squigglies.

TypeScript is not meant to be pretty. It is designed for the gradual migration of big JavaScript codebases. You can type some wildly dynamic code with a Turing complete type system.

  • Is TS type system absurdly powerful? Yes.
  • Can you write some hideous incomprehensible conditional types spaghetti in it? Yes.
  • Does this result in excellent editor support and precise types even for weird dynamic patterns? Absolutely.

We can posit the question: “Do we care if our dependencies are readable and easy to develop? Or do we just need them to be well-tested?”

Language Familiarity

It is easier to maintain code written in a way that’s familiar to us. “Legacy code” often means “code written by someone else.”, because we’re afraid of the unknown.

TypeScript, built for adoption in existing JS codebases, promises that all JavaScript developers already know some TypeScript. It is a superset, right? Just rename tada file, check if the few squigglies that appeared indicate bugs, maybe tweak your config a little bit, to make it less strict. Tada. It’s a tempting promise for people who don’t want to learn another language, who want to improve their existing codebase, who want to get shit done.

There are legions of them. Web-development agencies are full of JavaScript developers. On top of it, C-like syntax and the short feedback loop of JavaScript make it a language suitable for beginners. The number of JS developers will grow.

Statically typed JavaScript is doomed to be popular.

Cheap to produce

I find the “Build or Install” decision interesting. (Build vs Buy / Make Or Buy)

How much of our code should be other people’s code? The answer can be “all of it” if we’re using no-code tools. It can also be “zero” if you’re building a language or “next to none” for a library for building user interfaces.

For most web apps built in software houses, I believe the answer to be “as much as we’re comfortable with.” Gluing together libraries that fix parts of your problem is a valid way to build a product. Using no-code and gluing together entire products is also an entirely correct way. It isn’t as glorious as Real Man C++ Programming, but it solves people’s problems, and this is the important part. More so, it allows building better products in a limited time. Time to swallow our pride, and stand on the shoulder of giants, my friend.

But hey! Popular open source projects proceed in entirely different way. React has almost no dependencies, TypeScript and Mitt have exactly zero.

React contains a purpose-built min heap implementation.

twitter conversation explaining the choice

Whenever we install a library, we trade off control for time.

Libraries need more control than apps do. Few kB of difference in bundle size or one corner case vulnerability may be a difference between mass adoption and oblivion.

Another argument for zero-dependency policy might be, that the dependencies are not expected. We’d like to avoid the question “What is this? Why is it in our bundle?“.
You are invited to a party. Is it appropriate to bring your friends? It depends. Are you the party person, and the hosts expect you to bring your squad (redux-toolkit)? Or are you expected to come alone?

Languages and OSes need even more control. Fuchsia devs pin all dependencies to an exact version by policy. It is important to notice the difference in scale. I mentioned projects which plan for years, not months. They aim to topple the giants.

If you know you are going to compose your app from chunks of existing code, choosing a language with the bigger ecosystem is a good idea.

How do the ecosystems of these two languages compare? As I am writing this, DefinitelyTyped has 10,669 contributors[1] and 26,828 stars with a 0.4 contributors to stars ratio.

For comparison:

  • Vue: 290 👩‍💻 and 158k ⭐ ➡ 0.00182 ratio
  • TypeScript: 448 👩‍💻 and 59k ⭐ ➡ 0.00765 ratio
  • ReasonML: 124 👩‍💻 and 8.6k ⭐ ➡ 0.01455 ratio

Look at the ratio! Four tenths! DefinitelyTyped is not a fun thing people star, because they like it! I hate contributing to DT! Yet, there are many excellent libraries in the JavaScript ecosystem, and I want to use them in my statically typed projects.

ReasonML ecosystem is growing, but there’s a long way ahead of it.

There are 10 thousand people who introduce the best JavaScript libraries[2] to TypeScript. The sheer number is astonishing. There are more people who actively improve TypeScript ecosystem than people who declare that they like Reason! The numbers speak in favor of TypeScript. I realize that this is a cold argument and that I am betting for Goliath against David.


State of the Octoverse reports different numbers. GitHub isn’t clear what “contributor” means. I believe that repository pages indicate the number of commit authors, and the State of the Octoverse reports the number of issue authors.


Kind of. Many libraries “introduce themselves.” JavaScript ecosystem is a superset of the TypeScript ecosystem. Your favorite libraries you use in JavaScript might be written in TypeScript. Notable mentions: Jest, XState, both Redux and MobX.


Both TypeScript and ReasonML have a positive effect on the industry. They ease the transition for web developers who’d like to build bigger and more complex applications and for programmers with no love for JavaScript who’d like to build for the web without WASM.

I am not fond of the fact, but for my ordinary work projects choosing the uglier, more popular language seems to be the reasonable choice.

One exception. My gaming laptop explodes when I open 4th Electron-based project. ReveryUI is love.

Loose notes
  • import/export vs module/open/all files are modules
  • ReasonReact feels underpowered
    • (props-spread) Spreading props is not an “anti-pattern” in TypeScript, and it’s tremendously useful for design systems and libraries. It feels especially weird for me because Reason Records support spread.

Edited 9 times
  1. 49a3e89Sep 14, 2020More work on brain-style notes
  2. 5b51651Sep 13, 2020Initial work on Brain-style notes
  3. a2f5aafMar 16, 2020Add epistemic status
  4. 6d0c812Mar 16, 2020Expand "Language Familiarity" paragraph
  5. 66572bbMar 11, 2020Remove redundant article
  6. 7e19622Mar 09, 2020Polish /reasonable-language-choice
  7. 03883cdMar 09, 2020Fix a sentence
  8. 22e31adMar 09, 2020Finish(?) the Reasonable Choice
  9. 7dfd4b5Mar 07, 2020Draft some notes for Reasonable Language Choice