Author here, with a few comments on things that I see coming up through the discussion—

1. This is not a post bashing dynamic types. When I write that I now value type systems, that doesn’t imply or entail thinking that dynamically-typed languages are bad, wrong, etc. I have a preference for (a certain variety of) static types these days, but some of the best and smartest engineers I know have a preference for dynamically-typed languages—and as my conclusion notes, I take that much more seriously than I did earlier in my career. I’m particularly interested in spending time with Erlang/Elixir and Clojure in the future, as both of them take very different approaches to robustness from the dynamically-typed languages I’ve worked with in the past.

2. Folks have asked about what specifically I found lacking in the type systems I encountered earlier (Fortran, C, Java). I alluded to this in the post, but I’ll elaborate a bit. For my part, I found that all of those—really, anything in that line without influences from e.g. Standard ML—required a great deal of the work of a type system without the degree of benefit I wanted from it. There has been some progress here in languages like Java, C++, and C with `var`/`let`/`auto` with inference, and more by adding in closures and the like. But even today, the gap in what I can express and have the compiler check for me between Java and even TypeScript (much less TS, Rust, Elm, Haskell, etc.) is large. That gap is where a ton of the value is, at least for me.

3. Other folks allude to coming from e.g. PHP to C and Java and finding even C and Java’s relatively limited type systems to be a blessing. I can see how that would be the case, especially given Java’s really great tooling for refactoring. I actually worked with C, C++, Fortran, PHP, JS, and Python all in parallel for the first four years of my career (and semi-regularly poked at Java; I had some early-2010s interest in Android dev that never went anywhere), so it was certainly not from lack of exposure that I didn’t find the tradeoffs all that valuable in the type systems of C, Fortran, Java, etc.

4. I strongly suspect that “cognitive style” (for lack of a better way of putting it) plays an enormous role in how one feels about and approaches types. The literature on types is not very conclusive or robust, and while it finds (e.g in studies of TS) that it does eliminate certain categories of bugs, advocates like me would do well to remember that the effects are relatively small and relatively limited even so. And as I alluded to in (1), I know a bunch of brilliant developers who recognize the value of types in principle—who have worked with Haskell or other similarly robust languages!—but don’t prefer them, and from discussions with them it’s very clear to me that we just think differently, and therefore approach building systems differently!

5. I think there are places where not using the best and most robust type system imaginable is negligent: a TLS implementation, for example. But in those areas, I also think that you’d better be doing an enormous degree of testing, and putting formal methods to use, and doing multiple audits, and basically throwing every possible solution at the problem. Same thing for aircraft or spaceships or medical hardware. Your average CRUD app… probably doesn’t need that level of effort, though. One of the problems in any discussions or debates about these kinds of things is failing to distinguish appropriately between the things which are necessary for different domains and indeed which may be better suited for those domains.

6. Finally, a meta-point: I find it entirely predictable, but a little bit sad, that this post blew up here on HN, when I tossed it off in under 15m… while a deep dive on a cutting-edge JS reactivity system I spent 3 weeks writing and revising got far less attention (and I’m sure that goes for many such careful write-ups). I get it: we all have opinions about things like types. But it’s still too bad!