[acdha]

Sure, my point was simply that it’s not as simple as the author assumes. This is a common failure mode in FP advocacy and it’s disappointing because it usually means that a more interesting conversation doesn’t happen because most readers disengage.

[rastrian]

I get why it reads like FP evangelism, but I don’t think it’s “ignoring decades of prior art.” I’m not claiming these ideas are exclusive to FP. I’m claiming FP ecosystems systematized a bundle of practices (ADT/state machines, exhaustiveness, immutability, explicit effects) that consistently reduce a specific failure mode: invalid state transitions and refactor breakage.

Rust is actually aligned with the point: it delivers major reliability wins via making invalid states harder to represent (enums, ownership/borrowing, pattern matching). That’s not “FP-first,” but it’s very compatible with functional style and the same invariants story.

If the TS example came off as “types instead of validation,” that’s on me to phrase better, the point wasn’t “types eliminate validation,” it’s “types make the shape explicit so validation becomes harder to forget and easier to review.”

[acdha]

I would keep in mind how much the title communicates your intentions on future posts. The conversation about preventing invalid states has to be somewhat inferred when it could have been explicitly stated, and that’d be really useful comparing other approaches - e.g. the classic OOP style many people learned in school also avoid these problems as would something like modern Python using Pydantic/msgspec so it’d be useful to discuss differences in practice, and especially with a larger scope so people who don’t already agree with you can see how you came to that position.

For example, using the input parsing scenario, a Java 1.0 tutorial in 1995 would have said that you should create a TimeDuration class which parses the input and throws an exception when given an invalid value like “30s”. If you say that reliability requires FP, how would you respond when they point out that their code also prevents running with an invalid value? That discussion can be far more educational, especially because it might avoid derails around specific issues which are really just restating the given that JavaScript had lots of footgun opportunities for the unwary developer, even compared to some languages their grandmother might have used.