[mmastrac]

Postel's law is considered more and more harmful as the industry evolved.

[CodesInChaos]

That depends on how Postel's law is interpreted.

What's reasonable is: "Set reserved fields to 0 when writing and ignore them when reading." (I heard that was the original example). Or "Ignore unknown JSON keys" as a modern equivalent.

What's harmful is: Accept an ill defined superset of the valid syntax and interpret it in undocumented ways.

[treve]

Good modern protocols will explicitly define extension points, so 'ingoring unknown JSON keys' is in-spec rather than assumed that an implementer will do.

[tuetuopay]

Funny I never read the original example. And in my book, it is harmful, and even worse in JSON, since it's the best way to have a typo somewhere go unnoticed for a long time.

[yxhuvud]

I disagree. I find accepting extra random bytes in places to be just as harmful. I prefer APIs that push back and tell me what I did wrong when I mess up.

[n2d4]

Very much so. A better law would be conservative in both sending and accepting, as it turns out that if you are liberal in what you accept, senders will choose to disobey Postel's law and be liberal in what they send, too.

[mikestorrent]

It's an oscillation. It goes in cycles. Things formalize upward until you've reinvented XML, SOAP and WSDLs; then a new younger generation comes in and says "all that stuff is boring and tedious, here's this generation's version of duck typing", followed by another ten years of tacking strong types onto that.

MCP seems to be a new round of the cycle beginning again.

[Gigachad]

The modern view seems to be you should just immediately abort if the spec isn't being complied with since it's possibly someone trying to exploit the system with malformed data.

[esafak]

I think it is okay to accept liberally as long as you combine it with warnings for a while to give offenders a chance to fix it.

[hdjrudni]

"Warnings" are like the most difficult thing to 'send' though. If an app or service doesn't outright fail, warnings can be ignored. Even if not ignored... how do you properly inform? A compiler can spit out warnings to your terminal, sure. Test-runners can log warnings. An RPC service? There's no standard I'm aware of. And DNS! Probably even worse. "Yeah, your RRs are out of order but I sorted them for you." where would you put that?

[dotancohen]

The Python 3 community was famously divided on that matter, wrt Python 3. Now that it is over, most people on the "accept liberally" side of the fence have jumped sides.

[psnehanshu]

Warnings are ignored. It's much better to fail fast.

[wolrah]

Warnings only work if the person receiving them is either capable of and motivated to do something about it, or capable of motivating the person/people capable of doing something about it.

A weak warning that's just an entry in a scrolling console means nothing to end users and can be ignored by devs. A strong warning that comes out as a modal dialog can still be ignored by devs and then just annoys users. See the early era of Windows UAC for possibly the most widespread example of a strong warning added after the fact.