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.

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tuetuopay 9 hours ago | parent | next [-]

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.

	▲	sweetjuly 7 hours ago \| parent [-]
		The original example is very common in ISAs at least. Both ARMv8 and RISC-V (likely others too but I don't have as much experience with them) have the idea of requiring software to treat reserved bits as if they were zero for both reading and writing. ARMv8 calls this RES0 and an hardware implementation is constrained to either being write ignore for the field (eg read is hardwired to zero) or returning the last successful write. This is useful as it allows the ISA to remain compatible with code which is unaware of future extensions which define new functionality for these bits so long as the zero value means "keep the old behavior". For example, a system register may have an EnableNewFeature bit, and older software will end up just writing zero to that field (which preserves the old functionality). This avoids needing to define a new system register for every new feature.

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treve 10 hours ago | parent | prev | next [-]

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

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yxhuvud 9 hours ago | parent | prev [-]

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.