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Comment by inopinatus | Hacker News Reader

> I should have used "side-effect-free" instead of "idempotent" in my tweets

The HTTP term is "safe method". Although you weren't even wrong because section 4.2.2 of RFC7231 (i.e. HTTP) defines all safe methods, including GET, as idempotent.

I think they use this language because nothing is truly side-effect free. In fact GETs can have side-effects, the most obvious of which is writing the fact of it to a logfile, and that's the most harmless side-effect of all, right until you run out of disk space.

Being a language arse I think the high precision descriptor is actually nullipotent. https://en.wiktionary.org/wiki/nullipotent but I'd never say it out loud.

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'Side-effect free' means that doing it once, twice or n >= 3 times (with same parameters) yields the same result, i.e. what it returns doesn't depend on any remote state that is altered by the call itself.

However, an idempotent HTTP call is certainly not a pure function which some people seem to be mixing up. Pure functions don't work with I/O.

REST is bit more specific and explicitly requires GET to be nullipotent which really means "effect free" - it just reads and doesn't alter the state on the remote system at all.

Side-effects like log files, rate-limiting, etc. will always exist, but they do belong to a different 'layer', so to speak. That is, these should be unobservable side-effects (also think about minuscule effects on the power grid, the fact that a request might write something to an ARP-cache, etc. - they all happen at different layers, so the quantum world state keeps changing, but that's not what this is about). Whether an X-Request-Count header violates the requirements or not depends on interpretation. From the garage door perspective, I wouldn't care...

> Pure functions don't work with I/O.

there is a comment above that argue this point in better details.

https://news.ycombinator.com/item?id=16966046

Basically it depends on the many nuances you have on "pure", "functions" and "idempotent"

When discussing HTTP behaviours I think it’s easiest to stick to the definitions of these terms given in the HTTP standard. Anything else is fruitless bikeshedding.

IMO "side-effect free" is always a statement constrained by the operating level of abstraction. Logging is not an effect at the level of the application, but rather some subset of it's context (system, db).

I love that term and am going to use it as much as possible

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