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Comment by pards | Hacker News Reader

It's not idempotent - the same source URL entered multiple times yields different shortened URLs. Not sure if that's a feature or a bug ...

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That just means it’s not a well-defined function (i.e. each input is mapped to only one unique output, which is not the case here).

As other posters write, “idempotent” means that recursively applying the function returns the same result, e.g. f(f(x)) = f(x). (The site is also indeed not idempotent, since entering a shady URL will not return the same shady URL.)

In particular, it's not well-defined because it's nondeterministic/randomized.

Coincidentally I like your username.

Today I learned the mathy definition of idempotent (used above) which is different than the one more commonly used when talking about code: https://en.m.wikipedia.org/wiki/Idempotence

It’s not clear to me what the difference in definition is - even reading the CS definition on Wikipedia, both cases reference the mathematical definition. What am I missing?

In the imperative sense of idempotent, a method is idempotent with respect to system state, not just its arguments.

It's about repeated calls e.g.

    a.f(x).f(x)

results in the same state as

    a.f(x)

Not about repeated application. E.g.

    f(f(x)) = f(x)

Right. If you think about it as a function of the state itself, the 2 definitions agree. If the function is `f(x, state) -> (y, state')`, then f is "imperitively idempotent" when `g(state) = f(fixedX, state).state'` is "mathematically idempotent"

From your link:

> the property of certain operations in mathematics and computer science whereby they can be applied multiple times without changing the result beyond the initial application

Perhaps my maths background is influencing me here, but isn't that the commonly used definition when talking about code?

Not the replied-to user, but I also wasn't aware of the mathematical definition and my understanding of idempotency was that the main point was that performing the same idempotent operation multiple times wouldn't make a difference in terms of __side effects__ (for instance, not charging a user twice when accidentally submitting an idempotent request twice).

Yep, this is the meaning I was referring to.

The function f is idempotent, if f(x) = f(f(x)).

For this URL shortener, if http://www.5z8.info/super-nsfw_g0v1ba_hot-older-goats.mov (from above) points to http://shadyurl.com/, then shortening http://www.5z8.info/super-nsfw_g0v1ba_hot-older-goats.mov should also return http://www.5z8.info/super-nsfw_g0v1ba_hot-older-goats.mov. It's not enough for it to always return the same URL for the same input (which it also doesn't appear to do).

Seems like a feature since it says try again when you create a link.

Since I typically find short urls suspicious and frightening, especially in text messages from short phone numbers not in my contacts, this headline illustrates a third form of idempotentcy.

Not deterministic then

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