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I've heard Idempotency confused with Consistency. Idempotency is where a function f can be applied (as a composition) multiple times and it gives the same resul

I've heard Idempotency confused with Consistency. Idempotency is where a function f can be applied (as a composition) multiple times and it gives the same result, so Idempotency: f(f(x)) = f(x). Whereas Consistency: f(x) = f(x) = f(x). An example of an idempotent function f is RaiseToThePowerZero(x) on x > 0. We usually take consistency for granted, so it's perhaps better to think of an example of a non-consistent function f which would be RandomNumberGenerator().

JoeAltmaier 7y ago
Better examples: Write("filename", position, data) is idempotent; Append(handle, data) is not.
- riazrizvi 7y ago
  Write("filename", position, data) inputs 3 arguments and outputs probably only 1, a file. So is not a function that you can convolute, you cannot pass the output of Write() as the input to another Write(). So instead, f: SetFirstWordOfFileToZeros(file)=>file, would be a better example if we want file functions. Then SetFirstWordOfFileToZeros(SetFirstWordOfFileToZeros(file))=SetFirstWordOfFileToZeros(file).
  - JoeAltmaier 7y ago
    Yes mathematically composability is part of it. That's because math functions return the same type. Its easy to express the 'state' of a function as the value of the operation. There's a wider idea of idempotency in computing - that state changes in general respond to operations. Repeating the same function more than once (because the network stuttered or the sender resent the message) and arriving at the same state is a common example.
mehrdadn 7y ago
> as a convolution Sorry, what does idempotency have to do with convolution?
- riazrizvi 7y ago
  Should be composition, not convolution, my brain fart.
stefco_ 7y ago
If you count the filesystem stats on a file as part of the system state, it is certainly not idempotent. You need to be aware of that effect on system state before you can decide whether to ignore it. Idempotency requires that the function map from a space back onto itself. Touch is only roughly idempotent in that you can define a statistic S on the overall system state discarding filesystem stats such that S(f(f(x))) = S(f(x)).

JoeAltmaier 7y ago

Better examples: Write("filename", position, data) is idempotent; Append(handle, data) is not.

riazrizvi 7y ago
Write("filename", position, data) inputs 3 arguments and outputs probably only 1, a file. So is not a function that you can convolute, you cannot pass the output of Write() as the input to another Write(). So instead, f: SetFirstWordOfFileToZeros(file)=>file, would be a better example if we want file functions. Then SetFirstWordOfFileToZeros(SetFirstWordOfFileToZeros(file))=SetFirstWordOfFileToZeros(file).
- JoeAltmaier 7y ago
  Yes mathematically composability is part of it. That's because math functions return the same type. Its easy to express the 'state' of a function as the value of the operation. There's a wider idea of idempotency in computing - that state changes in general respond to operations. Repeating the same function more than once (because the network stuttered or the sender resent the message) and arriving at the same state is a common example.

mehrdadn 7y ago

> as a convolution Sorry, what does idempotency have to do with convolution?

riazrizvi 7y ago
Should be composition, not convolution, my brain fart.

stefco_ 7y ago

If you count the filesystem stats on a file as part of the system state, it is certainly not idempotent. You need to be aware of that effect on system state before you can decide whether to ignore it. Idempotency requires that the function map from a space back onto itself. Touch is only roughly idempotent in that you can define a statistic S on the overall system state discarding filesystem stats such that S(f(f(x))) = S(f(x)).