[rcfox]

The Linux shell command 'touch foo' is idempotent, but not side-effect free.

Reading a variable shared between threads without a lock is side-effect free but not idempotent.

[zomglings]

Interestingly, `touch` is not idempotent because it modifies timestamps. Not being pedantic, just an interesting consideration. `mkdir -p` is idempotent, I believe.

[mytailorisrich]

You're absolutely correct, it's not idempotent for the crucial reason you mention. It's crucial because updating the timestamps is the main purpose of 'touch' in the first place!

[omegalulw]

Reading a variable isn't side-effect free either :P

[rcfox]

In the sense that you're moving data in to a register and updating the program counter?

[sokoloff]

And potentially changing the contents of processor caches.

[zomglings]

Opening Schrodinger's box isn't side-effect free. :)