Let’s put your uncertainty to rest: at the extreme, any function execution spends both time and energy, both of which are observable side-effects. So yes, you’

Let’s put your uncertainty to rest: at the extreme, any function execution spends both time and energy, both of which are observable side-effects. So yes, you’re right that there no such thing as an absolutely pure function. “Pure” always assumes that all dependencies are pure themselves. Where it’s a reasonable assumption and whether it’s still useful or not depends on your program: assuming an API call to be pure is certainly not reasonable for many use cases, but it is reasonable enough to be useful for others.