My argument is that C++, a multi-paradigm language, benefits from not being bound to purity and that being able to mix imperative and functional code is a good thing. We aren't stuck in pure-land so we can insert IO where it needs to be, not where it propagates, but neither do we need to be stuck in state-land. Functional programming is not against the "nature" of the language, it gives us the freedom to choose.
> Because the first does not copy the container and its elements...
This is a quality of implementation detail. It would have been detrimental to the point of the article to waste time showing optimizations such as "if F is a function X to X, use the in-place version". But I assure you, my optimization testing has shown there is little difference between the code generated by this and vanilla <algorithm>. In fact, it is at times better.
https://github.com/splinterofchaos/Pure/blob/master/samples/...