When we say “pure functions are free” out loud, of course it seems categorically false, and I don’t think anyone has ever credibly argued for this claim. And yet I think it is an implicit assumption of a lot of functional programming development, specifically that which attempts to build “referentially transparent” systems on top of an imperative, impure systems and languages, rather than starting with a new language from scratch.

My example of React.js is the pinnacle of this, all features like hooks, contexts, and concurrent mode only make sense when they estimate the cost of re-executing pure functions to be free, or at least negligible, and yet real-world experience indicates this is definitely not the case.

> One benefit (with respect to the CPU) of pure functions is that they can be cached because you know you'll get the same result for each call (of course, this impacts memory), which you can't guarantee with impure functions. But that's a tradeoff of memory for time.

In my personal experience, I’ve become pretty skeptical of trading memory for execution time. Especially in garbage-collected languages, what you save in execution time you lose in GC pauses, so the end result is you get neither.