In principle, though, a smart compiler could take a program written in terms of pure functions and transform it into a program using mutable state for better pe

In principle, though, a smart compiler could take a program written in terms of pure functions and transform it into a program using mutable state for better performance. The advantage would be to start with a program about which you can make strong claims, and then break apply transformations that invalidate those claims, replacing them with weaker claims in a predictable, methodical fashion. Languages like C allow programmers to easily write programs that are hard to reason about mechanically, and the programmer can use that power to hand-tune the program to be very fast. The programmer can also use that power to crank out fragile programs without thinking through all of the edge cases. An ideal compiler working on pure functions could say "No, listen, write this so I can understand it, and then I'll start using shortcuts to make it run quick". Of course, even if your compiler is smart enough to actually beat competent hand-optimization, there are some negative side effects. If you have optimized a program by hand, you know what properties your optimizations rely on, and so you'll know when a new feature requires that code to be rewritten at a performance cost. You can take that cost into account making the decision about adding that feature. But if the optimization is done automatically without your involvement, you'll just see a sudden performance hit when you add a feature, and figuring out why will involve arcane knowledge of compiler internals.