One nice thing that can be done with pure functional programming (not sure what - if any - languages or runtimes implement it) is the caching of the results of

One nice thing that can be done with pure functional programming (not sure what - if any - languages or runtimes implement it) is the caching of the results of expensive calls. If there's any escape hatch to a method/function there's no guarantee that the result of f(q) will be a constant.