I dunno about slide 39 - where calling f(x) may or may not give x a value. From a code readability standpoint, a reader of the codebase needs to be wary of every function.

I see the issue is that iff you explicitly marked it as inout, it would limit the code from a logic unification point of view. I guess this is already explored in prolog, which I have little experience with - but it feels like it would inherently limit the ability to scale a codebase to huge sizes. Maybe a combination of convention and naming would solve the problem.