Comment by simpaticoder

I don't get it. Isn't this a runtime problem and not a compile-time problem? buy() or sell() is going to be called with dynamic parameters at runtime, in general. That is, calls with concrete values are NOT going to be hard-coded into your program. I would write the function to assert() invariants within the function, and avoid chasing compile-time safety entirely. If parameter order was a concern, then I'd modify the function to take a struct, or similar.