I believe it was always the case. I can remember books just like this in the 70's and 80's, teaching one how to write a Pascal interpreter in C, or then a GUI

I believe it was always the case. I can remember books just like this in the 70's and 80's, teaching one how to write a Pascal interpreter in C, or then a GUI system in Turbo Pascal, for example, which felt very much like this kind of introduction/tutorial to both the language, and the application of the language itself to an interesting component. Is this a Go book or a Monkey/Interpreter book? Its neither, but both.