If you were trying to implement a language with assignment statements like: x := y + z or f(y + z) or g(f(x, y)) you'd need to be able to evaluate e

If you were trying to implement a language with assignment statements like: x := y + z or f(y + z) or g(f(x, y)) you'd need to be able to evaluate expressions first. Maybe it's just simpler to illustrate a smaller language first?