> In contrast, layers of compilers tend to be rather loosely coupled, so you don't have to think about parsing at all while working on semantic analysis. I agr

> In contrast, layers of compilers tend to be rather loosely coupled, so you don't have to think about parsing at all while working on semantic analysis. I agree, this is a illustrative example of data coupling: Very loose, easily observable and there are even nice visualization techniques for certain steps. Each can be understood and tested in isolation. Also how vertical is the "vertical" approach really? In a first iteration you ought to lay the foundation of each component. Later you just add additional branches/structures so to speak. This is fine, but I think there is a minimal sub-language that you want to implement on the first iteration, that is sufficiently small to not to get lost in the more hairy details, but large enough so it presses you to find a 'workable flexibility' for each of the components.