Note that this also happens pretty much whenever anybody has any sort of template system that they want to support substitution into. You decide that you want t

Note that this also happens pretty much whenever anybody has any sort of template system that they want to support substitution into. You decide that you want to support substitution and suddenly you are building static analysis and documenting and writing syntax highlighting and all this mess. So for example when you want to create a validation system for something, and then you realize that you want to have generics/type-variables/polymorphism, you want validation containers. Bam, the complexity suddenly shoots up! Or maybe you just want to let someone configure with yaml, and then you find yourself with substitutions and substitutions in substitutions and all that mess. Why? The abstract reason is that the underlying mathematics of template substitution is lambda calculus, which is Turing complete. It's hard to learn lambda calculus because it's very abstract, it takes place in a mathematical ideal realm where nothing happens, but this also means that the same pattern reappears in a whole bunch of different contexts, and it is best if we just regard it as a design pattern and conscientiously adopt it. “Oh, this is a Lisp, I have seen the 20 line lisp-in-lisp evaluator, let's just code that in whatever host language and someone else will have thought of the details.”