To that end, there are type systems in some languages (turnstile in racket comes to mind) that are implemented entirely using macros. Turnstile isn't enough to

To that end, there are type systems in some languages (turnstile in racket comes to mind) that are implemented entirely using macros. Turnstile isn't enough to model certain type systems (for reasons too complex to reasonably cover here), but it's interesting to see that types-as-macros-over-the-untyped-lambda-form has been done to a reasonable degree of success.