They show the “type-safe printf” example there, but they don't seem to explain anywhere, if I can write my own type-safe variadic function or not. Is printf a b

They show the “type-safe printf” example there, but they don't seem to explain anywhere, if I can write my own type-safe variadic function or not. Is printf a built-in that is built with magic not allowed for mere mortals or do they just not dive into the topic because it's complex? Can any F# affectionados elaborate? Also, they only talk about units of measurement in the context of floats. Can I not have integers or rationals with units?