[bam365]

Not an expert on spec, but I suspect that there are a lot of data that are not representable with combinations of predicates: GADTs, rank-n types, existential types, type families, data families, and other more advanced type system features.

Either way, the tradeoff is this: statically typed languages are guaranteed to be free of type errors, but no such guarantees can be made with runtime assertions. Whether or not you use a tool like spec to help you make those assertions, the fact that they happen at runtime prevents them from making any guarantees about type safety. Even if you use spec literally everywhere, as you suggest (and no one actually comes close to doing anyway), you are still not guaranteed of type safety. Dijkstra explains why: https://www.cs.utexas.edu/users/EWD/transcriptions/EWD03xx/E...