In theory, practice is the same as theory. In practice, it isn't. I've seen only toy examples of some non-trivial program properties being encoded in type systems. It nowhere constituted proof of an entire program. Nobody actually does that. A lot can remain wrong in code which passes type checks. There is no "if it compiles, ship it". Not to mention that compilers can have bugs, and that hidden performance and resource problems can lie latent in high level languages.