Type systems are, in essence, automated mathematical theorem provers designed to prevent certain classes of bugs.[1] The functional style is also a bug-reducer in that you can reason more accurately about the state of your program, because you have limited state-changing code to particular places. Certain language features like Lisp's restarts or garbage collection are bug-reducing because they give you concise ways of expressing features you might otherwise have had to implement by hand, and every line of code you write is another line for a bug to hide in.
Yes, testing is invaluable and should not be omitted. No, none of these are silver bullets that eliminate the need for testing. But it is false to say that "...the only economically practical way to reduce bugs is to test thoroughly." There are many ways to reduce bugs that can help alongside testing.
[1]: If you've only ever used C++ or Java, this sounds hilariously weak, but in languages like the ML, the type system prevents null pointer exceptions, and in Haskell, the type system goes further and separates effectful and non-effectful code to ensure you don't cause side effects where you don't expect. Even more powerful are languages like ATS, which give you compile-time type errors when you've forgotten to allocate space for a null terminator for strings.
Yes, testing is invaluable and should not be omitted. No, none of these are silver bullets that eliminate the need for testing. But it is false to say that "...the only economically practical way to reduce bugs is to test thoroughly." There are many ways to reduce bugs that can help alongside testing.
[1]: If you've only ever used C++ or Java, this sounds hilariously weak, but in languages like the ML, the type system prevents null pointer exceptions, and in Haskell, the type system goes further and separates effectful and non-effectful code to ensure you don't cause side effects where you don't expect. Even more powerful are languages like ATS, which give you compile-time type errors when you've forgotten to allocate space for a null terminator for strings.