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Strong versus Weak Typing
A Conversation with Guido van Rossum, Part V
by Bill Venners with Frank Sommers
February 10, 2003

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Summary
Python creator Guido van Rossum talks with Bill Venners about the robustness of systems built with strongly and weakly typed languages, the value of testing, and whether he'd fly on an all-Python plane.

Guido van Rossum is the author of Python, an interpreted, interactive object-oriented programming language. In the late 1980s, Van Rossum began work on Python at the National Research Institute for Mathematics and Computer Science in the Netherlands, or Centrum voor Wiskunde en Informatica (CWI) as it is known in Dutch. Since then, Python has become very popular among developers, who are attracted to its clean syntax and reputation for productivity.

In this interview, which is being published in six weekly installments, Van Rossum gives insights into Python's design goals, the source of Python programmer productivity, the implications of weak typing, and more:

In this installment, Van Rossum discusses the robustness of systems built with strongly and weakly typed languages, the value of testing, and whether he'd fly on an all-Python plane.

Bill Venners: I once asked James Gosling about programmer productivity. His answer, which I consider to be the strong-typers view of weak typing, was, "There's a folk theorem out there that systems with very loose typing are very easy to build prototypes with. That may be true. But the leap from a prototype built that way to a real industrial-strength system is pretty vast."

Guido van Rossum: The leap is also a folk theorem as far as I can tell. That the leap from prototype to system is so big is just as poorly documented as the ease of prototyping.

Bill Venners: Gosling also said, "Anything that tells you about a mistake earlier not only makes things more reliable because you find the bugs, but the time you don't spend hunting bugs is time you can spend doing something else." When I asked Josh Bloch about strong and weak typing, he said, "It's always beneficial to detect programming errors as quickly as possible."

Guido van Rossum: Of course. I'm not arguing with that.

Bill Venners: Josh Bloch continued, "There's no doubt that you can prototype more quickly in an environment that lets you get away with murder at compile time, but I do think the resulting programs are less robust. I think that to get the most robust programs, you want to do as much static type checking as possible."

That all sounds fine to me in theory. It makes sense that the sooner I find programming errors the better. Strong typing helps me find errors at compile time. Weak typing makes me wait until a runtime exception is thrown. The trouble is that I use Mailman, a mailing list manager written completely in Python. Mailman works fine. It isn't a prototype. It's an application, and it seems quite robust to me. So where does theory miss practice?

Guido van Rossum: That attitude sounds like the classic thing I've always heard from strong-typing proponents. The one thing that troubles me is that all the focus is on the strong typing, as if once your program is type correct, it has no bugs left. Strong typing catches many bugs, but it also makes you focus too much on getting the types right and not enough on getting the rest of the program correct.

Strong typing is one reason that languages like C++ and Java require more finger typing. You have to declare all your variables and you have to do a lot of work just to make the compiler happy. An old saying from Unix developers goes something like, "If only your programs would be correct if you simply typed them three times." You'd gladly do that if typing your programs three times was enough to make them work correctly, but unfortunately it doesn't work that way.

All that attention to getting the types right doesn't necessarily mean you don't have other bugs in your program. A type is a narrow piece of information about your data. When you look at large programs that deal with a lot of strong typing, you see that many words are spent working around strong typing.

The container problem is one issue. It's difficult in a language without generics to write a container implementation that isn't limited to a particular type. And all the strong typing goes out the door the moment you say, "Well, we're just going to write a container of Objects, and you'll have to cast them back to whatever type they really are once you start using them." That means you have even more finger typing, because of all those casts. And you don't have the helpful support of the type system while you're inside your container implementation.

Python doesn't require you to write the cast, and its containers are completely generic. So it has the plus side of generic containers without the downside. It doesn't have the plus side that the C++ folks claim to get with their templates and other generics. But in practice that mechanism turns out to be very cumbersome. Even compiler writers have difficulty getting templates to work correctly and efficiently, and the programmers certainly seem to have a lot of trouble learning how to use it correctly. Templates are a whole new language that has enormous complexity.

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