Tuesday, July 31, 2007

Choosing a programming language: Is type safety worth it?

I'm a big fan of the strongly typed language vs weakly typed language debate. It's old, but it's also important.

I'm revisiting this topic because I'm trying to decide exactly that problem on a project that I'm working on. In my case, I'm torn between client side JavaScript on the web browser and Google Web Toolkit (GWT) compiled to JavaScript. The only reason I have to mention this is because I want to eliminate the quality of the libraries from the debate. GWT has fairly rudimentary libraries, but they're growing, and you can add JavaScript libraries if you really have to.

I'm going to try to have a debate with myself about it. I can't claim that I'm unbiased, and I'm going to ignore arguments that I don't consider significant. I'd love to hear further arguments from you in the comments.

Argument for weak typing

Weak typing allows you to write code faster. Well, that's what the proponents of it claim, anyway. I haven't seen any good measurements of this. There's certainly less typing to do, though. You don't have to declare the type of your variables. (In JavaScript, you do have to declare your local variables, however.) (It is worth pointing out that I have never written software and been limited by my typing speed. I'm skeptical of this argument.)

If you're designing a scripting language, then weak typing is easier to explain to inexperienced programmers. There's a whole step that doesn't have to be done. In some circumstances, this is a clear win. (In my case, this is irrelevant.)

Argument for strong typing

Strong typing allows the compiler to do more work for you. Essentially, you're constraining how variables can be used, and the compiler can use those constraints to detect errors or perform optimizations.

The optimization part is important for a few people. The error detection is great for everyone.

Counterargument from weak typing.

Unit testing is the weakly typed solution to error checking. (It's also the strongly typed solution, but for a different class of error.) It's either hard or impossible to write a unit test that checks that function arguments have the correct properties before going into a function. However, you can write a unit test that ensures that functions behave correctly in a variety of different circumstances. That's what programmers really care about, anyway.

Also, suppose we want to have a function that supports multiple types? In a strongly typed language like Java, we can only support different types if they share an interface. In a weakly typed language, we can support multiple types if they support the same operations. If I want a function that adds all the elements of a collection together, I can write one function that supports both strings and complex numbers correctly. (This is possible with generic programming as well in a strongly typed language, but that's not really what we're arguing about here.)

Rebuttal from strong typing

Unit testing is a good guarantee, and it's one that's required by strongly typed languages as well. However, it's a fundamentally different guarantee than you get by having the compiler check the types of variables as they're used by the code. First, a compiler checks every line of code even if it's hard to get to or you forgot to write one of the unit tests. Second, the combination of tests and strong typing is a better guarantee than tests alone.

Here's a Ruby example of some dangerous code:

function foo( a, b )
  if a < 0.0
    print( b )
foo( -12, 'This string is printed' )
foo( 5 )

This code has two "overloads". One takes a string and a negative number. The other takes a non-negative number and might take a string. The problem is, there is no good way to tell this based on the function signature. You can only tell by looking at the implementation. This means that you can't safely change the implementation of your function.

Unit testing partially guarantees that functions have working implementations. Strong typing partially guarantees that they are used correctly.


I'm going to go with Google Web Toolkit, which imposes strong typing on top of JavaScript. The mediocre library support may bite me later, but I'll be happier knowing that my compiler knows a bit about what my code will do.

I'm hoping I get flamed by someone, so please post your comments!

This originally appeared on PC-Doctor's blog.

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