Design by Contract Part 2

This is a continuation of thoughts after reading Design by Contract by Example by Richard Mitchell and Jim McKim. This time I want to write a little about design by contract without direct language support. What do you give up without language support, and is it still worth it to struggle through defining contracts without that support?

Design by Contract Part 1

Benefits of Language Support

These are some of the benefits of having language support for design by contract:

  • Automatic invariant checking upon exiting methods, without any extra effort like calling a helper routine or creating some kind of automatic object that checks invariants when it goes out of scope.
  • An “old” keyword that provides easy syntax for referencing the state of variables as they were before the current method began executing.
  • Contracts are automatically cumulative down an inheritance tree. In other words, if the base class has a contract, any subclasses automatically have that contract.
  • Related to the point above, language support can also help ensure that contracts on overridden methods follow the rules of contract inheritance: preconditions can only be the same or weaker, and postconditions can only be the same or stronger.
  • Contracts can be part of declarations. For example, if you are trying to simulate DBC in C++, you can’t put the contracts in the header file except as comments.
  • Contracts will probably be understood by a debugger, which might be more convenient than debugging something like the output of a preprocessor in the absence of language support.

Languages Supporting Design By Contract

Obviously Eiffel supports design by contract. I haven’t personally used Eiffel for a programming project. There is, though, a version of Eiffel that compiles to MSIL, so if you’re a .NET shop, maybe it’s a real possibility.

Another language with contract support is the D Programming Language. D is kind of a cool language in its own right. It has a lot of the conveniences of “modern” languages like Java and C#, but it compiles to native binary code instead of an intermediate code requiring a big run-time environment to execute. It is still a bit immature, and lacks much industry support, but I kind of wish it would catch on. The creator of the language, Walter Bright, has written an excellent essay about the design by contract support, comparing it to how one would attempt similar functionality in C++.

Preprocessors and Libraries

In the book, Mitchell and McKim give examples in Java using a free preprocessor called iContract. It lets you do DBC by writing special Javadoc comments which the preprocessor then converts to Java code. I’m not sure what the current status of iContract is, though, since the book’s reference to iContract’s source, http://www.reliable-system.com, has a big for sale sign on it. Links to the book’s own web site are also broken, though if you’re persistent you can find the publisher’s site for the book that has even more broken links on it. There’s an iContract Ant task, but I’m not sure how to get a hold of the preprocessor itself such that the task will work. The Ant documentation points to the same broken links as elsewhere. 

For C++, the best DBC library I’ve come across was posted on The Code Project by Jarl Lindrud. I haven’t had a chance to use it, but it looks promising. It makes heavy use of the magical Boost bind and lambda modules to pretty good effect.

Adding Language Support

There is research going on to add DBC facilities to existing languages. There is a proposal to extend standard C++ to provide contract support. It adds preconditions and postconditions to methods and invariants to classes and namespaces. Here is an example from the proposal:

int factorial( int n ) 
precondition 
{ 
    0 <= n && n <= 12; 
} 
postcondition( result ) 
{ 
    result >= 1; 
} 
{ 
    if ( n < 2 ) 
        return 1; 
    else 
        return n * factorial( n - 1 ); 
}

The proposal also includes an oldof keyword to support writing postconditions.

In addition to C++, there is also work being done to potentially add contract support to C#. Spec# is the name of the extended language. The researchers claim that contracts in Spec# will be more robust than those in Eiffel, handling some situations that that language didn’t do so well. One really cool aspect of Spec# is its integration with Visual Studio. It does static contract checking on the fly. If you type code that violates a contract, you get the red squiggle. You can download the current implementation from the project page and take it for a spin. In my brief experience, there were some more complicated constructs than are found in Eiffel (such as a concept known as “exposure”), and the documentation was virtually non-existent, but I still remain excited about the potential.

Syntactically Spec# has some nice features. One of the most common preconditions is that some argument is not null. Spec# expresses this with a single character, an exclamation point:

public static void Clear(int[]! xs) 
{ 
  for (int i = 0; i < xs.Length; i++) 
  { 
    xs[i] = 0; 
  } 
}

In this example, you can safely dereference xs without checking for null because the exclamation point in the method signature guarantees that it is non-null.

Spec# also uses keywords requires and ensures, which feel more natural to me than precondition and postcondition, or even Eiffel’s imperative require and ensure. Here’s the C++ example from above translated to Spec# (possibly even correctly?):

public static int factorial( int n ) 
  requires 0 <= n && n <= 12; 
  ensures result >= 1; 
{ 
    if ( n < 2 ) 
        return 1; 
    else 
        return n * factorial( n - 1 ); 
}

There was an MSDN webcast last year on Spec# that I sat in on that gave a good overview of where the project is going, as well as how it is addressing problems beyond what Eiffel does. It is now archived for on-demand viewing.

Conclusion

If you don’t have language support, or some kind of library or preprocessor, you could at the very least assert preconditions. That’s the advice of Mitchell and McKim (see p.205). Much of the benefit of contracts comes from simply thinking about them. Sometimes I’ve thought that the behavior of a function I’m writing is obvious until I stop to consider what the function requires and what it ensures. It adds a level of precision to your work to do that extra thinking.

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