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Why Is "Design by Contract" Not More Popular?
Coryoth writes "Design by Contract, writing pre- and post-conditions on functions, seemed like straightforward common sense to me. Such conditions, in the form of executable code, not only provide more exacting API documentation, but also provide a test harness. Having easy to write unit tests, that are automatically integrated into the inheritance hierarchy in OO languages, 'just made sense'. However, despite being available (to varying degrees of completeness) for many languages other than Eiffel, including Java, C++, Perl, Python, Ruby, Ada, and even Haskell and Ocaml, the concept has never gained significant traction, particularly in comparison to unit testing frameworks (which DbC complements nicely), and hype like 'Extreme Programming'. So why did Design by Contract fail to take off?"
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In a phrase, the niche is professional coders, rather than hack-a-day cowboys.
.NET or the JVM. Compilers and analysers can use such contracts to perform extra typechecking of e.g. arguments to methods, where the methods have preconditions. "Runtime cost" doesn't exist if the runtime has support, especially where verification also exists. Consider the old conundrum: do you check your parameters, or do you expect your caller to check the arguments before they're passed? What if the parameters being wrong leads to a subtle error that manifests later rather than a dramatic failure? With properly stated contracts, these checks can be inserted at the lowest level and automatically propagated higher statically by the compiler or dynamically by the runtime as necessary.
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Assertions, whether invariants, preconditions or postconditions, can be viewed as extensions to the type system [1]. Since they are expressions of a boolean type and don't modify state (at least, no sane ones would mutate state), they're very easily analysed.
Combine that with a runtime like
The niche that DbC is supposed to fill is formalising what is already recognised good practice - stating your assumptions while writing your code. If you have much experience as a programmer, you'll know that it's useful to use assert, ASSERT, or some other feature of your environment at choice places where you make certain assumptions that you think can't be broken. And if you've read The Pragmatic Programmer, you'll know why you'll want to leave those assertions in the final release.
Most popular languages, such as C#, Java, C++ etc., capture very little programmer intent in their type systems. There's a lot more potential there - preconditions and postconditions can e.g. state that a variable will be in a certain range dependent on the values of other variables, and then automatically detect range mismatches with e.g. loop invariants. Basically, assertions are really useful extensions to the type system.
[1] For example, consider that a 'requires x != null' constraint and a notional non-nullable reference type from C# or Java, analogous to 'MyType!' in C-w (C Omega):
http://blogs.msdn.com/cyrusn/archive/2004/06/03/1
Two reasons: first, the more people use it, the wider it will be supported. It's nice to be involved with something on the upswing that more people are interested in than bored with. Second, it's nice to know why something's unpopular. Maybe it's hard to use, but you're a genius and will be able to put it to work. Or, maybe, everyone else realized that it's awful and moved on to something less heinous.
Dewey, what part of this looks like authorities should be involved?
assert(condition) is your friend. It's not called a contract, it's not design (but a very good practice!) and it does the job well.