The Beauty of Closures

What's the big deal?

In the example, we've only seen a little bit of benefit in using a closure. Sure, we've separated out the control structure side from the logic required in the filtering itself, but that's not made the code that much simpler on its own. This is a familiar situation - a new feature often looks less-than-impressive when used in simplistic examples. The benefit that closures often bring, however, is composability. If that sounds like a bit of a stretch, I agree - and that's part of the problem. Once you're familiar with closures and possibly a little addicted to them, the connection seems very obvious. Until that time, it seems obscure.

Closures don't inherently provide composability. All they do is make it simpler to implement delegates (or single-method interfaces - I'll stick to using the term delegates for simplicity). Without some support for closures, it's easier to write a small loop than it is to call another method to do the looping, providing a delegate for some piece of the logic involved. Even with "just add a method to an existing class" support for delegates, you still end up losing the locality of the logic, and you often need more contextual information than it's easy to provide.

So, closures make it simpler to create delegates. That means it's more worthwhile to design APIs which use delegates. (I don't believe it's a coincidence that delegates were almost solely used for starting threads and handling events in .NET 1.1.) Once you start thinking in terms of delegates, the ways to combine them become obvious. For instance, it's trivial to create a Predicate which takes two other predicates, and represents either the logical AND or OR of them (or other boolean operations, of course).

A different sort of combination comes when you feed the result of one delegate into another, or curry one delegate to create a new one. All sorts of options become available when you start thinking of the logic as just another type of data.

The use of composition doesn't end there though - the whole of LINQ is built on it. The filter we built using lists is just one example of how one sequence of data can be transformed into another. Other operations include ordering, grouping, combining with another sequence, and projecting. Writing each of those operations out longhand hasn't been too painful historically, but the complexity soon mounts up by the time your "data pipeline" consists of more than a few transformations, In addition, with the deferred execution and data streaming provided by LINQ to Objects, you incur significantly less memory overhead than with in the straightforward implementation of just running one transformation when the other has finished. The complexity isn't taken out of the equation by the individual transformations being particularly clever - it's removed by the ability to express small snippets of logic inline with closures, and the ability to combine operations with a well-designed API. Conclusion

Closures are a little underwhelming to start with. Sure, they let you implement an interface or create a delegate instance pretty simply (depending on language). Their power only becomes evident when you use them with libraries which take advantage of them, allowing you to express custom behaviour at just the right place. When the same libraries also allow you to compose several simple steps in a natural manner to implement significant behaviour, you get a whole which is only as complex as the sum of its parts - instead of as complex as the product of its parts. While I don't quite buy into the idea of composability as the silver bullet against complexity which some would advocate, it's certainly a powerful technique, and one which is applicable in many more situations due to closures.

One of the key features of lambda expressions is the brevity. When you compare the Java code from earlier with the C# code, Java looks extremely clumsy and heavyweight. This is one of the issues which the various proposals for closures in Java aim to address. I will give my perspective on these different proposals in a blog post in the not-too-distant future.

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About the author

Jon Skeet United Kingdom

C# MVP currently living in Reading and working for Google.

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