Thoughts on programming by composition

Programming by composition is about putting together a large, complex program from small pieces. In the past, when we talk about composition in the object-oriented world, we declare interfaces. We then pass objects from classes that implement interfaces to anyplace that accepts these interfaces. However, there are some limitations in this approach:

  1. Interfaces live in namespaces. So except for a few well known interfaces, most interfaces live in their private namespaces so components have to be explicitly aware of each other to compose.
  2. Interfaces with multiple members are often too big a unit for composition. We often desire smaller composition units.

Composition is much easier in the functional world because the basic units are functions (or lambdas). Functions do not have namespace and functions are smallest units possible. For as long as the function signature matches, we can adapt them together.  In addition, some functional language relies on remarkably smaller number of types, for example, Cons in Scheme. Is there a middle ground we can do the same in C# which is an object-oriented language with some functional features?

As the Linq library has demonstrated, types implementing IEnumerable<T> can be easily composed in a fluent style. In additional, my blog on Linq to graph shows that some more complicated data structures can be flattened to IEnumerable<T> and thus consumed by Linq. This takes care the problems where T is a simple type or a well known type. For more complicated data types, a type that implements IDictionary<string, object> can store arbitrary complex data structure as demonstrated in scripting languages like Javascript. Many data sources, such as a record from a comma separated file, or IDataReader are naturally Dictionary<string, object>. Many outputs of the programs such as JSON and XML are also IDictionary. The remaining question is whether we can use IDictionary in the heart of the programs.

Unfortunately, C# code to access dictionary is ugly; there are currently no syntactic sugars to make them pretty (while VB.NET has). A combination of C# dynamic keyword and DynamicObject is the closest thing to make IDictionary access pretty but DynamicObject carries a huge overhead. So the solution that I propose is to create a strongly-typed wrapper over the underlying dictionary when strong-type is needed. This is not a new idea; strongly-typed dataset already uses it. We can in fact use C# dynamic to duck-typing to strongly-typed wrapper from any namespaces. The duck-typing code generated by C# compiler is far more efficient than DynamicObject.  It is fairly easy to generate strongly-typed wrapper code from the meta data and templates and I intent to demonstrate that in my later blog posts.

For an example of IDictionary implementation over delimited file record, see the Record class in SkyLinq. For an example of strongly-typed wrapper, see the W3SVCLogRecord class. I do not have many composition example yet, but I promise that as the SkyLinq project unfold the entire system would build on composition.

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