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Showing posts from November, 2010

Abstracting Computation: Arrows in C#

I just committed an implementation of Arrows to my open source Sasa library. It's in its own dll and namespace, Sasa.Arrow, so it doesn't pollute the other production quality code.

The implementation is pretty straightforward, and it also supports C#'s monadic query pattern, also known as LINQ. It basically boils down to implementing combinators on delegates, like Func<T, R>. It's not possible to implement the query pattern as extension methods for Func<T, R> because type inference fails for even the simplest of cases. So instead I wrapped Func<T, R> in a struct Arrow<T, R>, and implemented the query pattern as instance methods instead of extension methods. This removes a number of explicit type parameters that the inference engine struggles with, and type inference now succeeds.

Of course, type inference still fails when calling Arrow.Return() on a static method, but this is a common and annoying failure of C#'s type inference [1].

What is thi…

Factoring Out Common Patterns in Libraries

A well designed core library is essential for building concise, maintainable programs in any programming language. There are many common, recurrent patterns when writing code, and ideally, these recurring uses should be factored into their own abstractions that are distributed as widely as possible throughout the core library.

Consider for instance, an "encapsulated value" pattern:
/// <summary>
/// A read-only reference to a value.
/// </summary>
/// <typeparam name="T">The type of the encapsulated value.</typeparam>
public interface IValue<T>
{
/// <summary>
/// A read-only reference to a value.
/// </summary>
T Value { get; }
}
This shows up everywhere, like Nullable<T>, Lazy<T>, and Task<T> (property called "Result"), IEnumerator<T> (property called "Current"), and many many more.

However, the common interface of a value encapsulated in an object has not been factored out into a common …