I've always tried to contribute to open source software whenever I've seen the opportunity, but becoming a maintainer of an open source library gave me some completely new perspectives.

I've used Emacs for almost a decade, and I used Vim before that. Maybe the fact that vi key bindings are just another package in Emacs, called Evil, was one of the reasons I switched to Emacs. I like modal editing, but I don't use Evil. Instead, I use God mode with Emacs key bindings.

At some point, I discovered Doom Emacs, which packed a lot of functionality that interested me compared to other Emacs distributions.

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Another way to solve the expression problem in C# and other object-oriented languages is by using object algebras^[1]. Object algebras define a parameterised interface, and a factory that implements that interface.

First, let's define the interface of the Eval operation.

public interface IEval
{
    double Eval();
}

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Let's explore how extension methods in C# can solve the expression problem. For an introduction to the expression problem, refer to my first post in this series. Extension methods allow operations to be defined over a type without modifying the type's original definition.

First, we define IExpr as a marker interface and implement it in the Const and Add types. The implementations of Const and Add here are identical to those in the previous post.

public interface IExpr { }

public class Const : IExpr
{
    public double Value { get; }

    public Const(double value) =>
        Value = value;
}

public class Add : IExpr
{
    public IExpr Left { get; }
    public IExpr Right { get; }

    public Add(IExpr left, IExpr right) =>
        (Left, Right) = (left, right);
}

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Programmers are always defining types and operations to use these types. It's the essence of developing features in working software. The expression problem^[1] asks how easy it is to define types and operations in a given programming language or paradigm, and is stated as follows:

"The goal is to define a datatype by cases, where one can add new cases to the datatype and new functions over the datatype, without recompiling existing code, and while retaining static type safety (e.g., no casts)."

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