Why OOP when you can NOOP?

gistfile1.d

private auto typeDispatch(T, Cs...)(T value, Cs callbacks) {
    /*static*/ foreach(i, C; Cs) {
        if((cast(ParameterTypeTuple!C[0]) value) !is null) {
            return callbacks[i](cast(ParameterTypeTuple!C[0]) value);
        }
    }
    throw new SemanticError("Can't compile that!");
}

// ...

    return typeDispatch(
        e,
        (Variable var) {
            // Do stuff with var...
        },
        (Application app) {
            // Do stuff with app...
        },
        (Expression expr) {
            // Do stuff with expr...
        });