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openmethods.d

Open methods are a simple, expressive, and efficient mechanism to dynamically dispatch functions depending on the type of one or more objects. They combine the advantages of virtual methods with those of the Visitor pattern while avoiding their disadvantages (such as tight coupling, intricate dependencies, awkward setup, and extensive boilerplate requirements). Open methods scale up naturally to multiple dispatch in a pay-as-you-go manner.

TL;DR

If you are familiar with the concept of open multi-methods, or prefer to learn by reading code, go directly to the synopsis

Introduction

This library implements fast, open multi-methods for D.

Imagine that you are writing a matrix math library. Matrices come in different subtypes - dense (i.e. ordinary), diagonal, tridiagonal, etc. Some operations can be implemented more efficiently for certain types - for example, transposing a diagonal matrix is a no-op; adding two diagonal matrices does not require adding the zeroes that are outside of the diagonals; etc.

At one point you want to write a demo program, which needs to output matrices to the console in a format that clearly identifies which category they belong to. For example, just print the diagonal for a diagonal matrix, or just the non zero elements for a sparse matrix. You need a polymorphic print function.

At this point you don't have any good option in traditional OO languages like D, Java, C++ and any others that follow the message passing metaphor, as initiated by Simula and popularized for the first time by Smalltalk.

The most obvious approach is to stick a virtual print function in the Matrix base class and override it in the subclasses. It will work but it has severe drawbacks. Different application may want to display matrices in different ways. Some applications may not need to display matrices at all - but they will still pull the print functions from your library, because of the way virtual functions are implemented.

Or you may resort on a "type switch": have the application test for each category and print accordingly. This is tedious, error prone and, above all, not extensible. Adding a new matrix subclass requires updating all the type switches.

Then there is the binary operation problem. To implement, say, matrix addition efficiently, you need to take the type of two objects into account. In addition to the problems described above, you will have to resort on hacks like double dispatch or - again - type switches.

This library neatly solves this problem. It brings you the flexibility and the power of open multi-methods, as found in languages like Lisp, Clojure, Dylan or TADS. They are fast too, comparable to ordinary virtual function calls. And, because they use compressed dispatch tables, the memory footprint remains reasonable even in presence of multiple virtual arguments. Hey, you can even add methods to Object if you need to.

Example

The full code for the examples can be found in the following files:

matrix.d, densematrix.d and diagonalmatrix.d - the matrix "library"
app.d - the "application"

matrix.d defines the Matrix interface:

interface Matrix
{
  @property int rows() const;
  @property int cols() const;
  @property double at(int i, int j) const;
}

The DenseMatrix subclass stores the elements in a single array. So does DiagonalMatrix, but it only stores the elements on the diagonal.

The print method in app.d is declared as follows:

import openmethods;

void print(virtual!Matrix m);

The virtual! qualifier indicates that the right version of print will be selected depending on the runtime type of its argument. In effect, print is a virtual function - except that it's defined outside of the matrix class.

This is just a declaration. Here is a catch-all implementation that works for all matrix types:

@method
void _print(Matrix m)
{
  const int nr = m.rows;
  const int nc = m.cols;
  for (int i = 0; i < nr; ++i) {
    for (int j = 0; j < nc; ++j) {
      writef("%3g", m.at(i, j));
    }
    writeln();
  }
}

Note two things:

the @method attribute marks the function as a method implementation
the method name is prefixed by an underscore

Here is an implementation for DiagonalMatrix:

@method
void _print(DiagonalMatrix m)
{
  import std.algorithm;
  import std.format;
  import std.array;
  writeln("diag(" ~ m.elems.map!(x => format("%g", x)).join(" ") ~ ")");
}

Every module that declares methods or define implementations must include the following line:

mixin(registerMethods);

I like to place it just after the import methods directive.

To recap:

import std.stdio;

import matrix;
import diagonalmatrix;
import densematrix;

import openmethods;
mixin(registerMethods);

void print(virtual!Matrix m);

@method
void _print(Matrix m)
{
  const int nr = m.rows;
  const int nc = m.cols;
  for (int i = 0; i < nr; ++i) {
    for (int j = 0; j < nc; ++j) {
      writef("%3g", m.at(i, j));
    }
    writeln();
  }
}

@method
void _print(DiagonalMatrix m)
{
  import std.algorithm;
  import std.format;
  import std.array;
  writeln("diag(" ~ m.elems.map!(x => format("%g", x)).join(" ") ~ ")");
}

void main()
{
  Matrix dense1 =
    new DenseMatrix(2, 3,
                    [ 1, 2, 3,
                      4, 5, 6 ]);
  writeln("dense1 =");
  print(dense1);
  writeln();

  Matrix diag1 = new DiagonalMatrix([ 7, 8, 9]);
  write("diag1 = ");
  print(diag1);
  writeln();

  // ...
}

Multiple Dispatch

Let's now look at matrix addition...

In matrix.d, a plus method is declared with two virtual parameters:

Matrix plus(virtual!Matrix, virtual!Matrix);

densematrix.d contains a "catch-all" implementations that uses the Matrix interface to access the elements of the operands, and returns a DenseMatrix:

@method
Matrix _plus(Matrix m1, Matrix m2)
{
  const int nr = m1.rows;
  const int nc = m1.cols;
  assert(nr == m2.rows);
  assert(nc == m2.cols);
  double[] result;
  result.length = nr * nc;
  int o = 0;
  for (int j = 0; j < nc; ++j) {
    for (int i = 0; i < nr; ++i) {
      result[o++] = m1.at(i, j) + m2.at(i, j);
    }
  }
  return new DenseMatrix(nr, nc, result);
}

While this override returns the correct result if both operands are dense matrices, it is inefficient. Thus a better override is also provided:

@method
Matrix _plus(DenseMatrix a, DenseMatrix b)
{
  const int nr = a.rows;
  const int nc = a.cols;
  assert(a.nr == b.nr);
  assert(a.nc == b.nc);
  auto result = new DenseMatrix;
  result.nr = nr;
  result.nc = nc;
  result.elems.length = a.elems.length;
  result.elems[] = a.elems[] + b.elems[];
  return result;
}

diagonalmatrix.d provides an override that just adds the diagonals - and returns a new DiagonalMatrix.

@method
Matrix _plus(DiagonalMatrix a, DiagonalMatrix b)
{
  assert(a.rows == b.rows);
  double[] sum;
  sum.length = a.elems.length;
  sum[] = a.elems[] + b.elems[];
  return new DiagonalMatrix(sum);
}

Conclusion

Because open methods live outside class hierarchies, they make it possible to extend libraries to a degree that virtual member functions cannot provide.

If a function is missing from the matrix library - say, transpose -, it can be added without changing the library's code. I can vary the implementation of transpose depending on its exact type - transposing a diagonal matrix is a no-op.

The library can also be extended with new classes. I can add a TriDiagonalMatrix class and specialize plus to handle cases like the addition of a diagonal and a tri-diagonal matrix.

Behavior that belongs in application code need not be forced into classes anymore, therefore avoiding the God Object problem.

Tying polymorphism to membership is a mistake that most OOP languages have made in the wake of Simula and Smalltalk. OOP has been under a lot of criticism during the last decade. Indeed, OOP promised a lot, especially in terms of modularity and extensibility - but failed to deliver. Open methods rectify this mistake - to the extent that some would not even consider the open method approach OOP. Indeed, with methods algorithms take the front stage again, and method calls look more like "rule matching" than "message passing".