Implementing Indexer in F#

Question

I am trying to convert this C# code to F#:

double[,] matrix;

public Matrix(int rows, int cols)
    {
        this.matrix = new double[rows, cols];
    }

 public double this[int row, int col]
    {
        get
        {
            return this.matrix[row, col];
        }
        set
        {
            this.matrix[row, col] = value;
        }
    }

Basically my biggest problem is creating the indexer in F#. I couldn't find anything that I could apply in this situation anywhere on the web. I included a couple of other parts of the class in case incorporating the indexer into a Matrix type isn't obvious. So a good answer would include how to make a complete type out of the three pieces here, plus anything else that may be needed. Also, I am aware of the matrix type in the F# powerpack, however I am trying to learn F# by converting C# projects I understand into F#.

Thanks in advance,

Bob

Answer 1

F# calls them "indexed properties"; here is the MSDN page. In F# they work slightly differently - each indexed property has a name.

However, there is a default one called "Item". So an implementation of your example would look like this:

member this.Item
  with get(x,y) = matrix.[(x,y)]
  and  set(x,y) value = matrix.[(x,y)] <- value

Then this is accessed via instance.[0,0]. If you have named it something other than "Item", you would access it with instance.Something[0,0].