Adding Overloaded Constructor That Requires Initialization Code to Implicit F# Type

Question

I currently have the following code:

type Matrix(sourceMatrix:double[,]) =
      let rows = sourceMatrix.GetUpperBound(0) + 1
      let cols = sourceMatrix.GetUpperBound(1) + 1
      let matrix = Array2D.zeroCreate<double> rows cols
      do
        for i in 0 .. rows - 1 do
        for j in 0 .. cols - 1 do
          matrix.[i,j] <- sourceMatrix.[i,j]

  new (rows, cols) = Matrix( Array2D.zeroCreate<double> rows cols)

  new (boolSourceMatrix:bool[,]) = Matrix(Array2D.zeroCreate<double> rows cols)
        for i in 0 .. rows - 1 do
        for j in 0 .. cols - 1 do
            if(boolSourceMatrix.[i,j]) then matrix.[i,j] <- 1.0
            else matrix.[i,j] <- -1.0

My problem lies in the last constructor that takes a bool[,] parameter. The compiler isn't letting me get away with the two for loops I'm trying to use for initialization in this constructor. How can I make this work?

Answer 1

The easiest solution would be to just do this instead:

new (boolSourceMatrix) = Matrix(Array2D.map (fun b -> if b then 1.0 else -1.0) boolSourceMatrix)

The specific issue that you were running into is that the let-bound fields from the primary constructor aren't available in alternate constructors. To work around this, you could use an explicitly defined field, if you wanted. However, in this case it's better to take advantage of the additional functionality in the Array2D module.