Dynamically changing format statements in the F# ISE

This had me stuck yesterday – how to change the width of  some formatted printf output. In the end with a bit of searching I realised the TextWriterFormat member gives you a way to define the format such that you can change it in your program. An example is a better viewer for the Pascals Triangle sequence generator I made recently – all related to a school math project. Also – where does it tell you that to print a % you need to double it? Why didn’t they do as with speech marks and use a leading \?

let rec PascalsTriangle = seq {
    yield [1];
    for aLine in PascalsTriangle ->
        let newLine =
            aLine
            |> Seq.pairwise
            |> Seq.map (fun (x,y) -> x+y)
            |> Seq.toList
        List.append (1::newLine) [1]
    }

PascalsTriangle |> Seq.take 10
//|> Seq.iter (fun i -> printfn "%A" i)
|> Seq.iter (fun v -> 
                        v |> Seq.iter (fun w -> printf "%3i" w)
                        printfn ""
            )

// smarten up the printing...
let samplePT = PascalsTriangle |> Seq.take 12
let maxDigits = (Seq.concat samplePT |> Seq.max).ToString().Length
let numberFormatter digits = Printf.TextWriterFormat<int->unit>(sprintf "%%%dd" digits)
let spaceFormatter digits = Printf.TextWriterFormat<string->unit>(sprintf "%%%ds" digits)
let rows = Seq.length samplePT
samplePT |> Seq.iteri   ( fun u v ->
                                    for i in 1..((rows / 1) - u) do
                                        printf (spaceFormatter ((int)((maxDigits+1)/2))) " "
                                    v |> Seq.iter (fun w -> printf (numberFormatter (maxDigits+1)) w)
                                    printfn ""
                        )

The output is like this:

                      1
  
                    1   1

  
                  1   2   1

  
                1   3   3   1

  
              1   4   6   4   1

  
            1   5  10  10   5   1

  
          1   6  15  20  15   6   1

  
        1   7  21  35  35  21   7   1

  
      1   8  28  56  70  56  28   8   1

  
    1   9  36  84 126 126  84  36   9   1

  
  1  10  45 120 210 252 210 120  45  10   1

  
1  11  55 165 330 462 462 330 165  55  11   1

Wish I knew how to do this in WPF…

Wish I knew a better way to post into the blog…