Additional constructors in F#

f#

Solution

As Daniel pointed out, the F# design is that you have one main constructor that typically takes all the arguments needed by the class and performs the initialization. Other constructors can either provide default values for the arguments or can calculate them from some other information.

In your case, I think that the best design would be to pass `rows` as constructor argument. Then you can add two additional constructors (one that loads a file and other that provides empty list).

This makes the code a bit simpler, because you do not have to check if the argument is supplied (as in Daniel's version). I also did a few other simplifications (i.e. calculate `width` functionally and use sequence comprehensions - if `Sheet` does not modify the data, you could also avoid using `ResizeArray`):

type Sheet private (rows) =  
  // The main constructor is 'private' and so users do not see it,
  // it takes columns and calculates the maximal column length
  let width = rows |> Seq.map Seq.length |> Seq.fold max 0

  // The default constructor calls the main one with empty ResizeArray
  new() = Sheet(ResizeArray<_>())

  // An alternative constructor loads data from the file
  new(fileName:string) =
    let lines = System.IO.File.ReadLines fileName 
    Sheet(ResizeArray<_> [ for line in lines -> ResizeArray<_> (line.Split ',') ])

Problem

I am trying to create an additional constructor in F# that does some extra work (i.e. reads a basic csv file) as follows: ``` type Sheet () = let rows = new ResizeArray<ResizeArray<String>>() let mutable width = 0 new(fileName) as this = Sheet() then let lines = System.IO.File.ReadLines fileName for line in lines do let cells = line.Split ',' rows.Add(new ResizeArray<String> (cells)) //line 16 if cells.Length > width then width <- cells.Length ``` but I get the following errors: ``` Error 1 The namespace or module 'rows' is not defined C:\Users\ga1009\Documents\PhD\cpp\pmi\fsharp\pmi\Csv.fs 16 Error 2 The value or constructor 'width' is not defined C:\Users\ga1009\Documents\PhD\cpp\pmi\fsharp\pmi\Csv.fs 17 Error 3 The value or constructor 'width' is not defined C:\Users\ga1009\Documents\PhD\cpp\pmi\fsharp\pmi\Csv.fs 17 ``` What am I doing wrong?

Original source