Problem Statement

CSV spreadsheet files are suitable for storing tabular data in a relatively portable way. The CSV format is flexible but somewhat ill-defined.
For present purposes, authors may assume that the data fields contain no commas, backslashes, or quotation marks.

Read a CSV file, change some values and save the changes back to a file. For this task we will use the following CSV file: Suggestions

Let's start with the solution:

open System.IO

[<EntryPoint>]
let main _ =
    let input = File.ReadAllLines "test_in.csv"
    let output = 
        input
        |> Array.mapi (fun i line -> 
            if i = 0 then line + ",SUM" 
            else
                let sum = Array.sumBy int (line.Split(','))
                sprintf "%s,%i" line sum)
    File.WriteAllLines ("test_out.csv", output)
    0