https://github.com/ChrisMarinos/FSharpKoans/blob/fsharp/FSharpKoans/AboutTheStockExample.fs

FSharpKoansStock.fs

open System
open System.Globalization
//---------------------------------------------------------------
// Apply Your Knowledge!
//
// Below is a list containing comma separated data about 
// Microsoft's stock prices during March of 2012. Without
// modifying the list, programatically find the day with the
// greatest variance between the opening and closing price.
//
// The following functions may be of use:
// 
// abs - takes the absolute value of an argument
// 
// System.Double.Parse - converts a string argument into a 
//                       numerical value.
//
// Hint: Use CultureInfo.InvariantCulture to always parse '.' as 
// the decimal point.
//
// The following function will convert a comma separated string
// into an array of the column values.
//                       
// let splitCommas (x:string) =
//     x.Split([|','|])
//---------------------------------------------------------------


let stockData =
    [ "Date,Open,High,Low,Close,Volume,Adj Close";
      "2012-03-30,32.40,32.41,32.04,32.26,31749400,32.26";
      "2012-03-29,32.06,32.19,31.81,32.12,37038500,32.12";
      "2012-03-28,32.52,32.70,32.04,32.19,41344800,32.19";
      "2012-03-27,32.65,32.70,32.40,32.52,36274900,32.52";
      "2012-03-26,32.19,32.61,32.15,32.59,36758300,32.59";
      "2012-03-23,32.10,32.11,31.72,32.01,35912200,32.01";
      "2012-03-22,31.81,32.09,31.79,32.00,31749500,32.00";
      "2012-03-21,31.96,32.15,31.82,31.91,37928600,31.91";
      "2012-03-20,32.10,32.15,31.74,31.99,41566800,31.99";
      "2012-03-19,32.54,32.61,32.15,32.20,44789200,32.20";
      "2012-03-16,32.91,32.95,32.50,32.60,65626400,32.60";
      "2012-03-15,32.79,32.94,32.58,32.85,49068300,32.85";
      "2012-03-14,32.53,32.88,32.49,32.77,41986900,32.77";
      "2012-03-13,32.24,32.69,32.15,32.67,48951700,32.67";
      "2012-03-12,31.97,32.20,31.82,32.04,34073600,32.04";
      "2012-03-09,32.10,32.16,31.92,31.99,34628400,31.99";
      "2012-03-08,32.04,32.21,31.90,32.01,36747400,32.01";
      "2012-03-07,31.67,31.92,31.53,31.84,34340400,31.84";
      "2012-03-06,31.54,31.98,31.49,31.56,51932900,31.56";
      "2012-03-05,32.01,32.05,31.62,31.80,45240000,31.80";
      "2012-03-02,32.31,32.44,32.00,32.08,47314200,32.08";
      "2012-03-01,31.93,32.39,31.85,32.29,77344100,32.29";
      "2012-02-29,31.89,32.00,31.61,31.74,59323600,31.74"; ]

// Feel free to add extra [<Koan>] members here to write
// tests for yourself along the way. You can also try 
// using the F# Interactive window to check your progress.
//[ "Date, Open,High,Low, Close,Volume,Adj Close";
type Row = 
        { DOE : string
          Open : float
          High : float
          Low : float
          Close : float
          Volume : float
          AdjClose : float }

let splitCommas (x:string) =
    x.Split([|','|])

let createRowFromCSV (x : string) = 
        let rowData = splitCommas x
        { DOE = rowData.[0]
          Open = System.Double.Parse rowData.[1]
          High = System.Double.Parse rowData.[2]
          Low = System.Double.Parse rowData.[3]
          Close = System.Double.Parse rowData.[4]
          Volume = System.Double.Parse rowData.[5]
          AdjClose = System.Double.Parse rowData.[6] }

(*
let rec checkForGreatestVal index max dt = 
    let row = splitCommas stockData.[index]
    let diff = abs( row.[1] - row.[5])
    let diffDate = row.[0]
    if diff > max then 
        checkForGreatestVal (index-1) diff diffDate 
    else 
        checkForGreatestVal (index-1) max dt
*)        

let YouGotTheAnswerCorrect() =
    let rows = Seq.map (fun row -> createRowFromCSV row )  stockData.Tail
    let diff = Seq.map (fun item -> (item.DOE, abs (item.Open - item.Close))) rows
    let result = Seq.maxBy snd diff |> fst 
    result 
    

let output= YouGotTheAnswerCorrect()
output.Dump()