// Solution

// In this sample, the goal was to compute the length of the list

// Method 1: Library Function

let ourList =
 [
   2;
   4;
   13;
   26;
   15;
   29;
   130;
   0;
   -7;
 ]

let sum1 = List.sumBy(fun el ->
  if el % 13 = 0 then
    el
  else
    0
  )

// Method 2: Recursion
let rec sum2 = function
  | []   -> 0
  | h::t ->
    if h % 13 = 0 then sum2(t) + h
    else sum2(t)

// Method 3: Tail Rec
let sum3 =
  let rec sum3' acc i = function
  | [] -> acc
  | h::t ->
    if i % 2 = 0 then sum3' (acc+h) (i+1) t
    else sum3' (acc) (i+1) t
  sum3' 0 0

let main =
    printfn "%10d simple" (sum1 ourList)
    printfn "%10d recursion" (sum2 ourList)
    printfn "%10d tail recursion" (sum3 ourList)