https://projecteuler.net/problem=8 completed in F#

A.fs

(* declare module *)
module A

(* declare import *)
open System

(* declare number *)
let num = "73167176531330624919225119674426574742355349194934\
96983520312774506326239578318016984801869478851843\
85861560789112949495459501737958331952853208805511\
12540698747158523863050715693290963295227443043557\
66896648950445244523161731856403098711121722383113\
62229893423380308135336276614282806444486645238749\
30358907296290491560440772390713810515859307960866\
70172427121883998797908792274921901699720888093776\
65727333001053367881220235421809751254540594752243\
52584907711670556013604839586446706324415722155397\
53697817977846174064955149290862569321978468622482\
83972241375657056057490261407972968652414535100474\
82166370484403199890008895243450658541227588666881\
16427171479924442928230863465674813919123162824586\
17866458359124566529476545682848912883142607690042\
24219022671055626321111109370544217506941658960408\
07198403850962455444362981230987879927244284909188\
84580156166097919133875499200524063689912560717606\
05886116467109405077541002256983155200055935729725\
71636269561882670428252483600823257530420752963450"

let digits = num
          |> Seq.map Char.GetNumericValue
          |> Seq.map bigint
          |> Seq.toList

let rec adjacents xs toTake =
  match xs with
  | [] -> []
  | xs -> if   List.length xs >= toTake
          then List.take toTake xs :: adjacents (List.tail xs) toTake
          else []

let product = List.fold (fun acc n -> acc * n) (bigint 1)

(* declare theGreatest!!!! *)
let theGreatest xs toTake = (xs, toTake)
                        ||> adjacents
                         |> List.map product
                         |> List.max

(* declare entrypoint main *)
[<EntryPoint>]
let main args =
  Console.WriteLine (string (theGreatest digits 13))
  0