Tweaks just because.

diamond.fsx

open System
// We can not work with radius of the diamond outside 0 to 25
let createRadius i = 
    match i with 
        | i when i >= 0 && i < 26 -> Some i
        | _ -> None

let radius = createRadius 20 |> Option.get

let axis = [|-radius .. radius|]

let TaxicabNorm (x, y) =  
    abs x + abs y

let indexedLetters =
    ['A' .. 'Z'] |> Seq.mapi (fun i l -> i,l) 

let findLetter y =
    indexedLetters |> Seq.find(fun (i, _) -> i = y) |> snd

let createAllPoints vector = 
    seq { for x in vector do for y in vector do yield (x, y) }

let diamond = createAllPoints axis |> Seq.filter (fun p -> TaxicabNorm p = radius) 

let axisTo2D axis = 
    [| for _ in axis -> (Array.map (fun _ -> '.') axis) |]

let drawing = axisTo2D axis 

let findIndexInAxis elem axis =
    Array.findIndex (fun x -> x = elem) axis

for (x, y) in diamond do
    let yindex = findIndexInAxis y axis
    let xindex = findIndexInAxis x axis
    drawing.[xindex].[yindex] <- abs y |> findLetter

let printArray (arr: char array) = new string (arr) |> sprintf "%A" |> Console.WriteLine 

for line in drawing do
    printArray line