Convert integers to Roman Numerals

RomanNumerals.fs

let rec repeat x (s : string) : string = 
    match x with 
    | 0 -> ""
    | x -> s + (repeat (x-1) s)

let roman_numeral_ordering n (one : string) (five : string) (ten : string) =
    match n with
    | 0 -> ""
    | n when n < 4 -> repeat n one
    | n when n = 4 -> one + five
    | n when n = 5 -> five
    | n when 5 < n && n < 9 -> five + (repeat (n - 5) one)
    | n when n = 9 -> one + ten
    | n -> "unknown"

let century_roman m = 
    match m with
    | 0 -> ""
    | 1 -> "M"
    | m -> "unknown"

let hundreds_roman m = 
    match m with 
    | m when m < 10 -> roman_numeral_ordering m "C" "D" "M"
    | m -> "unknown"

let tens_roman m = 
    match m with 
    | m when m < 10 -> roman_numeral_ordering m "X" "L" "C"
    | m -> "unknown"

let ones_roman m = 
    match m with 
    | m when m < 10 -> roman_numeral_ordering m "I" "V" "X"
    | m -> "unknown"

let rec convert_to_roman x : string =
    match x with
    | x when (float x)/1000.0 >= 1.0 -> (century_roman (x/1000)) + convert_to_roman (x%1000)
    | x when (float x)/100.0 >= 1.0 -> hundreds_roman (x/100) + convert_to_roman (x%100)
    | x when (float x)/10.0 >= 1.0 -> tens_roman (x/10) + convert_to_roman (x%10)
    | x when (float x) >= 1.0 -> ones_roman x
    | x -> ""

let rec print_roman_numerals i = 
    match i with
    | 1999 -> printfn "Done"
    | i -> printfn "%i : %s\n" i (convert_to_roman (i)); print_roman_numerals (i+1)

// single example
convert_to_roman 1984 |> printfn "%A" 
      
// print all
print_roman_numerals 1;;