Define a function romanDigitToInt that converts a single Roman digit to a decimal integer. This function takes an argument as a element of the RomanDigit union type and transforms it into the decimal integer using patter matching. This function should have signature: RomanDigit -> int.

| (f,s) when (convertRomanToInt f) < (convertRomanToInt s) -> (convertRomanToInt s) - (convertRomanToInt f) + (romanNumberToInteger romanNumber.[2..])

f atuple

let charToRomanDigit c =

printfn "%c" c

match c with

| 'I' -> 'I'

| 'V' -> 'V'

| 'X' -> 'X'

| 'L' -> 'L'

| 'C' -> 'C'

| 'D' -> 'D'

| 'M' -> 'M'

| _ -> failwith "Invalid Entry"

let stringToNumber str =

let upperDigit (x:string) = x.ToUpper()

let upperString = upperDigit str

printfn "%s" upperString

String.collect (fun c -> sprintf "%c" (charToRomanDigit c)) upperString

let romanNumber = "XIX"

let upperDigit (x:string) = x.ToUpper()

printfn "a: %s" (upperDigit romanNumber)

romanNumber = stringToNumber romanNumber

printfn "\nValue = %d" (romanNumberToInteger romanNumber)

Explanation:

Create a function called convertRomanToInt for pattern matching to convert romanNumber to valid integer
Create a recursive function called romanNumberToInteger and check if length of romanNumber is 1 or 0 .
Check whether the first character is less than second character, then its reverse otherwise it's normal .
Create tuple of first and second character for pattern matching .
Create a function charToRomanDigit and use String.collect to go through each character present in string and reconstruct them again .
System.Char.ToUpper is used to convert a character to uppercase and stringToNumber to convert string in valid Roman number .
Finally print the results.