script.fsx

let e numberStr = 
    match numberStr with
    | "" -> numberStr
    | _  -> numberStr + " e "
    

let writeNumber nr : string =
    let unity = ["zero"; "um"; "dois"; "três"; "quatro"; "cinco"; "seis"; "sete"; "oito"; "nove"; "dez"; "onze"; "doze"; "treze"; "quatorze"; "quinze"; "dezesseis"; "dezessete"; "dezoito"; "dezenove"]
    let decimals = [""; "cem"; "vinte"; "trinta"; "quarenta"; "cinquenta"; "sessenta"; "setenta"; "oitenta"; "noventa"]
    let hundreds = [""; "cento"; "duzentos"; "trezentos"; "quatrocentos"; "quinhentos"; "seissentos"; "setecentos"; "oitocentos"; "novecentos"]
    let thousand = "mil"

    let mutable number = nr
    let mutable numberStr = ""
    
    let thousandNr = number / 1000
    if(thousandNr <> 0) then
        if(thousandNr = 1)  then
            numberStr <- thousand
        else
            numberStr <- unity.[thousandNr] + " " + thousand
    
    number <- number - thousandNr * 1000
    
    if(number <> 0 || numberStr = "") then
        let hundredNr = number / 100
        number <- number - hundredNr * 100
        if(hundredNr <> 0) then
            if(number = 0) then
                numberStr <- (e numberStr) + decimals.[hundredNr]
            else
                if(numberStr <> "") then
                    numberStr <- numberStr + " "

                numberStr <- numberStr + hundreds.[hundredNr]
    
        if(number < 20) then
            numberStr <- (e numberStr) + unity.[number]
        else
            let decimalNr = number / 10
            number <- number - decimalNr * 10
            if(number = 0) then
                numberStr <- (e numberStr) + decimals.[decimalNr]
            else
                numberStr <- (e numberStr) + decimals.[decimalNr] + " e " + unity.[number]
    numberStr

let writeNumber2 nr =
    let unity = ["zero"; "um"; "dois"; "três"; "quatro"; "cinco"; "seis"; "sete"; "oito"; "nove"; "dez"; "onze"; "doze"; "treze"; "quatorze"; "quinze"; "dezesseis"; "dezessete"; "dezoito"; "dezenove"]
    let decimals = [""; "cem"; "vinte"; "trinta"; "quarenta"; "cinquenta"; "sessenta"; "setenta"; "oitenta"; "noventa"]
    let hundreds = [""; "cento"; "duzentos"; "trezentos"; "quatrocentos"; "quinhentos"; "seissentos"; "setecentos"; "oitocentos"; "novecentos"]
    let thousand = "mil"

    let fthousand number =
        let thousandNr = number / 1000
        let discounted = number - thousandNr * 1000
        match thousandNr with 
        | 0 -> (number, "")
        | 1 -> (discounted, thousand)
        | _ -> (discounted, unity.[thousandNr] + " " + thousand)

    let fhundred (number, prefix) =
        let hundredNr = number / 100
        let discounted = number - hundredNr * 100

        match hundredNr with 
        | 0 -> (number, prefix)
        | _ -> (discounted, prefix + (if prefix <> "" then " " else "") + hundreds.[hundredNr])

    let inline e numberStr = 
        match numberStr with
        | "" -> numberStr
        | _  -> numberStr + " e "

    let fdecimal (number, prefix) =
        if number < 20
        then (number, prefix)
        else 
            let decimalNr = number / 10
            let discounted = number - decimalNr * 10
            (discounted, (e prefix) + decimals.[decimalNr])

    let funit (number, prefix) =
        if number = 0 && prefix <> "" 
        then prefix
        else (e prefix) + unity.[number]

    nr |> fthousand |> fhundred |> fdecimal |> funit

printf "Digite um número: \n"
let number = int32(System.Console.ReadLine())
let str = writeNumber2 number

printf "\n%s\n" str