Function that takes a bigint as input and outputs its word form

English-Integer.fs

let english_integer ( i : bigint ) : string  =
 
    let ZERO_TO_19 =
        [   "zero"     ;    "one"      ;    "two"      ; "three"   ;  "four"    ;  "five"       ;
            "six"      ;    "seven"    ;    "eight"    ; "nine"    ;  "ten"     ;  "eleven"     ;
            "twelve"   ;    "thirteen" ;    "fourteen" ; "fifteen" ;  "sixteen" ;  "seventeen"  ;    
            "eighteen" ;    "nineteen"                                                          ]
   
    let HYPHEN_ONES =
        [   ""     ; "-one"   ; "-two"    ; "-three" ; "-four" ; "-five"     ;
            "-six" ; "-seven" ; "-eight"  ; "-nine"                          ]
 
    let TWENTY_UP =
        [   ""      ; ""      ; "twenty"  ; "thirty" ; "forty"      ;
            "fifty" ; "sixty" ; "seventy" ; "eighty" ; "ninety"     ]
   
    let THOUSAND_UP =
        [   ""              ;   " thousand"        ;    " million"       ;  " billion"              ;
            " trillion"     ;   " quadrillion"     ;    " quintillion"   ;  " sextillion"           ;    
            " septillion"   ;   " octillion"       ;    " nonillion"     ;  " decillion"            ;
            " undecillion"  ;   " duodecillion"    ;    " tredecillion"  ;  " quattuordecillion"    ;
            " sexdecillion" ;   " septendecillion" ;    " octodecillion" ;  " novemdecillion"       ;  
            " vigintillion"                                                                         ]
 
    let rec toEnglish i step  =
        match i with
        | x     when x < 0I    ->   "negative " + toEnglish -x 0  
 
        | x     when x < 20I   ->   ZERO_TO_19.Item (x|>int)  + THOUSAND_UP.Item (step)
 
        | x     when x < 100I  ->   TWENTY_UP.Item   (( x|>int ) / 10 ) +
                                    HYPHEN_ONES.Item (( x|>int ) % 10 ) +
                                    THOUSAND_UP.Item ( step    )
 
        | x     when x < 1000I ->   let  s = ( HYPHEN_ONES.Item (( x|> int) / 100 )).Substring 1 + " hundred"
                                    if   i % 100I > 0I
                                    then s + " and " + toEnglish ( x % 100I ) 0 + THOUSAND_UP.Item ( step )
                                    else s + THOUSAND_UP.Item ( step )
 
        | x     ->  let   current = i % 1000I
                    match current  with
                    | x when x = 0I ->  toEnglish (i/1000I) ( step + 1 )
 
                    | x             ->  toEnglish (i/1000I) ( step + 1 ) + " " +
                                        toEnglish current   ( step     )
    in  toEnglish i 0