ISBN13 check digit: Difference between revisions

===Easy to read version===

<syntaxhighlight lang="sml">(* these type decorations are optional, you could just as well put:

fun isValidISBN s =

*)

fun isValidISBN (s : string) : bool =

let

val digits = List.filter Char.isDigit (explode s)

val nums = map (fn x => ord x - ord #"0") digits

val len = length nums

fun sumISBN [] = raise Domain

| sumISBN [x] = x

| sumISBN (x1::x2::xs) = x1 + 3*x2 + sumISBN xs

in

len = 13 andalso sumISBN nums mod 10 = 0

end

val test = ["978-1734314502", "978-1734314509",

"978-1788399081", "978-1788399083"]

fun printTest (s : string) : unit =

(print s; print (if isValidISBN s then ": good\n" else ": bad\n"))

fun main () = app printTest test</syntaxhighlight>

===Original version===

<syntaxhighlight lang="sml">let