simple caesar cipher module implemented with SML

caesar.sml

(* see. http://d.hatena.ne.jp/a-hisame/20120217/1329499672 *)

(* ===========================================================
 * for using
 * > sml
 * > use "caesar.sml";
 * > Caesar.encode "hoge";
 * val it = "mtlj" : string
 * > _
 ========================================================== *)
structure Caesar :
sig
  val encode : int -> string -> string
  val decode : int -> string -> string
end =
struct
  fun let2int c = ord c - ord #"a"

  fun int2let n = chr (ord #"a" + n)

  fun shift n c = if Char.isLower c
                  then int2let((let2int c + n) mod 26)
                  else c

  (* assert (encode 1 "hal") = "ibm" *)
  fun encode n = implode o map (shift n) o explode

  fun decode n = encode (~n)
end (* structure Caesar *)