如何递归地使用具有多个字母标识符和多个数字in的char列表(SML中的词法分析)

fun lexAnalysis(chars: char list): string list =
  let
    fun lexHelper(chars: char list, current: string, result: string list): string list =
      case chars of
        [] => result
      | c::rest =>
          if Char.isAlpha c then
            lexHelper rest (current ^ Char.toString c) result
          else if Char.isDigit c then
            lexHelper rest (current ^ Char.toString c) result
          else if current <> "" then
            lexHelper rest "" (result @ [current])
          else
            lexHelper rest "" result
  in
    lexHelper chars "" []
  end;

val input = explode "abc123def456ghi"
val result = lexAnalysis input