open TextIO;

     datatype intOrEof = Eof | Int of int;

     fun digit(c) = c >= #"0" andalso c <= #"9";

     fun startInt(file) = startInt1(file, input1(file))

     and startInt1(file, NONE) = Eof
     |   startInt1(file, SOME c) =
             if digit(c) then Int(ord(c)-ord(#"0"))
             else startInt(file);

     fun finishInt(i,file) =
             if i = Eof then Eof
             else finishInt1(i,file,input1(file))

     and finishInt1(i, file, NONE) = i
     |   finishInt1(i as Int(j), file, SOME c) =
             if digit(c) then
                     finishInt(Int(10*j+ord(c)-ord(#"0")), file)
             else i;

     fun getInt(file) = finishInt(startInt(file), file)

     fun sumInts1(file) =
             let
                 val i = getInt(file)
             in
                 if i = Eof then 0
                 else let
                          val Int(j) = i
                      in
                          j + sumInts1(file)
                      end
             end;

     fun sumInts(filename) = sumInts1(openIn(filename));