open TextIO;

     val END = ~1;

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

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

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

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

     and finishInt1(i, file, NONE) = i
     |   finishInt1(i, file, SOME c) =
             if digit(c) then
                     finishInt(10*i+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 = END then 0
                 else i + sumInts1(file)
             end;

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