open TextIO; (* loads the structure TextIO in which I/O functions such 
	        openIn, inputN, and endOfStream are defined *)
exception BadChar;

val END = ~1;

fun white(c) = c=" " orelse c="\n" orelse c="\t";

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

fun startInt(file) = (* get the first digit from file;
			return END if there is no integer *)
  if endOfStream(file) then END
  else let
           val c = inputN(file,1)
       in
           if digit(c) then ord(String.sub(c,0))-ord(#"0")
	   else if white(c) then startInt(file)
	        else raise BadChar
       end;

fun finishInt(i,file) = (* return the integer whose first digits
			   have value i and whose remaining digits
			   are found on file, up to the end of the first
			   while space *)
  if endOfStream(file) then i
  else let
          val c = inputN(file,1)
       in
          if digit(c) then finishInt(10*i+ord(String.sub(c,0))-ord(#"0"), file)
	  else if white(c) then i
	       else raise BadChar
       end;

fun getInt(file) = (* read an integer from file *)
  finishInt(startInt(file),file);

fun sumInts1(file) = (* sum the integers on file *)
  let
      val i = getInt(file)
  in 
      if i=END then 0
      else i+sumInts1(file)
  end;

fun sumInts(filename) = (* sum the integers on file "filename"
			   relative to the current UNIX directory *)
  sumInts1(openIn(filename));