SML: Exception while reading a line of integers

fun parse file =
 let
(* A function to read an integer from specified input. *)
     fun readInt input =
   Option.valOf (TextIO.scanStream (Int.scan StringCvt.DEC) input)


 
          in
              (Array.update(acc,readInt inStream, f+1); readInts (i - 1) sumd acc has)
          end
 in
      (a, b, readInts b 0 empty [])
 end
(* ................................................................*)

I want to read from a text file that will contain 2 lines:
the first will contain 2 integers a, b
the second will contain b integers x1, x2,.. xb.
for example it will be in this form :

    a b 
    n1 n2 n3 .. nb

I get this error message :

uncaught exception Option
  raised at: Basis/Implementation/option.sml:19.25-19.31

Line 19: is basically readInts i sumd acc has if that helps.
The problem with my code is because of readInts function. I want to read integer by integer, and not the whole line at once (so I can not use functions like "explode") (it's not like I am forced to do it that way, but it's move time productive to read once and form my desired array acc and list has, instead of reading and storing the b integers in a buffer list and then re-run the list to form my wanted data). Can you see what's the problem with my code?

Solution

You call readInt twice in the second case of readInts, advancing the stream past two numbers instead of just one. As you have b numbers in your input, but do this b times, you attempt to read 2 * b numbers and eventually can't anymore. When this happens TextIO.scanStream (Int.scan StringCvt.DEC) input is NONE, so Option.valOf raises Option.