Why does my while loop not terminate in this functional language?

Question

I'm trying to build a standard ML program using both imperative and functional concepts, and write the contents to a file. But my while loop doesn't seem to terminate and instead prints the same value continuously.

fun writeImage image filename =

  let val ap = TextIO.openOut filename

      val (w,h) = size image

      val row = ref 0

      val stringToWrite = "";

  in

    while !row < h do

        TextIO.output(ap,"A");

        row := !row + 1;

     TextIO.closeOut ap

  end;  

If I remove the first line after the while loop, the loop terminates. But if I include TextIO.output(ap,"A");, it doesn't. Why is this the case?

Answer 1

Let's write your program with correct indentation, and then it becomes clear where the mistake is:

...
while !row < h do
    TextIO.output(ap,"A");
row := !row + 1;
TextIO.closeOut ap
...

You loop forever because the increment is outside of the body of the loop.

You intended to write this:

...
while !row < h do (
   TextIO.output(ap,"A");
   row := !row + 1
);
TextIO.closeOut ap
...

right?

I have long made a study of how people come to make mistakes when programming. I am curious to know how you came to make this mistake. If you believed that ; binds stronger than while then why did you believe that the TextIO.closeOut ap was after the loop? Surely if your belief was that the ; binds the increment to the loop then the ; should bind it to the loop as well. Did you perhaps think that ML is a language like Python, where the looping constructs use the whitespace as a guide to the extent of the body?

What are your beliefs about ; in ML more generally? Do you think of ; as a statement terminator, as it is in C-like languages? Or do you think of it as an infix sequencing operation on side-effecting expressions?

What was your thought process here, and how could the tooling have made it easier for you to solve your problem without having to ask for help?