Example: Mutable Counter

Here is code that implements a counter. Every time next_val is called, it returns one more than the previous time.

# let counter = ref 0;;
val counter : int ref = {contents = 0}

# let next_val = 
    fun () ->
      counter := (!counter) + 1;
      !counter;;
val next_val : unit -> int = <fun> 

# next_val();;
- : int = 1

# next_val();;
- : int = 2

# next_val();;
- : int = 3

In the implementation of next_val, there are two expressions separated by semi-colon. The first expression, counter := (!counter) + 1, is an assignment that increments counter by 1. The second expression, !counter, returns the newly incremented contents of counter.

This function is unusual in that every time we call it, it returns a different value. That's quite different than any of the functions we've implemented ourselves so far, which have always been deterministic: for a given input, they always produced the same output. On the other hand, we've seen some library functions that are nondeterministic, for example, functions in the Random module, and Pervasives.read_line. It's no coincidence that those happen to be implemented using mutable features.

We could improve our counter in a couple ways. First, there is a library function incr : int ref -> unit that increments an int ref by 1. Thus it is like the ++ operator in many language in the C family. Using it, we could write incr counter instead of counter := (!counter) + 1.

Second, the way we coded the counter currently exposes the counter variable to the outside world. Maybe we're prefer to hide it so that clients of next_val can't directly change it. We could do so by nesting counter inside the scope of next_val:

let next_val = 
  let counter = ref 0 
  in fun () ->
    incr counter;
    !counter

Now counter is in scope inside of next_val, but not accessible outside that scope.

When we gave the dynamic semantics of let expressions before, we talked about substitution. One way to think about the definition of next_val is as follows.

  • First, the expression ref 0 is evaluated. That returns a location loc, which is an address in memory. The contents of that address are initialized to 0.

  • Second, everywhere in the body of the let expression that counter occurs, we substitute for it that location. So we get:

    fun () -> incr loc; !loc
    
  • Third, that anonymous function is bound to next_val.

So any time next_val is called, it increments and returns the contents of that one memory location loc.

Now imagine that we instead had written the following (broken) code:

let next_val_broken = fun () ->
  let counter = ref 0
  in incr counter;
     !counter

It's only a little different: the binding of counter occurs after the fun () -> instead of before. But it makes a huge difference:

# next_val_broken ();;
- : int = 1

# next_val_broken ();;
- : int = 1

# next_val_broken ();;
- : int = 1

Every time we call next_val_broken, it returns 1: we no longer have a counter. What's going wrong here?

The problem is that every time next_val_broken is called, the first thing it does is to evaluate ref 0 to a new location that is initialized to 0. That location is then incremented to 1, and 1 is returned. Every call to next_val_broken is thus allocating a new ref cell, whereas next_val allocates just one new ref cell.

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