13 Refs

A reference in OCaml is a pointer to a typed location in memory. This is also called a "ref" or a "ref cell"

The binding of the variable to the pointer is immutable, but the contents of the memory location are mutable.

Recall immutable values in OCaml.

We can have ints:

3110;;

- : int = 3110

We can bind them to names:

let x = 3110;;

val x : int = 3110

Refs are created with the standard library function ref.

ref 3110;;

- : int ref = {contents = 3110}

This is a reference to in integer, whose current contents are 3110. Let's bind it to a name.

let y = ref 3110;;

val y : int ref = {contents = 3110}

So what is y bound to right now?

y;;

- : int ref = {contents = 3110}

It's a location in memory with the contents 3110. If we want to get the contents out, we can use the dereference operator, ! (pronounced "bang").

!y;;

- : int = 3110

Note that when we dereference y, we get an int, not an int ref.

x + y;;

File "[7]", line 1, characters 4-5:
1 | x + y;;
        ^
Error: This expression has type int ref
       but an expression was expected of type int

This doesn't type check, because y is a ref, and x is an int. We need to bang y to do this.

x + !y;;

- : int = 6220

We can use the assignment operator to change the value of y.

y := 2110;;

- : unit = ()

Note that the assignment operator returns a unit.

y;;

- : int ref = {contents = 2110}

Now, y points to the same location, but with different contents (2110). We can show this by banging y again.

x + !y;;

- : int = 5220

y := "Hello";;

File "[12]", line 1, characters 5-12:
1 | y := "Hello";;
         ^^^^^^^
Error: This expression has type string but an expression was expected of type
         int

We can't assign a different type to y, only integers.