Status of pointers in OCaml

Pointers exist in OCaml, and in fact they spread all over the place. They are used either implicitly (in the most cases), or explicitly (in the rare occasions where implicit pointers are not more handy). The vast majority of pointers usages that are found in usual programming languages simply disappear in OCaml, or more exactly, those pointers are totally automatically handled by the compiler. Thus, the OCaml programmer can safely ignore the existence of pointers, focusing on the semantics of their program.

For instance, lists or trees are defined without explicit pointers using a concrete datatype definition. The underlying implementation uses pointers, but this is hidden from the programmer since pointer handling is done by the compiler.

In the rare occasions where explicit pointers are needed (the most common case is when translating into OCaml an algorithm described in a classic imperative language), OCaml provides references that are full-fledged pointers, even first class citizen pointers (references can be passed as argument, embedded into arbitrary data structures, and returned as function results).

Explicit pointers are OCaml values of type ref

You can program directly with explicit references if you want to, but this is normally a waste of time and effort.

Let's examine the simple example of linked lists (integer lists to be simple). This data type is defined in C (or in Pascal) using explicit pointers, for instance:

/* Cells and lists type in C */
struct cell {
  int hd;
  struct cell *tl;

typedef struct cell cell, *list;
{Cells and lists type in Pascal}
 list = ^cell;
 cell = record
  hd: integer;
  tl: cell;

We can translate this in OCaml, using a sum type definition, without pointers:

# type list = Nil | Cons of int * list;;
type list = Nil | Cons of int * list

Cell lists are thus represented as pairs, and the recursive structure of lists is evident, with the two alternatives, empty list (the Nilconstructor) and non empty list (the Cons constructor).

Automatic management of pointers and automatic memory allocation shine when allocating list values: one just writes Cons (x, l) to add x in front of the list l. In C, you need to write this function, to allocate a new cell and then fill its fields. For instance:

/* The empty list */
#define nil NULL

/* The constructor of lists */
list cons (element x, list l)
  list result;
  result = (list) malloc (sizeof (cell));
  result -> hd = x;
  result -> tl = l;
  return (result);

Similarly, in Pascal:

{Creating a list cell}
function cons (x: integer; l: list): list;
  var p: list;
    p^.hd := x;
    p^.tl := l;
    cons := p

We thus see that fields of list cells in the C program have to be mutable, otherwise initialization is impossible. By contrast in OCaml, allocation and initialization are merged into a single basic operation: constructor application. This way, immutable data structures are definable (those data types are often referred to as “pure” or “functional” data structures). If physical modifications are necessary for other reasons than mere initialization, OCaml provides records with mutable fields. For instance, a list type defining lists whose elements can be in place modified could be written:

# type list = Nil | Cons of cell
  and cell = { mutable hd : int; tl : list };;
type list = Nil | Cons of cell
and cell = { mutable hd : int; tl : list; }

If the structure of the list itself must also be modified (cells must be physically removed from the list), the tl field would also be declared as mutable:

# type list = Nil | Cons of cell
  and cell = { mutable hd : int; mutable tl : list };;
type list = Nil | Cons of cell
and cell = { mutable hd : int; mutable tl : list; }

Physical assignments are still useless to allocate mutable data: you write Cons {hd = 1; tl = l} to add 1 to the list l. Physical assignments that remain in OCaml programs should be just those assignments that are mandatory to implement the algorithm at hand.

Very often, pointers are used to implement physical modification of data structures. In OCaml programs this means using vectors or mutable fields in records.

In conclusion: You can use explicit pointers in OCaml, exactly as in C, but this is not natural, since you get back the usual drawbacks and difficulties of explicit pointers manipulation of classical algorithmic languages. See a more complete example below.

Defining pointers in OCaml

The general pointer type can be defined using the definition of a pointer: a pointer is either null, or a pointer to an assignable memory location:

# type 'a pointer = Null | Pointer of 'a ref;;
type 'a pointer = Null | Pointer of 'a ref

Explicit dereferencing (or reading the pointer's designated value) and pointer assignment (or writing to the pointer's designated memory location) are easily defined. We define dereferencing as a prefix operator named !^, and assignment as the infix ^:=.

# let ( !^ ) = function
    | Null -> invalid_arg "Attempt to dereference the null pointer"
    | Pointer r -> !r;;
val ( !^ ) : 'a pointer -> 'a = <fun>

# let ( ^:= ) p v =
    match p with
     | Null -> invalid_arg "Attempt to assign the null pointer"
     | Pointer r -> r := v;;
val ( ^:= ) : 'a pointer -> 'a -> unit = <fun>

Now we define the allocation of a new pointer initialized to point to a given value:

# let new_pointer x = Pointer (ref x);;
val new_pointer : 'a -> 'a pointer = <fun>

For instance, let's define and then assign a pointer to an integer:

# let p = new_pointer 0;;
val p : int pointer = Pointer {contents = 0}
# p ^:= 1;;
- : unit = ()
# !^p;;
- : int = 1

Integer Lists

Now we can define lists using explicit pointers as in usual imperative languages:

# type ilist = cell pointer
  and cell = { mutable hd : int; mutable tl : ilist };;
type ilist = cell pointer
and cell = { mutable hd : int; mutable tl : ilist; }

We then define allocation of a new cell, the list constructor and its associated destructors.

# let new_cell () = {hd = 0; tl = Null};;
val new_cell : unit -> cell = <fun>
# let cons x l =
    let c = new_cell () in
    c.hd <- x;
    c.tl <- l;
    (new_pointer c : ilist);;
val cons : int -> ilist -> ilist = <fun>
# let hd (l : ilist) = !^l.hd;;
val hd : ilist -> int = <fun>
# let tl (l : ilist) = !^l.tl;;
val tl : ilist -> ilist = <fun>

We can now write all kind of classical algorithms, based on pointers manipulation, with their associated loops, their unwanted sharing problems and their null pointer errors. For instance, list concatenation, as often described in literature, physically modifies its first list argument, hooking the second list to the end of the first:

# let append (l1 : ilist) (l2 : ilist) =
  let temp = ref l1 in
  while tl !temp <> Null do
    temp := tl !temp
  !^ !temp.tl <- l2;;
val append : ilist -> ilist -> unit = <fun>

# let l1 = cons 1 (cons 2 Null);;
val l1 : ilist =
   {contents = {hd = 1; tl = Pointer {contents = {hd = 2; tl = Null}}}}

# let l2 = cons 3 Null;;
val l2 : ilist = Pointer {contents = {hd = 3; tl = Null}}

# append l1 l2;;
- : unit = ()

The lists l1 and l2 are effectively catenated:

# l1;;
- : ilist =
 {contents =
   {hd = 1;
    tl =
      {contents = {hd = 2; tl = Pointer {contents = {hd = 3; tl = Null}}}}}}

Just a nasty side effect of physical list concatenation: l1 now contains the concatenation of the two lists l1 and l2, thus the list l1 no longer exists: in some sense append consumes its first argument. In other words, the value of a list data now depends on its history, that is on the sequence of function calls that use the value. This strange behaviour leads to a lot of difficulties when explicitly manipulating pointers. Try for instance, the seemingly harmless:

# append l1 l1;;
- : unit = ()

Then evaluate l1:

# l1;;
- : ilist =
 {contents =
   {hd = 1;
    tl =
      {contents = {hd = 2; tl = Pointer {contents = {hd = 3; tl = <cycle>}}}}}}

Polymorphic lists

We can define polymorphic lists using pointers; here is a simple implementation of those polymorphic mutable lists:

# type 'a lists = 'a cell pointer
  and 'a cell = { mutable hd : 'a pointer; mutable tl : 'a lists };;
type 'a lists = 'a cell pointer
and 'a cell = { mutable hd : 'a pointer; mutable tl : 'a lists; }
# let new_cell () = {hd = Null; tl = Null};;
val new_cell : unit -> 'a cell = <fun>
# let cons x l =
    let c = new_cell () in
    c.hd <- new_pointer x;
    c.tl <- l;
    (new_pointer c : 'a lists);;
val cons : 'a -> 'a lists -> 'a lists = <fun>
# let hd (l : 'a lists) = !^l.hd;;
val hd : 'a lists -> 'a pointer = <fun>
# let tl (l : 'a lists) = !^l.tl;;
val tl : 'a lists -> 'a lists = <fun>
# let append (l1 : 'a lists) (l2 : 'a lists) =
  let temp = ref l1 in
  while tl !temp <> Null do
    temp := tl !temp
  !^ !temp.tl <- l2;;
val append : 'a lists -> 'a lists -> unit = <fun>

Null pointers

So you've got a survey on your website which asks your readers for their names and ages. Only problem is that for some reason a few of your readers don't want to give you their age - they stubbornly refuse to fill that field in. What's a poor database administrator to do?

Assume that the age is represented by an int, there are two possible ways to solve this problem. The most common one (and the most wrong one) is to assume some sort of "special" value for the age which means that the age information wasn't collected. So if, say, age = -1 then the data wasn't collected, otherwise the data was collected (even if it's not valid!). This method kind of works until you start, for example, calculating the mean age of visitors to your website. Since you forgot to take into account your special value, you conclude that the mean age of visitors is 7½ years old, and you employ web designers to remove all the long words and use primary colours everywhere.

The other, correct method is to store the age in a field which has type "int or null". Here's a SQL table for storing ages:

create table users
  userid serial,
  name text not null,
  age int             -- may be null

If the age data isn't collected, then it goes into the database as a special SQL NULL value. SQL ignores this automatically when you ask it to compute averages and so on.

Programming languages also support nulls, although they may be easier to use in some than in others. In Java, any reference to an object can be null, so it might make sense in Java to store the age as an Integer and allow references to the age to be null. In C pointers can, of course, be null, but if you wanted a simple integer to be null, you'd have to first box it up into an object allocated by malloc on the heap.

OCaml has an elegant solution to the problem of nulls, using a simple polymorphic variant type defined (in Stdlib) as:

type 'a option = None | Some of 'a

A "null pointer" is written None. The type of age in our example above (an int which can be null) is int option (remember: backwards like int list and int binary_tree).

# Some 3;;
- : int option = Some 3

What about a list of optional ints?

# [None; Some 3; Some 6; None];;
- : int option list/2 = [None; Some 3; Some 6; None]

And what about an optional list of ints?

# Some [1; 2; 3];;
- : int list/2 option = Some [1; 2; 3]

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