Pointers in OCaml
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}
type
list = ^cell;
cell = record
hd: integer;
tl: cell;
end;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;
begin
new(p);
p^.hd := x;
p^.tl := l;
cons := p
end;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:
# (* Physical append *)
let append (l1 : ilist) (l2 : ilist) =
let temp = ref l1 in
while tl !temp <> Null do
temp := tl !temp
done;
!^ !temp.tl <- l2;;
val append : ilist -> ilist -> unit = <fun>
# (* An example: *)
let l1 = cons 1 (cons 2 Null);;
val l1 : ilist =
Pointer
{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 =
Pointer
{contents =
{hd = 1;
tl =
Pointer
{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 =
Pointer
{contents =
{hd = 1;
tl =
Pointer
{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
done;
!^ !temp.tl <- l2;;
val append : 'a lists -> 'a lists -> unit = <fun>