]>
Finally I think I've done the ``Further work part'' I mentioned for
vector initialization!
We end up with a new initialization function for arrays that ensures
the proper and unique initialisation of each element of the array, but
is simpler to use and understand:
(*
val initialize :
int -> 'a -> ((int -> 'a) -> (int -> 'a -> unit) -> unit) -> 'a array
[initialize n x f] returns a fresh array of length [n],
with elements initialized by function [f].
All the elements of the new array must be assigned once and only
once by the function [f]. [f] received two functions as arguments,
one to access elements of the new array, and the other to set the
elements of the new array. [f] can access to element [i] of the new
array provided [f] has already properly initialized element [i].
Raise [Not_yet_initialized i] if element [i] is accessed before being
assigned.
Raise [Already_initialized i] if element [i] is assigned twice.
Raise [Never_initialized i] if element [i] has never been assigned at
the end of initialization.
[Array.initialize n x f] uses [2 n] words of heap space.
*)
exception Not_yet_initialized of int;;
exception Already_initialized of int;;
exception Never_initialized of int;;
let initialize n x0 f =
if n = 0 then [||] else
let init_v = Array.make n false in
let v = Array.make n x0 in
let get i = if init_v.(i) then v.(i) else raise (Not_yet_initialized i) in
let set i ei =
if not init_v.(i) then (v.(i) <- ei; init_v.(i) <- true) else
raise (Already_initialized i) in
(f get set : unit);
for i = 0 to n - 1 do if not init_v.(i) then raise (Never_initialized i) done;
v;;
The examples can now be written more naturally, with a minimum of
modification (beside correction of original bugs in chooses) :
let fibs n =
let init_fibs get set =
set 0 1; set 1 1;
for i = 2 to n - 1 do
set i (get (i - 1) + get (i - 2))
done in
initialize n 0 init_fibs;;
let chooses n =
let init_chooses get set =
let set_ei i ei =
set i ei;
if n - i <> i then set (n - i) ei in
set_ei 0 1;
for i = 1 to n / 2 do
set_ei i (get (i - 1) * (n - i + 1) / i)
done in
initialize (n + 1) 0 init_chooses;;
let invs f n =
let init_inv get set =
for i = 0 to n - 1 do set (f i) i done in
initialize n 0 init_inv;;
Pierre Weis
INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://cristal.inria.fr/~weis/