Re: reference initialization
 Pierre Weis
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Date:   (:) 
From:  Pierre Weis <Pierre.Weis@i...> 
Subject:  Re: reference initialization 
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/