Re: reference initialization

[Max Skaller <maxs@in.ot.com.au>]

Pierre Weis wrote:
 
> So, adding a test to detect this case we can initialize the vector
> properly, without using Obj.magic.
> 
> exception Not_yet_initialized of int;;
> exception Already_initialized of int;;
> exception Never_initialized of int;;
> 
> let initialize n f =
>  if n = 0 then [||] else
>  let init_v = Array.make n false in
>  let v = ref [||] in
>  let get i = if init_v.(i) then !v.(i) else raise (Not_yet_initialized i) in
>  let set i ei =
>    if !v = [||] then v := Array.make n ei;

Hmmm. This should work, even if 'ei' has a finaliser or mutable
field: 'ei' isn't a 'dummy' value, but a real value that the client
wanted in the array. On the other hand, a dummy value the client
is forced to supply may have dire consequences where the type
is either a class instance , or finalised: here
either construction or destruction may have arbitrary
semantics.

So this (the code you gave) is much better than having to supply a dummy
value.

The same problem occurs with 'forced' variable initialisation:
dummy values may have unwanted side-effects.
There is a tension here, since ocaml is not a purely
functional language.

-- 
John (Max) Skaller at OTT [Open Telecommications Ltd]
mailto:maxs@in.ot.com.au      -- at work
mailto:skaller@maxtal.com.au  -- at home