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From: Pierre Weis <pierre.weis@inria.fr>
Message-Id: <200309090932.LAA05613@pauillac.inria.fr>
Subject: Re: [Caml-list] GC Question
In-Reply-To: <3F5CE84F.4030609@univ-savoie.fr> from Christophe Raffalli at "Sep 8, 103 10:36:31 pm"
To: christophe.raffalli@univ-savoie.fr (Christophe Raffalli)
Date: Tue, 9 Sep 2003 11:32:01 +0200 (MET DST)
Cc: Damien.Doligez@inria.fr, caml-list@inria.fr
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> in the following kind code :
> 
> let l = ... a function building a long list ... in
> let l' = List.map fn l in (* or fold or anything similar *)
> ... no more reference to l ...
> 
> Once the beginning of l has been read to compute l' (assuming List.map 
> starts from the beginning of l) is the GC able to collect the beginning 
> of l ?

No it cannot since, AFAIK, l is not freed (i.e. assigned to 0) into
the stack (or local environment) before you leave the block where your
code occurs (normally the activation block of the function where it
appears in).

> If not how to write the code to ensure this behaviour of the GC ?
>
> Christophe Raffalli

Do not use let bindings, write function applications or function
compositions instead.
 (Instead of writing
  let l1 = f l0 in
  let l2 = g l1 in
  h l2
  write
  let i l = h (g (f l)) in
  i l0)

Alternatively, use explicit references to lists and free them
explicitely in your code ! This is tractable when the flow of control
of the code is simple enough (for instance in the case of a
clear succession of passes where data from previous passes are clearly
unused in the following steps).

Hope this helps,

Pierre Weis

INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://pauillac.inria.fr/~weis/


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