Date: Mon, 15 Mar 1999 10:06:45 +0100
Message-Id: <199903150906.KAA29053@lsun565.lannion.cnet.fr>
From: Jean-Francois Monin <JeanFrancois.Monin@cnet.francetelecom.fr>
To: Alexey Nogin <nogin@cs.cornell.edu>
Subject: Re: List.filter in Ocaml 2.02
In-Reply-To: <36E95F00.129D35DB@CS.Cornell.EDU>
<36E854D1.E52CD73B@CS.Cornell.EDU>
Alexey Nogin wrote :
> > exception Identity
> >
> > let share f x = try f x with Identity -> x
> >
> > let filter f l =
> > let rec fil = function
> > | [] -> raise Identity
> > | h :: t ->
> > if f h then h :: fil t else share fil t in
> > share fil l
> [...]
> 2) Even if the list is not changed, you first allocate the space and put
> its copy there, then raise an exception and return the old value. That
> means that you first waste time in memory allocation and then you waste
> it in garbage collection.
I don't agree, because this construction
> > if f h then h :: fil t else share fil t in
^^
should never be evaluated when Identity is raised in the "fil t"
occurring just after. At least, if you replace "h :: fil t" with
"h $$ fil t", where
let ($$) x l = x :: l
you get the expected behavior (for mem alloc concerns).
It would be very surprising that the above program behaves not the same.
-- Jean-Francois
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