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From: Alessandro Baretta <alex@baretta.com>
To: "Scott J," <jscott@planetinternet.be>, Ocaml <caml-list@inria.fr>
Subject: Re: [Caml-list] Runtime overflow and what to do
Date: Fri, 11 Oct 2002 12:16:13 +0200	[thread overview]
Message-ID: <3DA6A4ED.5050700@baretta.com> (raw)
In-Reply-To: <021501c270aa$ca400660$0100a8c0@janxp>



Scott J, wrote:
> Hi,
>  
> So it is not easy, perhaps even impossible if the processor doesn't set 
> a flag .
>  
> e.g. fact n begins first with a lis of negative numbers (after overflow) 
> then comes again a list with positive numbers and then it remains = 0
>  
> Scott

If your problem is only the factorial function, then you can 
calculate statically (i.e. at source-code writing time) the 
value of the maximum integer whose factorial will fit in 
O'Caml's representation of integers, and test against that, 
once and for all, before entering the recursive calculation.

exception overflow
let max_fact_arg = ...

let fact =
let rec rec_fact n =
   if n <= 0 then 1
   else n * rec_fact (n-1)
in function
| x when x <= max_fact_arg -> rec_fact n
| _ -> raise Overflow

This strategy allows to incur in hardly any runtime costs 
for the check, and it also allows you prove more easily that 
your program will not raise runtime Overflow exceptions (the 
precondition for not entering the exception raising branch 
does not have to be propagated out of a recursive function 
call).

Alex

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  parent reply	other threads:[~2002-10-11 10:05 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2002-10-10 15:01 Scott J,
2002-10-10 15:07 ` Maxence Guesdon
2002-10-10 15:11 ` Maxence Guesdon
2002-10-10 15:21 ` Luc Maranget
2002-10-10 18:13   ` Sven LUTHER
2002-10-10 18:57     ` Xavier Leroy
2002-10-10 22:17       ` Scott J,
2002-10-11  8:00         ` Sven LUTHER
2002-10-11 10:16         ` Alessandro Baretta [this message]
2002-10-12 16:13           ` John Carr
2002-10-12 16:58             ` Sven LUTHER
2002-10-12 17:12               ` John Prevost
2002-10-13  9:31               ` Xavier Leroy
2002-10-13  9:52                 ` Sven LUTHER
2002-10-13  9:57                   ` Xavier Leroy
2002-10-13 10:15                     ` Sven LUTHER
2002-10-13 13:25                   ` John Carr
2002-10-13  9:28             ` Xavier Leroy
2002-10-14  0:56               ` Chris Hecker
2002-10-14  9:46                 ` Jacques Garrigue
2002-10-14 11:02                   ` Scott J,
2002-10-14 14:05                   ` Tim Freeman
2002-10-14 15:32                     ` Stefano Zacchiroli
2002-10-14 16:12                       ` Alain Frisch
2002-10-14 16:49                   ` [Caml-list] new ocaml switches (was Re: Runtime overflow and what to do) Chris Hecker
2002-10-13  9:25         ` [Caml-list] Runtime overflow and what to do Xavier Leroy
2002-10-13 10:04           ` Daniel de Rauglaudre
2002-10-13 10:29             ` Pierre Weis
2002-10-13 10:47               ` Daniel de Rauglaudre
2002-10-13 12:38                 ` Pierre Weis
2002-10-13 16:14               ` Xavier Leroy
2002-10-13 17:06                 ` Michel Quercia
2002-10-15 19:15                 ` Pierre Weis
2002-10-15 19:25                   ` Xavier Leroy
2002-10-16  8:24                     ` Pierre Weis
2002-10-13 10:19           ` Pierre Weis
2002-10-11  8:02       ` Sven LUTHER
2002-10-11  6:42 ` Florian Hars

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