Date: Thu, 9 Dec 1993 20:36:02 +0100
From: lsanchez@dit.upm.es (Luis Sanchez Fernandez)
Message-Id: <199312091936.AA04537@yeti.dit.upm.es>
To: cr@dcs.edinburgh.ac.uk, lsanchez@dcs.edinburgh.ac.uk
Subject: Re: Lazy evaluation and caml-light
Hi,
this e-mail is related to the answer to another one that I sent
a month (more or less) ago. Sorry for the delay. Because of the
delay, I'm including the previous one to help to fix the ideas.
The problem in your program is this function~:
let rec recursive sb sd=selec({state=Evaluated (append true sb)},sd,
F(fun ()->distr2(sb,{state=Evaluated (recursive sb sd)})));;
With the recursive call you compute an infinite number of time
the value of
(recusive sb sd) !!!!!
You are right. That was the reason because I sent the first e-mail.
I didn't know how to do it better.
Here is a better way to write the same program:
**********************************************************************
type 'a stream_aux = Str of ('a * 'a stream)
| Unevaluated of (unit -> 'a stream)
| Not_Ready (* use for fix_point only *)
and 'a stream == 'a stream_aux ref;;
let rec Eval p = p := Eval' !p; !p
where Eval' = function
Str _ as s -> s
| Unevaluated f -> Eval (f ())
| Not_Ready -> raise (Failure "Can't evaluate any more")
;;
let fix_point f =
let fp = ref Not_Ready in
fp := !(f fp);
fp
;;
Mmm. I think that now I understand how fix_point works. The trick
is that the system keeps the local variable fp AFTER
fix_point is called. If I call twice to fix_point,
two different local variables are kept. Am I right?
let append x ls = ref (Str (x,ls));; (* not usefull *)
let append' x f a = ref (Str(x, ref (Unevaluated (fun () -> f a))));;
let stop_eval f a = ref (Unevaluated (fun () -> f a));;
let rec selec (f,g,h) =
match (Eval f) with Str (sf,rf) ->
if sf then
match (Eval g) with Str(sg,rg) -> append' sg selec (rf,rg,h)
else
match (Eval h) with Str(sh,rh) -> append' sh selec (rf,g,rh);;
let rec distr2 (f,g)=
let (Str(sf,rf)) = (Eval f) and (Str(sg,rg)) = (Eval g) in
if not sf then
append' sg distr2 (rf,rg)
else
distr2 (rf,rg);;
let rec recursive (sb,sd) =
fix_point f where
f = function x -> selec (
(append true sb),sd,
stop_eval distr2 (sb,x))
;;
Thanks for the help,
Luis Sanchez.