Message-Id: <9210260033.AA01662@aitgw.ge.com>
Date: Sun, 25 Oct 92 19:33:12 EST
From: HEDDEN@ESDSDF.dnet.ge.com
Subject: Streams in Caml Light 0.5
To: caml-list@margaux
I am having a small difficulty using streams in Caml Light v0.5.
The example below might be a bug, but is more likely due to a
lack of understanding on my part.
The exception in the following toplevel session indicates that
there is a problem:
> Caml Light version 0.5
#include "test";;
map_stream : ('a -> 'b) -> 'a stream -> 'b stream = <fun>
filter_stream : ('a -> bool) -> 'a stream -> 'a stream = <fun>
int_stream : int stream = <abstract>
- : int = 1
- : int = 9
- : int = 25
odd_sq_stream : int stream = <abstract>
- : int = 49
Uncaught exception: Parse_failure
- : unit = ()
#quit();;
The file "test.ml" (that is loaded above) contains the following:
(* Maps a function over a stream *)
let rec map_stream func = function
[< 'x >] -> [< 'func x >]
| [< >] -> [< >]
;;
(* Filters a stream according to a predicate *)
let rec filter_stream pred strm =
match strm with
[< 'x >] -> if pred x then [< 'x >]
else filter_stream pred strm
| [< >] -> [< >]
;;
(* The stream of positive integers *)
let int_stream = ints 1
where rec ints n = [< 'n; ints (succ n) >]
;;
(* Use `filter_stream' and `map_stream' to create a
stream of the squares of the odd integers *)
(* These work correctly *)
stream_next (map_stream (function x->x*x)
(filter_stream (function n->(n mod 2)= 1)
int_stream));;
stream_next (map_stream (function x->x*x)
(filter_stream (function n->(n mod 2)= 1)
int_stream));;
stream_next (map_stream (function x->x*x)
(filter_stream (function n->(n mod 2)= 1)
int_stream));;
(* Encapsulate the above *)
let odd_sq_stream =
map_stream (function x->x*x)
(filter_stream (function n->(n mod 2)= 1)
int_stream)
;;
(* Test the above encapsulation *)
(* This works correctly *)
stream_next odd_sq_stream;;
(* This causes an error *)
stream_next odd_sq_stream;;
Why is it that the first time `odd_sq_stream' is invoked it returns
the correct value, but on subsequent calls it causes an error? Is
this a bug, or am I doing something wrong?
Jerry D. Hedden
hedden@esdsdf.dnet.ge.com