[Caml-list] mixing fast and [<>] stream: illegal?

[Caml-list] mixing fast and [<>] stream: illegal?

[Stefano Zacchiroli]

  From the ocaml 3.02 documentation:

           Stream builders

   Warning: these functions create streams with fast access; it is
   illegal to mix them with streams built with [< >]; would raise
   Failure when accessing such mixed streams.

   val from : (int -> 'a option) -> 'a t;;

   Stream.from f returns a stream built from the function f. To create a
   new stream element, the function f is called with the current stream
   count. The user function f must return either Some <value> for a
   value or None to specify the end of the stream. 

I have tried to mix fast and normal streams as follows:

           Objective Caml version 3.02

   # let i = ref 0;;
   val i : int ref = {contents=0}
   # let gen (foo:int) =
       let nextint = !i in
       i:=!i+1;
       Some nextint;;
   val gen : int -> int option = <fun>
   # let s = Stream.from gen;;
   val s : int Stream.t = <abstr>
   # Stream.next s;;
   - : int = 0
   # match s with parser [<'1>] -> true | [<>] -> false;;
   - : bool = true
   # match s with parser [<'1>] -> true | [<>] -> false;;
   - : bool = false

Seems to me that no illegal action was performed ...
Am I wrong?
Is really dangerous mixing the two streams?

TIA.
Cheers!

-- 
Stefano "Zack" Zacchiroli <zack@cs.unibo.it> ICQ# 33538863
Home Page: http://www.cs.unibo.it/~zacchiro
Undergraduate student of Computer Science @ University of Bologna, Italy
                 - Information wants to be Open -
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Re: [Caml-list] mixing fast and [<>] stream: illegal?

[Daniel de Rauglaudre]

Hi,

On Sun, Nov 04, 2001 at 06:59:52PM +0100, Stefano Zacchiroli wrote:

> Seems to me that no illegal action was performed ...
> Am I wrong?
> Is really dangerous mixing the two streams?

The doc meant "streams expressions built with [< >]", not parsers. The [< >]
in parsers are stream patterns, not stream expressions. Your code is ok.

What does not work are things like [< '1; Stream.from gen; '2 >].
It is not dangerous, it will just fail:

  # let gen i = Some i;;       
  val gen : 'a -> 'a option = <fun>
  # let s = [< '1; Stream.from gen; '2 >];;
  val s : int Stream.t = <abstr>
  # Stream.next s;;
  - : int = 1
  # Stream.next s;;
  Uncaught exception: Failure "illegal stream concatenation".

-- 
Daniel de RAUGLAUDRE
daniel.de_rauglaudre@inria.fr
http://cristal.inria.fr/~ddr/
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