Queue.fold give wrong order?

Queue.fold give wrong order?

[Tom Wilkie]

Dear all

Is Queue.fold going over items in the wrong order?  It says "equivalent to List.fold_left" but I would expect the behaviour to be, when inserting items 1, then 2, then 3 a fold would be given items in that order?

Is there a good reason for this?  Could be have a rev_fold for the opposite order please?

Thanks

Tom

# ocaml
        Objective Caml version 3.10.2

# open Queue;;
# let q = Queue.create ();;
val q : '_a Queue.t = <abstr>
# Queue.add 1 q;;
- : unit = ()
# Queue.add 2 q;;     
- : unit = ()
# Queue.add 3 q;;
- : unit = ()
# Queue.fold (fun acc a -> a::acc) [] q;;
- : int list = [3; 2; 1]
# Queue.iter (fun a -> print_endline (string_of_int a)) q;;
1
2
3
- : unit = ()

RE: [Caml-list] Queue.fold give wrong order?

[David Allsopp]

Tom Wilkie wrote:
> Dear all
> 
> Is Queue.fold going over items in the wrong order?  It says "equivalent
> to List.fold_left" but I would expect the behaviour to be, when
> inserting items 1, then 2, then 3 a fold would be given items in that
> order?
> 
> Is there a good reason for this?  Could be have a rev_fold for the
> opposite order please?
> 
> Thanks
> 
> Tom
> 
> # ocaml
>         Objective Caml version 3.10.2
> 
> # open Queue;;
> # let q = Queue.create ();;
> val q : '_a Queue.t = <abstr>
> # Queue.add 1 q;;
> - : unit = ()
> # Queue.add 2 q;;
> - : unit = ()
> # Queue.add 3 q;;
> - : unit = ()
> # Queue.fold (fun acc a -> a::acc) [] q;;
> - : int list = [3; 2; 1]
> # Queue.iter (fun a -> print_endline (string_of_int a)) q;;
> 1
> 2
> 3
> - : unit = ()

List.fold_left (fun acc a -> a::acc) [] [1; 2; 3];;
List.iter (fun a -> print_endline (string_of_int a)) [1; 2; 3];;

If that still puzzles you then please, with respect, re-post to the
beginners' list.


David

Re: [Caml-list] Queue.fold give wrong order?

[Bruno Verlyck, Bruno.Verlyck]

Hi,

From: Tom Wilkie <tom@acunu.com>
Date: Tue, 19 Jan 2010 15:10:43 +0000
> Is Queue.fold going over items in the wrong order?  It says "equivalent to
> List.fold_left" but I would expect the behaviour to be, when inserting items 1,
> then 2, then 3 a fold would be given items in that order?
It's already the case.

> # Queue.fold (fun acc a -> a::acc) [] q;;
> - : int list = [3; 2; 1]
Your (fun acc a -> a::acc) is first called with args [] and 1, and
yields a next value for accu of [1]; and so on.

HTH,
Bruno.