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Here is the latest Caml Weekly News, for the week of 30 August to 06 September, 2005.

  1. Restarting a piece of code
  2. ocaml-gui mailing list
  3. GUIs and ocaml
  4. Polymorphic variants and scene graphs
  5. Unix.localtime not threadsafe?
  6. Existential types
  7. Offre d'emploi a l'IRISA

Restarting a piece of code


Andrej Bauer asked and Alex Baretta answered:
> I have a problem which I do not know how to program in The Right Way.
> I would be grateful to hear ideas on how to do it.
> Suppose I have a bunch of functions (the actual example is exact real
> number arithmetic) which optimistically compute results (e.g. we compute
> numbers up to some precision and hope it will be good enough) that may
> turn out not to be good enough later during the computation. When this
> happens, the entire computation needs to be restarted (e.g. we need to
> recompute the numbers again with higher precision).

This is a very interesting idea. Something similar comes up in my
AS/Xcaml application server when an HTTP resource which has already
computed a portion of the HTTP response realizes that it cannot continue
and must delegate the response to a different HTTP resource. An
exception is raised which is caught by the AS/Xcaml runtime system,
which aborts the current computation and restarts the computation of the
request by activating the HTTP resource whose URL is provided in the
exception body.


Back to your question, I suggest not to represent computations as thunks
but to use continuation passing style. This programming paradigm
provides enough flexibility to allow for different overflow thresholds
in different parts of the computation, without need for globals and side
effects (yes, you should worry about them, they are the bad guys).

exception Overflow

let grow_threshold old_value = ...

let compute threshold threshold_transformation computation =
    computation (threshold_transformation threshold)
  with Overflow ->
    let new_threshold = grow_threshold threshold in
      computation (threshold_transformation new_threshold)

Recursive computations occur through the compute driver, which also
receives a threshold_transformation function determining the value of
the threshold for the next computation. If you want your code to be
parametric with respect to the threshold growing policy, you might want
to modify the code as follows (-rectypes required!)

let rec compute grow_threshold = fun
      (compute grow_threshold)
      (threshold_transformation threshold)
  with Overflow ->
    let new_threshold = grow_threshold threshold in
        (compute grow_threshold)
        (threshold_transformation new_threshold)

In this case the driver for recursive computations is no longer the
global compute function but the first parameter of the computation function.
Andrej Bauer then asked and Alex Baretta answered:
> Can we avoid having to pass treshold around directly? You can imagine
> that users won't be too thrilled about this sort of thing.

I think you can. See arithmetic functions as building blocks for
computations---rather than computations themselves---which are
parametric with respect to the threshold value.

let unit_expression _ = assert false
let imaginary_unit_expression _ = assert false
let addition_expression _ = assert false
let negative_expression _ = assert false
let multiplication_expression _ = assert false
let inverse_expression _ = assert false
let exponential_expression _ = assert false
let logarithm_expression _ = assert false

exception Overflow

let rec compute grow_threshold threshold_transformation = fun
      (compute grow_threshold)
      (threshold_transformation threshold)
  with Overflow ->
    let new_threshold = grow_threshold threshold in
        (compute grow_threshold)
        (threshold_transformation new_threshold)

(* Here we define the primitive operations of the algebraic structure *)
(* We don't mind if the primitive operations are a little heavy, so   *)
(* long as it is easy to compose them to form complex computations.   *)

let one = fun compute threshold -> unit_expression threshold
let i = fun compute threshold -> imaginary_unit_expression threshold

let add x y = fun compute threshold ->
  let x' = compute x threshold in
  let y' = compute y threshold in
    addition_expression x y threshold

let neg x = fun compute threshold ->
  let x' = compute x threshold in
    negative_expression x threshold

let mul x y = fun compute threshold ->
  let x' = compute x threshold in
  let y' = compute y threshold in
    multiplication_expression x y threshold

let inv x = fun compute threshold ->
  let x' = compute x threshold in
    inverse_expression x threshold

let exp x = fun compute threshold ->
  let x' = compute x threshold in
    exponential_expression x threshold

let log x = fun compute threshold ->
  let x' = compute x threshold in
    logarithm_expression x threshold

(* Let's say these are all the basic computations we need. Now we can *)
(* start building more computations on top of these.                  *)

let sub x y = add x (neg y)
let div x y = mul x (inv y)
let pow x y = exp (mul (log x) y)
let root x y = exp (mul (log x) (inv y))
let two = add one one
let twoi = add i i
let cos x = div (add (exp (mul i x)) (exp (neg (mul i x)))) two
let sin x = div (sub (exp (mul i x)) (exp (neg (mul i x)))) twoi

> I sense monads. Or am I looking for dynamic binding?

You are looking for partial evaluation/multistage programming, but you
don't necessarily have to delve into MetaOcaml to solve your problem. As
you can see you can generate a homomorphism from the calculus of
imaginary numbers to the calculus of computations of imaginary numbers
which can be directly represented in Ocaml.

ocaml-gui mailing list


David Mentre announced:
I have created a Google group to discuss about GUI issues in/with
OCaml. For those interested by the subject...

Group email:  
Description:            Mailing list to discuss design and development
of a cross-platform Graphic User Interface for the OCaml language

GUIs and ocaml


Continuing the thread from last week, Jon Harrop asked and Olivier Andrieu answered:
> May I add another question to this thread: What do people think of
> OCaml's GUI libraries in terms of stability?

Well there are not that many libraries for GUIs in ocaml. We have :

 - LablTk/CamlTk
 - LablGTK
 - Wxcaml [1]
 - OCaml-Win32 [2]

Are there any others ?

Jon Harrop said and Olivier Andrieu answered:
> If anyone is seriously considering writing bindings for Cairo from
> OCaml then I'd like to know.

Gah! they already exist :

(No tarballs, only in the CVS repository).
Jon Harrop said and David Mentre answered:
> I had better clarify before continuing. I see two viable approaches here:
> 1. Write OCaml bindings to the native GUIs and then an OCaml wrapper that
> abstracts the nativeness away.
> 2. Write a new widget toolkit designed to render via OpenGL.
> If we do (1) then it will handle skinning and the intersection of the sets of
> functionality of the native GUIs.
> If we do (2) then customisation and skinning will be ignored but you have the
> advantage of real time 2D and 3D graphics.
> Option (2) seems much easier and more useful to me. I've never cared about
> skinning...

Well, in option (1), as somebody else underlined it, it is a lot more
than just skinning. How do you handle i18n and l10n issues? How to
display bidirectionnal languages? How to write Arabic, Hebrew or
Chinese characters? How to take input in Japanese or Tamil? How to
handle copy/paste and drag & drop with other applications of the
platform? Or to print PDF with Arabic and Indian characters in the
same document?

Maybe you don't have those issues for you local market, Jon. But as
soon as you are writting graphical applications to be used worldwide,
those issues are coming out pretty quickly. This is at least a
requirement for my application.

I really fear your under estimate the amount of work needed to
accomplish such a job. And the OCaml community seems pretty fragmented
on this GUI front.

To improve on the situation, I see following options:

 1. improve Labgtk2 documentation and use Gtk2 on all patforms. As our
try on Win32 have shown, compiling Gtk2 on Windows seems pretty
complicated and a moving target, albeit not impossible to do. I don't
know for MacOS X. And Gtk2 behaviour is different on Win32 than native
Win32 applications;

 2. write a *simple* GUI front end, in OCaml, with only simple modules
and/or simple objects. No fancy use of OCaml type system. Stay close
to ML core. Use Labgtk2, native Win32 and MacOS X libraries to render
the GUI. However I don't know the complexity of handling gory details,
like encoding of strings;

 3. (Jon option) write a pure GUI from scratch, in pure OCaml. A
project similar to Qt or WxWidgets for C++. It seems doable to have a
basic GUI but handling all i18n and desktop interoperability issues
seems pretty complicated;

 4. Use Labltk. I can't really comment on it, as I have never used it
and can't say about its graphical behaviour or desktop integration
(copy/paste and drag&drop);

 5. drop OCaml to write GUI. Use other languages (usually C++) to make
front-ends and use OCaml code only as backend, communicating through
sockets or shared memory. Maybe the most viable long term solution.

Personnally, I don't know what decision to take. My current GUI code
is in Lablgtk2 but, as stated in other emails, use of Gtk2 is too
difficult for me. And I don't want to go into another reinvent the
wheel syndrom. (sigh)
Matt Gushee said, Skaller answered, and Chris Campbell said:
> > I'm not sold on the idea of a "simple" GUI front end.
> I am. My idea is this: GET RID OF CALLBACKS.
> The idea is to use (user space) threads instead.
> The big problem with GUI's is that they're passive.
> You write 'event handlers' and have to store the current
> state manually. By control inverting the code, you can
> just use a thread for each widget.

Joe Armstrong did something like this with Erlang and X called Ex11. 
Can't remember how it works exactly, but it seems each widget is a
thread and can process events independantly of the others.

Polymorphic variants and scene graphs


Jon Harrop said:
Here's my first attempt at using the technique described in Jacques' paper 
"Code reuse using polymorphic variants" to make an extensible scene graph. 
This could be useful for GUI code because users could easily add their own 

First, we define functions to render a polygon or a group (list of scene 

  type polygon = [ `Polygon of vec2 list ]

  let render_polygon (`Polygon rs) =
    GlDraw.begins `polygon;
    List.iter vertex rs;
    GlDraw.ends ()

  type 'a group = [ `Group of 'a list ]

  let rec render_group render (`Group l) =
    List.iter (render_group render) l

  type 'a t =
      [ `Polygon of vec2 list 
      |`Group of 'a list ]

  let rec render1 self = function
    | #polygon as polygon -> render_polygon polygon
    | #group as group -> render_group (render1 self) group

This "render1" function needs a Y combinator to implement recursion:

  let rec render1' t = render1 render1' t

That "render1'" function can now be used to render scene graphs containing 
polygons and groups.

Next, we define a function to render an axis-aligned rectangle (e.g. for a 

  type rectangle = [ `Rectangle of vec2 * vec2 ]

  let render_rectangle (`Rectangle (l, u)) =
    render_polygon (`Polygon [vec2 l.x l.y; vec2 u.x l.y;
                              vec2 u.x u.y; vec2 l.x u.y])

  let render2 self = function
    | #rectangle as rectangle -> render_rectangle rectangle
    | #t as t -> render1 self t

By dispatching over type "t", this render function can now render rectangles 
as well as polygons and groups. Here's the final "render2'" function:

  let rec render2' t = render2 render2' t

Do polymorphic variants look like the way to go for scene graphs? They are 
slower than ordinary variants but performance is not so important in scene 
graph code and, particularly, not in GUI code.

Now, we want to add:

1. GL naming to identify which GUI element has been clicked on.

2. A "get_bound" function.

3. Layout functions, e.g. "hbox" and "vbox".

4. GL picking to search for the first GUI element under a mouse click.

5. Rendering of text.

and, finally, a selection of widgets.

Unix.localtime not threadsafe?


Yaron Minsky asked:
I was looking at the Unix.localtime implementation, and it appears to
me that it's not threadsafe.  I'm wondering if anyone can confirm.

First off, the underlying localtime call is definitely not re-entrant.
 The tm data structure is shared among all calls, leading to the
possibility of races.  What I'm not sure of is whether there can be a
race given the locking of the OCaml runtime.  Here's the code from
gmtime.c in the ocaml distribution:

static value alloc_tm(struct tm *tm)
  value res;
  res = alloc_small(9, 0);
  Field(res,0) = Val_int(tm->tm_sec);
  Field(res,1) = Val_int(tm->tm_min);
  Field(res,2) = Val_int(tm->tm_hour);
  Field(res,3) = Val_int(tm->tm_mday);
  Field(res,4) = Val_int(tm->tm_mon);
  Field(res,5) = Val_int(tm->tm_year);
  Field(res,6) = Val_int(tm->tm_wday);
  Field(res,7) = Val_int(tm->tm_yday);
  Field(res,8) = tm->tm_isdst ? Val_true : Val_false;
  return res;

CAMLprim value unix_localtime(value t)
  time_t clock;
  struct tm * tm;
  clock = (time_t) Double_val(t);
  tm = localtime(&clock);
  if (tm == NULL) unix_error(EINVAL, "localtime", Nothing);
  return alloc_tm(tm);

If I understand the way ocaml's locking works, another thread could
make a call to localtime where alloc_small is called.  If there are
mixed C threads and Ocaml threads, then a call by the C thread at that
point could muck up the tm data structure before switching back to
OCaml, thus leading to bad data getting into the tm data structure.

So, does this seem like a going bug?  We think we may have encountered
this in the wild, so this may be more than a theoretical bug.
Bardur Arantsson answered and Florian Weimer added:
> I don't think the glibc/Linux localtime() man page explicitly states 
> this, but I expect that it returns a pointer to a *thread-local* 
> statically allocated struct tm... in which case there's no problem.
> Most other system functions whose API looks non-threadsafe do the same. 
> ('errno' would be the standard example).

Thread-local storage is a recent innovation in the Linux camp.
Previously, developers seem to think that no efficient implementation
was possible.  That's why there all the *_r variants (such as

I believe Solaris does something in the direction you suggest.
Xavier Leroy also answered:
As others mentioned, the C library implementation of this function
could use thread-local storage to avoid this particular race.  I don't
think this is guaranteed, though.

> If I understand the way ocaml's locking works, another thread could
> make a call to localtime where alloc_small is called.

No, at least not another Caml thread: GC triggered from C code
(e.g. alloc_small()) cannot cause context switches.  So, from a Caml
standpoint, the call to localtime() and the following allocation of
its result are atomic.

> If there are mixed C threads and Ocaml threads, then a call by the C
> thread at that point could muck up the tm data structure before
> switching back to OCaml, thus leading to bad data getting into the
> tm data structure.

Yes, this is possible in principle if 1- some non-Caml threads call
localtime(), and 2- the implementation of that function does not use
thread-local storage.
Damien Bobillot said:
For information : Mac OS X use thread-local storage since version 10.2.

Existential types


Lukasz Stafiniak asked and Jacques Garrigue answered:
> I use an abstract type and a one-way typecast operator to implement
> existential quantification; e. g.
> type 'a t
> type unknown_t
> let some (v : 'a t) = ((Obj.magic v) : unknown_t t)
> Good?

This is a standard technique that is used in labltk for instance.
There should be no problem as long as your ['a t] type has a uniform
representation (i.e. this use of magic only encodes more refined type
Note that if your values are implemented in ocaml, there is a
reasonable possibility that you could avoid the magic altogether.
And if this is not the case (i.e. implemented in ocaml but magic
required), there is a serious risk that this is actually unsound.
Remember that the function Obj.repr of type ['a -> Obj.t] is unsound,
contrary to the intuition.
Which leads to the rule of thumb: the only "safe" uses of  magic are
when dealing with "unsafe" values (implemented in C.)

By the way, I'm not sure I would call this existantial quantification,
as it is only one-way. This looks more like a simple form of

Offre d'emploi a l'IRISA


Thomas Genet announced (in French only):
L'unité de Recherche INRIA de RENNES recrute sur contrat à durée
déterminée (appellation ingénieur expert) : 

                         1 Ingénieur recherche & développement

Lieu de travail : INRIA-Rennes

Projet d'accueil : Lande

Début du contrat: entre le 1/09/05 et le 31/12/05

Durée du contrat : 12 mois

Formation requise : doctorat, DEA, DESS, M2R ou M2Pro en Informatique

Activité du Projet d'accueil: Le projet Lande est une équipe de
recherche qui s'intéresse à la vérification formelle de logiciels. En
particulier, Lande travaille en partenariat avec France Telecom R&D et
Thomson R&D sur la vérification de protocoles cryptographiques. Lande
développe une technique de vérification automatique, à l'état de
prototype, qui a déjà été appliquée avec succès à des protocoles
cryptographiques industriels.

Objectif du poste: A partir des techniques développées par l'équipe
Lande, l'objectif est de réaliser un outil complet, intégré et open
source pour la vérification de protocoles cryptographiques. Les
protocoles visés sont des protocoles de commerce électronique et de
diffusion de contenu video numérique. Ce travail sera réalisé en
collaboration avec France Telecom R&D et Thomson R&D. 

Qualités requises:

- solide expérience dans le développement d'applications 
  complexes en Objective Caml. 
- autonomie
- expérience du travail en équipe
- anglais courant
- des connaissances dans le domaine de la vérification et/ou des protocoles
  cryptographiques seraient un plus 

Contact:  ( (

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