[Caml-list] transactions?

[Caml-list] transactions?

[skaller]

Is there an Ocaml eqivalent of begin/end blocking section?
I.e. something that sets the global lock if threads
are enabled (but still compiles if they're not)?

So can write for example:
  begin_trx(); 
  if Hashtbl.mem h k then () else Hashtbl.add h k v;
  end_trx()

and know the table won't get changed by another thread
between the lookup and insertion?

-- 
John Skaller, mailto:skaller@users.sf.net
voice: 061-2-9660-0850, 
snail: PO BOX 401 Glebe NSW 2037 Australia
Checkout the Felix programming language http://felix.sf.net



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Re: [Caml-list] transactions?

[Benjamin Geer]

skaller wrote:
> Is there an Ocaml eqivalent of begin/end blocking section?
> I.e. something that sets the global lock if threads
> are enabled (but still compiles if they're not)?

You can use camlp4's pa_ifdef.cmo to create a global mutex only if 
threads are enabled.  The utility functions below take care of this, as 
well as unlocking the mutex if an exception is thrown.

---------------------------------------------------------------------
(* Implements try-with-finally. *)
let run_with_finally ~f ~(finally: unit -> unit) =
   let res =
     try
       f ()
     with e ->
       finally ();
       raise e
   in
     finally ();
     res ;;

(* Locks a mutex, calls a function, then unlocks the mutex.  The mutex
    is also unlocked if the function throws an exception. *)
ifdef THREADS then
   let run_in_mutex ~f ~(mutex : Mutex.t) =
     Mutex.lock mutex;
     run_with_finally ~f ~finally:(function () -> Mutex.unlock mutex) ;;

(* If threads are enabled, creates a mutex, otherwise returns unit. *)
let create_optional_mutex () =
   (ifdef THREADS then
      Mutex.create ()
    else
      ()) ;;

(* Calls a function, using the supplied mutex if threads are enabled. *)
let run_in_optional_mutex ~f ~mutex =
   (ifdef THREADS then
      run_in_mutex ~f ~mutex
    else
      f ()) ;;
---------------------------------------------------------------------

Example usage:

---------------------------------------------------------------------
let hash_mutex = create_optional_mutex () ;;

let add_to_hash h k v =
   run_in_optional_mutex
     ~f:(function () ->
         if Hashtbl.mem h k then () else Hashtbl.add h k v)
     ~mutex:hash_mutex ;;
---------------------------------------------------------------------

Ben

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Re: [Caml-list] transactions?

[skaller]

On Sun, 2004-05-02 at 23:36, Benjamin Geer wrote:
> skaller wrote:
> > Is there an Ocaml eqivalent of begin/end blocking section?
> > I.e. something that sets the global lock if threads
> > are enabled (but still compiles if they're not)?
> 
> You can use camlp4's pa_ifdef.cmo to create a global mutex only if 
> threads are enabled.  The utility functions below take care of this, as 
> well as unlocking the mutex if an exception is thrown.

Ah. OK. Thanks! 

-- 
John Skaller, mailto:skaller@users.sf.net
voice: 061-2-9660-0850, 
snail: PO BOX 401 Glebe NSW 2037 Australia
Checkout the Felix programming language http://felix.sf.net



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Re: [Caml-list] transactions?

[Benjamin Geer]

Benjamin Geer wrote:
>> Is there an Ocaml eqivalent of begin/end blocking section?
>> I.e. something that sets the global lock if threads
>> are enabled (but still compiles if they're not)?
> 
> You can use camlp4's pa_ifdef.cmo to create a global mutex only if 
> threads are enabled.  The utility functions below take care of this, as 
> well as unlocking the mutex if an exception is thrown.

Actually I've just realised that pa_ifdef.cmo seems to be deprecated 
(it's not mentioned in the camlp4 docs anymore), but you can use 
pa_macro.cmo[1].  Here's a revised version:

------------------------------------------------------------------------
(* Implements try-with-finally. *)
let try_with_finally ~(f : unit -> 'a) ~(finally: unit -> unit) =
   let res =
     try
       f ()
     with e ->
       finally ();
       raise e
   in
     finally ();
     res ;;

(* Locks a mutex, calls a function, then unlocks the mutex.  The mutex
    is also unlocked if the function throws an exception. *)
IFDEF THREADS THEN
   let call_in_mutex ~f ~(mutex : Mutex.t) =
     Mutex.lock mutex;
     try_with_finally ~f ~finally:(function () -> Mutex.unlock mutex) END ;;

(* If threads are enabled, creates a mutex, otherwise returns unit. *)
let create_optional_mutex () =
   (IFDEF THREADS THEN
      Mutex.create ()
    ELSE
      () END) ;;

(* This function's arguments are a function to be called, and a value
    returned by create_optional_mutex.  If threads are enabled, it
    calls the function in a mutex.  If threads are disabled, it
    just calls the function. *)
let call_in_optional_mutex ~f ~mutex =
   (IFDEF THREADS THEN
      call_in_mutex ~f ~mutex
    ELSE
      f () END) ;;
------------------------------------------------------------------------

Note that you have to take care of defining the THREADS symbol.

Ben

[1] http://caml.inria.fr/camlp4/manual/manual002.html#toc1

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[Caml-list] Camlp4 docs

[Martin Jambon]

On Sun, 2 May 2004, Benjamin Geer wrote:

> Actually I've just realised that pa_ifdef.cmo seems to be deprecated
> (it's not mentioned in the camlp4 docs anymore), but you can use
> pa_macro.cmo[1].
...
> [1] http://caml.inria.fr/camlp4/manual/manual002.html#toc1

But where is the documentation for defining macros?
The only source of information I found while searching for this a few
days ago was the comments in the source file pa_macro.ml.


Martin

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