--- Guard/Guard.pm	2008/12/13 17:37:22	1.1
+++ Guard/Guard.pm	2010/03/12 17:25:58	1.23
@@ -4,7 +4,21 @@
 
 =head1 SYNOPSIS
 
- use Guard;
+   use Guard;
+   
+   # temporarily chdir to "/etc" directory, but make sure
+   # to go back to "/" no matter how myfun exits:
+   sub myfun {
+      scope_guard { chdir "/" };
+      chdir "/etc";
+   
+      code_that_might_die_or_does_other_fun_stuff;
+   }
+
+   # create an object that, when the last reference to it is gone,
+   # invokes the given codeblock:
+   my $guard = guard { print "destroyed!\n" };
+   undef $guard; # probably destroyed here
 
 =head1 DESCRIPTION
 
@@ -16,16 +30,23 @@
 objects, which execute a given code block when destroyed, and scoped
 guards, which are tied to the scope exit.
 
+=head1 FUNCTIONS
+
+This module currently exports the C<scope_guard> and C<guard> functions by
+default.
+
 =over 4
 
 =cut
 
 package Guard;
 
+no warnings;
+
 BEGIN {
-   $VERSION = '0.01';
+   $VERSION = '1.021';
    @ISA = qw(Exporter);
-   @EXPORT = qw(guard scope_guard cancel);
+   @EXPORT = qw(guard scope_guard);
 
    require Exporter;
 
@@ -37,10 +58,19 @@
 
 =item scope_guard BLOCK
 
+=item scope_guard ($coderef)
+
 Registers a block that is executed when the current scope (block,
 function, method, eval etc.) is exited.
 
-This is similar to this code fragment:
+See the EXCEPTIONS section for an explanation of how exceptions
+(i.e. C<die>) are handled inside guard blocks.
+
+The description below sounds a bit complicated, but that's just because
+C<scope_guard> tries to get even corner cases "right": the goal is to
+provide you with a rock solid clean up tool.
+
+The behaviour is similar to this code fragment:
 
    eval ... code following scope_guard ...
    {
@@ -48,69 +78,81 @@
       eval BLOCK;
       eval { $Guard::DIED->() } if $@;
    }
+   die if $@;
 
 Except it is much faster, and the whole thing gets executed even when the
 BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.
 
-See B<EXCEPTIONS>, below, for an explanation of exception handling
-(C<die>) within guard blocks.
-
-Example: Temporarily change the directory to F</etc> and make sure it's
-set back to F</> when the function returns:
+If multiple BLOCKs are registered to the same scope, they will be executed
+in reverse order. Other scope-related things such as C<local> are managed
+via the same mechanism, so variables C<local>ised I<after> calling
+C<scope_guard> will be restored when the guard runs.
+
+Example: temporarily change the timezone for the current process,
+ensuring it will be reset when the C<if> scope is exited:
+
+   use Guard;
+   use POSIX ();
+
+   if ($need_to_switch_tz) {
+      # make sure we call tzset after $ENV{TZ} has been restored
+      scope_guard { POSIX::tzset };
+
+      # localise after the scope_guard, so it gets undone in time
+      local $ENV{TZ} = "Europe/London";
+      POSIX::tzset;
 
-   sub dosomething {
-      scope_guard { chdir "/" };
-      chdir "/etc";
-
-      ...
+      # do something with the new timezone
    }
 
 =item my $guard = guard BLOCK
 
+=item my $guard = guard ($coderef)
+
 Behaves the same as C<scope_guard>, except that instead of executing
 the block on scope exit, it returns an object whose lifetime determines
 when the BLOCK gets executed: when the last reference to the object gets
 destroyed, the BLOCK gets executed as with C<scope_guard>.
 
-The returned object can be copied as many times as you want.
-
-See B<EXCEPTIONS>, below, for an explanation of exception handling
-(C<die>) within guard blocks.
+See the EXCEPTIONS section for an explanation of how exceptions
+(i.e. C<die>) are handled inside guard blocks.
 
 Example: acquire a Coro::Semaphore for a second by registering a
-timer. The timer callback references the guard used to unlock it again.
+timer. The timer callback references the guard used to unlock it
+again. (Please ignore the fact that C<Coro::Semaphore> has a C<guard>
+method that does this already):
 
-   use AnyEvent;
+   use Guard;
+   use Coro::AnyEvent;
    use Coro::Semaphore;
 
    my $sem = new Coro::Semaphore;
 
-   sub lock_1s {
+   sub lock_for_a_second {
       $sem->down;
       my $guard = guard { $sem->up };
 
-      my $timer;
-      $timer = AnyEvent->timer (after => 1, sub {
-         # do something
-         undef $sem;
-         undef $timer;
-      });
+      Coro::AnyEvent::sleep 1;
+
+      # $sem->up gets executed when returning
    }
 
 The advantage of doing this with a guard instead of simply calling C<<
 $sem->down >> in the callback is that you can opt not to create the timer,
-or your code can throw an exception before it can create the timer, or you
-can create multiple timers or other event watchers and only when the last
-one gets executed will the lock be unlocked.
+or your code can throw an exception before it can create the timer (or
+the thread gets canceled), or you can create multiple timers or other
+event watchers and only when the last one gets executed will the lock be
+unlocked. Using the C<guard>, you do not have to worry about catching all
+the places where you have to unlock the semaphore.
 
-=item Guard::cancel $guard
+=item $guard->cancel
 
 Calling this function will "disable" the guard object returned by the
 C<guard> function, i.e. it will free the BLOCK originally passed to
 C<guard >and will arrange for the BLOCK not to be executed.
 
-This can be useful when you use C<guard> to create a fatal cleanup handler
-and later decide it is no longer needed.
+This can be useful when you use C<guard> to create a cleanup handler to be
+called under fatal conditions and later decide it is no longer needed.
 
 =cut
 
@@ -120,26 +162,30 @@
 
 =head1 EXCEPTIONS
 
-Guard blocks should not normally throw exceptions (e.g. C<die>), after
-all, they are usually used to clean up after such exceptions. However, if
-something truly exceptional is happening, a guard block should be allowed
-to die. Also, programming errors are a large source of exceptions, and the
-programmer certainly wants to know about those.
+Guard blocks should not normally throw exceptions (that is, C<die>). After
+all, they are usually used to clean up after such exceptions. However,
+if something truly exceptional is happening, a guard block should of
+course be allowed to die. Also, programming errors are a large source of
+exceptions, and the programmer certainly wants to know about those.
 
-Since in most cases, the block executing when the guard gets executes does
+Since in most cases, the block executing when the guard gets executed does
 not know or does not care about the guard blocks, it makes little sense to
 let containing code handle the exception.
 
-Therefore, whenever a guard block throws an exception, it will be caught,
-and this module will call the code reference stored in C<$Guard::DIED>
+Therefore, whenever a guard block throws an exception, it will be caught
+by Guard, followed by calling the code reference stored in C<$Guard::DIED>
 (with C<$@> set to the actual exception), which is similar to how most
 event loops handle this case.
 
+The default for C<$Guard::DIED> is to call C<warn "$@">, i.e. the error is
+printed as a warning and the program continues.
+
+The C<$@> variable will be restored to its value before the guard call in
+all cases, so guards will not disturb C<$@> in any way.
+
 The code reference stored in C<$Guard::DIED> should not die (behaviour is
 not guaranteed, but right now, the exception will simply be ignored).
 
-The default for C<$Guard::DIED> is to call C<warn "$@">.
-
 =head1 AUTHOR
 
  Marc Lehmann <schmorp@schmorp.de>
@@ -147,8 +193,17 @@
 
 =head1 THANKS
 
-To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to
-the problem of exceptions.
+Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
+solution to the problem of exceptions.
+
+=head1 SEE ALSO
+
+L<Scope::Guard> and L<Sub::ScopeFinalizer>, which actually implement
+dynamic guards only, not scoped guards, and have a lot higher CPU, memory
+and typing overhead.
+
+L<Hook::Scope>, which has apparently never been finished and can corrupt
+memory when used.
 
 =cut