[ViewVC] Annotation of: cvs/Guard/README

NAME
    Guard - safe cleanup blocks

SYNOPSIS
     use Guard;

DESCRIPTION
    This module implements so-called "guards". A guard is something (usually
    an object) that "guards" a resource, ensuring that it is cleaned up when
    expected.

    Specifically, this module supports two different types of guards: guard
    objects, which execute a given code block when destroyed, and scoped
    guards, which are tied to the scope exit.

FUNCTIONS
    This module currently exports the "scope_guard" and "guard" functions by
    default.

    scope_guard BLOCK
        Registers a block that is executed when the current scope (block,
        function, method, eval etc.) is exited.

        The description below sounds a bit complicated, but that's just
        because "scope_guard" tries to get even corner cases "right": the
        goal is to provide you with a rock solid clean up tool.

        This is similar to this code fragment:

           eval ... code following scope_guard ...
           {
              local $@;
              eval BLOCK;
              eval { $Guard::DIED->() } if $@;
           }
           die if $@;

        Except it is much faster, and the whole thing gets executed even
        when the BLOCK calls "exit", "goto", "last" or escapes via other
        means.

        See EXCEPTIONS, below, for an explanation of exception handling
        ("die") within guard blocks.

        Example: Temporarily change the directory to /etc and make sure it's
        set back to / when the function returns:

           sub dosomething {
              scope_guard { chdir "/" };
              chdir "/etc";

              ...
           }

    my $guard = guard BLOCK
        Behaves the same as "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
        "scope_guard".

        The returned object can be copied as many times as you want.

        See EXCEPTIONS, below, for an explanation of exception handling
        ("die") within 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.

           use AnyEvent;
           use Coro::Semaphore;

           my $sem = new Coro::Semaphore;

           sub lock_1s {
              $sem->down;
              my $guard = guard { $sem->up };

              my $timer;
              $timer = AnyEvent->timer (after => 1, sub {
                 # do something
                 undef $sem;
                 undef $timer;
              });
           }

        The advantage of doing this with a guard instead of simply calling
        "$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.

    Guard::cancel $guard
        Calling this function will "disable" the guard object returned by
        the "guard" function, i.e. it will free the BLOCK originally passed
        to "guard "and will arrange for the BLOCK not to be executed.

        This can be useful when you use "guard" to create a fatal cleanup
        handler and later decide it is no longer needed.

EXCEPTIONS
    Guard blocks should not normally throw exceptions (e.g. "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.

    Since in most cases, the block executing when the guard gets executes
    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
    $Guard::DIED (with $@ set to the actual exception), which is similar to
    how most event loops handle this case.

    The code reference stored in $Guard::DIED should not die (behaviour is
    not guaranteed, but right now, the exception will simply be ignored).

    The default for $Guard::DIED is to call "warn "$@"".

AUTHOR
     Marc Lehmann <schmorp@schmorp.de>
     http://home.schmorp.de/

THANKS
    To Marco Maisenhelder, who reminded me of the $Guard::DIED solution to
    the problem of exceptions.

Revision:	1.2
Committed:	Sat Dec 13 17:50:29 2008 UTC (15 years, 5 months ago) by root
Branch:	MAIN
CVS Tags:	rel-0_1
Changes since 1.1:	+119 -123 lines
Log Message:	0.1
#	User	Rev	Content
1	root	1.1	NAME
2	root	1.2	Guard - safe cleanup blocks
3	root	1.1
4			SYNOPSIS
5			use Guard;
6
7			DESCRIPTION
8	root	1.2	This module implements so-called "guards". A guard is something (usually
9			an object) that "guards" a resource, ensuring that it is cleaned up when
10			expected.
11
12			Specifically, this module supports two different types of guards: guard
13			objects, which execute a given code block when destroyed, and scoped
14			guards, which are tied to the scope exit.
15
16			FUNCTIONS
17			This module currently exports the "scope_guard" and "guard" functions by
18			default.
19
20			scope_guard BLOCK
21			Registers a block that is executed when the current scope (block,
22			function, method, eval etc.) is exited.
23
24			The description below sounds a bit complicated, but that's just
25			because "scope_guard" tries to get even corner cases "right": the
26			goal is to provide you with a rock solid clean up tool.
27
28			This is similar to this code fragment:
29
30			eval ... code following scope_guard ...
31			{
32			local $@;
33			eval BLOCK;
34			eval { $Guard::DIED->() } if $@;
35			}
36			die if $@;
37
38			Except it is much faster, and the whole thing gets executed even
39			when the BLOCK calls "exit", "goto", "last" or escapes via other
40			means.
41
42			See EXCEPTIONS, below, for an explanation of exception handling
43			("die") within guard blocks.
44
45			Example: Temporarily change the directory to /etc and make sure it's
46			set back to / when the function returns:
47
48			sub dosomething {
49			scope_guard { chdir "/" };
50			chdir "/etc";
51
52			...
53			}
54
55			my $guard = guard BLOCK
56			Behaves the same as "scope_guard", except that instead of executing
57			the block on scope exit, it returns an object whose lifetime
58			determines when the BLOCK gets executed: when the last reference to
59			the object gets destroyed, the BLOCK gets executed as with
60			"scope_guard".
61
62			The returned object can be copied as many times as you want.
63
64			See EXCEPTIONS, below, for an explanation of exception handling
65			("die") within guard blocks.
66
67			Example: acquire a Coro::Semaphore for a second by registering a
68			timer. The timer callback references the guard used to unlock it
69			again.
70
71			use AnyEvent;
72			use Coro::Semaphore;
73
74			my $sem = new Coro::Semaphore;
75
76			sub lock_1s {
77			$sem->down;
78			my $guard = guard { $sem->up };
79
80			my $timer;
81			$timer = AnyEvent->timer (after => 1, sub {
82			# do something
83			undef $sem;
84			undef $timer;
85			});
86			}
87
88			The advantage of doing this with a guard instead of simply calling
89			"$sem->down" in the callback is that you can opt not to create the
90			timer, or your code can throw an exception before it can create the
91			timer, or you can create multiple timers or other event watchers and
92			only when the last one gets executed will the lock be unlocked.
93
94			Guard::cancel $guard
95			Calling this function will "disable" the guard object returned by
96			the "guard" function, i.e. it will free the BLOCK originally passed
97			to "guard "and will arrange for the BLOCK not to be executed.
98
99			This can be useful when you use "guard" to create a fatal cleanup
100			handler and later decide it is no longer needed.
101
102			EXCEPTIONS
103			Guard blocks should not normally throw exceptions (e.g. "die"), after
104			all, they are usually used to clean up after such exceptions. However,
105			if something truly exceptional is happening, a guard block should be
106			allowed to die. Also, programming errors are a large source of
107			exceptions, and the programmer certainly wants to know about those.
108
109			Since in most cases, the block executing when the guard gets executes
110			does not know or does not care about the guard blocks, it makes little
111			sense to let containing code handle the exception.
112
113			Therefore, whenever a guard block throws an exception, it will be
114			caught, and this module will call the code reference stored in
115			$Guard::DIED (with $@ set to the actual exception), which is similar to
116			how most event loops handle this case.
117
118			The code reference stored in $Guard::DIED should not die (behaviour is
119			not guaranteed, but right now, the exception will simply be ignored).
120
121			The default for $Guard::DIED is to call "warn "$@"".
122	root	1.1
123			AUTHOR
124			Marc Lehmann <schmorp@schmorp.de>
125			http://home.schmorp.de/
126
127	root	1.2	THANKS
128			To Marco Maisenhelder, who reminded me of the $Guard::DIED solution to
129			the problem of exceptions.
130