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Manual Reference Pages  -  LOG::CONTEXTUAL (3)

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Log::Contextual - Simple logging interface with a contextual log



version 0.006005


 use Log::Contextual qw( :log :dlog set_logger with_logger );
 use Log::Contextual::SimpleLogger;
 use Log::Log4perl :easy;

 my $logger  = Log::Log4perl->get_logger;

 set_logger $logger;

 log_debug { program started };

 sub foo {

   my $minilogger = Log::Contextual::SimpleLogger->new({
     levels => [qw( trace debug )]

   my @args = @_;

   with_logger $minilogger => sub {
     log_trace { foo entered };
     my ($foo, $bar) = Dlog_trace { "params for foo: $_" } @args;
     # ...
     log_trace { foo left };


Beginning with version 1.008 Log::Dispatchouli also works out of the box with Log::Contextual:

 use Log::Contextual qw( :log :dlog set_logger );
 use Log::Dispatchouli;
 my $ld = Log::Dispatchouli->new({
    ident     => slrtbrfst,
    to_stderr => 1,
    debug     => 1,

 set_logger $ld;

 log_debug { program started };


Major benefits:
o Efficient

The logging functions take blocks, so if a log level is disabled, the block will not run:

 # the following wont run if debug is off
 log_debug { "the new count in the database is " . $rs->count };

Similarly, the D prefixed methods only Dumper the input if the level is enabled.

o Handy

The logging functions return their arguments, so you can stick them in the middle of expressions:

 for (log_debug { "downloading:\n" . join qq(\n), @_ } @urls) { ... }

o Generic

Log::Contextual is an interface for all major loggers. If you log through Log::Contextual you will be able to swap underlying loggers later.

o Powerful

Log::Contextual chooses which logger to use based on user defined CodeRefs. Normally you don’t need to know this, but you can take advantage of it when you need to later.

o Scalable

If you just want to add logging to your basic application, start with Log::Contextual::SimpleLogger and then as your needs grow you can switch to Log::Dispatchouli or Log::Dispatch or Log::Log4perl or whatever else.

This module is a simple interface to extensible logging. It exists to abstract your logging interface so that logging is as painless as possible, while still allowing you to switch from one logger to another.

It is bundled with a really basic logger, Log::Contextual::SimpleLogger, but in general you should use a real logger instead. For something more serious but not overly complicated, try Log::Dispatchouli (see SYNOPSIS for example.)


This module is certainly not complete, but we will not break the interface lightly, so I would say it’s safe to use in production code. The main result from that at this point is that doing:

 use Log::Contextual;

will die as we do not yet know what the defaults should be. If it turns out that nearly everyone uses the :log tag and :dlog is really rare, we’ll probably make :log the default. But only time and usage will tell.


See SETTING DEFAULT IMPORT OPTIONS for information on setting these project wide.


When you import this module you may use -logger as a shortcut for set_logger, for example:

 use Log::Contextual::SimpleLogger;
 use Log::Contextual qw( :dlog ),
   -logger => Log::Contextual::SimpleLogger->new({ levels => [qw( debug )] });

sometimes you might want to have the logger handy for other stuff, in which case you might try something like the following:

 my $var_log;
 BEGIN { $var_log = VarLogger->new }
 use Log::Contextual qw( :dlog ), -logger => $var_log;


The -levels import option allows you to define exactly which levels your logger supports. So the default, [qw(debug trace warn info error fatal)], works great for Log::Log4perl, but it doesn’t support the levels for Log::Dispatch. But supporting those levels is as easy as doing

 use Log::Contextual
   -levels => [qw( debug info notice warning error critical alert emergency )];


The -package_logger import option is similar to the -logger import option except -package_logger sets the logger for the current package.

Unlike -default_logger, -package_logger cannot be overridden with set_logger or with_logger.

 package My::Package;
 use Log::Contextual::SimpleLogger;
 use Log::Contextual qw( :log ),
   -package_logger => Log::Contextual::WarnLogger->new({
      env_prefix => MY_PACKAGE

If you are interested in using this package for a module you are putting on CPAN we recommend Log::Contextual::WarnLogger for your package logger.


The -default_logger import option is similar to the -logger import option except -default_logger sets the <B>defaultB> logger for the current package.

Basically it sets the logger to be used if set_logger is never called; so

 package My::Package;
 use Log::Contextual::SimpleLogger;
 use Log::Contextual qw( :log ),
   -default_logger => Log::Contextual::WarnLogger->new({
      env_prefix => MY_PACKAGE


Eventually you will get tired of writing the following in every single one of your packages:

 use Log::Log4perl;
 use Log::Log4perl :easy;
 BEGIN { Log::Log4perl->easy_init($DEBUG) }

 use Log::Contextual -logger => Log::Log4perl->get_logger;

You can set any of the import options for your whole project if you define your own Log::Contextual subclass as follows:

 package MyApp::Log::Contextual;

 use base Log::Contextual;

 use Log::Log4perl :easy;

 sub arg_default_logger { $_[1] || Log::Log4perl->get_logger }
 sub arg_levels { [qw(debug trace warn info error fatal custom_level)] }
 sub default_import { :log }

 # or maybe instead of default_logger
 sub arg_package_logger { $_[1] }

 # and almost definitely not this, which is only here for completeness
 sub arg_logger { $_[1] }

Note the $_[1] || in arg_default_logger. All of these methods are passed the values passed in from the arguments to the subclass, so you can either throw them away, honor them, die on usage, etc. To be clear, if you define your subclass, and someone uses it as follows:

 use MyApp::Log::Contextual -default_logger => $foo,
                            -levels => [qw(bar baz biff)];

Your arg_default_logger method will get $foo and your arg_levels will get [qw(bar baz biff)];

Additionally, the default_import method is what happens if a user tries to use your subclass with no arguments. The default just dies, but if you’d like to change the default to import a tag merely return the tags you’d like to import. So the following will all work:

 sub default_import { :log }

 sub default_import { :dlog }

 sub default_import { qw(:dlog :log ) }

See Log::Contextual::Easy::Default for an example of a subclass of Log::Contextual that makes use of default import options.



 my $logger = WarnLogger->new;
 set_logger $logger;


set_logger will just set the current logger to whatever you pass it. It expects a CodeRef, but if you pass it something else it will wrap it in a CodeRef for you. set_logger is really meant only to be called from a top-level script. To avoid foot-shooting the function will warn if you call it more than once.


 my $logger = WarnLogger->new;
 with_logger $logger => sub {
    if (1 == 0) {
       log_fatal { Non Logical Universe Detected };
    } else {
       log_info  { All is good };

Arguments: LOGGER CODEREF, CodeRef $to_execute

with_logger sets the logger for the scope of the CodeRef $to_execute. As with set_logger, with_logger will wrap $returning_logger with a CodeRef if needed.


Import Tag: :log

Arguments: CodeRef $returning_message, @args

log_$level functions all work the same except that a different method is called on the underlying $logger object. The basic pattern is:

 sub log_$level (&@) {
   if ($logger->is_$level) {

Note that the function returns it’s arguments. This can be used in a number of ways, but often it’s convenient just for partial inspection of passthrough data

 my @friends = log_trace {
   friends list being generated, data from first friend:  .
 } generate_friend_list();

If you want complete inspection of passthrough data, take a look at the Dlog_$level functions.

Which functions are exported depends on what was passed to -levels. The default (no -levels option passed) would export:


Import Tag: :log

Arguments: CodeRef $returning_message, Item $arg

This is really just a special case of the log_$level functions. It forces scalar context when that is what you need. Other than that it works exactly same:

 my $friend = logS_trace {
   I only have one friend:  .  Dumper($_[0]->TO_JSON)
 } friend();

See also: DlogS_$level.


Import Tag: :dlog

Arguments: CodeRef $returning_message, @args

All of the following six functions work the same as their log_$level brethren, except they return what is passed into them and put the stringified (with Data::Dumper::Concise) version of their args into $_. This means you can do cool things like the following:

 my @nicks = Dlog_debug { "names: $_" } map $_->value, $frew->names->all;

and the output might look something like:

 names: "fREW"

Which functions are exported depends on what was passed to -levels. The default (no -levels option passed) would export:


Import Tag: :dlog

Arguments: CodeRef $returning_message, Item $arg

Like logS_$level, these functions are a special case of Dlog_$level. They only take a single scalar after the $returning_message instead of slurping up (and also setting wantarray) all the @args

 my $pals_rs = DlogS_debug { "pals resultset: $_" }
   $schema->resultset(Pals)->search({ perlers => 1 });


Anywhere a logger object can be passed, a coderef is accepted. This is so that the user can use different logger objects based on runtime information. The logger coderef is passed the package of the caller, and the caller level the coderef needs to use if it wants more caller information. The latter is in a hashref to allow for more options in the future.

Here is a basic example of a logger that exploits caller to reproduce the output of warn with a logger:

 my @caller_info;
 my $var_log = Log::Contextual::SimpleLogger->new({
    levels  => [qw(trace debug info warn error fatal)],
    coderef => sub { chomp($_[0]); warn "$_[0] at $caller_info[1] line $caller_info[2].\n" }
 my $warn_faker = sub {
    my ($package, $args) = @_;
    @caller_info = caller($args->{caller_level});
 log_debug { test };

The following is an example that uses the information passed to the logger coderef. It sets the global logger to $l3, the logger for the A1 package to $l1, except the lol method in A1 which uses the $l2 logger and lastly the logger for the A2 package to $l2.

Note that it increases the caller level as it dispatches based on where the caller of the log function, not the log function itself.

 my $complex_dispatcher = do {

    my $l1 = ...;
    my $l2 = ...;
    my $l3 = ...;

    my %registry = (
       -logger => $l3,
       A1 => {
          -logger => $l1,
          lol     => $l2,
       A2 => { -logger => $l2 },

    sub {
       my ( $package, $info ) = @_;

       my $logger = $registry{-logger};
       if (my $r = $registry{$package}) {
          $logger = $r->{-logger} if $r->{-logger};
          my (undef, undef, undef, $sub) = caller($info->{caller_level} + 1);
          $sub =~ s/^\Q$package\E:://g;
          $logger = $r->{$sub} if $r->{$sub};
       return $logger;

 set_logger $complex_dispatcher;


Because this module is ultimately pretty looking glue (glittery?) with the awesome benefit of the Contextual part, users will often want to make their favorite logger work with it. The following are the methods that should be implemented in the logger:


The first six merely need to return true if that level is enabled. The latter six take the results of whatever the user returned from their coderef and log them. For a basic example see Log::Contextual::SimpleLogger.


In between the loggers and the log functions is a log router that is responsible for finding a logger to handle the log event and passing the log information to the logger. This relationship is described in the documentation for Log::Contextual::Role::Router.

Log::Contextual and packages that extend it will by default share a router singleton that implements the with_logger() and set_logger() functions and also respects the -logger, -package_logger, and -default_logger import options with their associated default value functions. The router singleton is available as the return value of the router() function. Users of Log::Contextual may overload router() to return instances of custom log routers that could for example work with loggers that use a different interface.


triddle - Tyler Riddle <>

voj - Jakob Voss <>


mst - Matt S. Trout <>


Arthur Axel fREW Schmidt <>


This software is copyright (c) 2015 by Arthur Axel fREW Schmidt.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

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perl v5.20.3 LOG::CONTEXTUAL (3) 2015-03-14

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