NAME

Carp - alternative warn and die for modules

SYNOPSIS

use Carp;# warn user (from perspective of caller)carp "string trimmed to 80 chars";# die of errors (from perspective of caller)croak "We're outta here!";# die of errors with stack backtraceconfess "not implemented";# cluck, longmess and shortmess not exported by defaultuse Carp qw(cluck longmess shortmess);cluck "This is how we got here!"; # warn with stack backtrace$long_message   = longmess( "message from cluck() or confess()" );$short_message  = shortmess( "message from carp() or croak()" );

DESCRIPTION

The Carp routines are useful in your own modules because they act likedie() orwarn(), but with a message which is more likely to be useful to a user of your module. In the case ofcluck() andconfess(), that context is a summary of every call in the call-stack;longmess() returns the contents of the error message.

For a shorter message you can usecarp() orcroak() which report the error as being from where your module was called.shortmess() returns the contents of this error message. There is no guarantee that that is where the error was, but it is a good educated guess.

Carp takes care not to clobber the status variables$! and$^E in the course of assembling its error messages. This means that a$SIG{__DIE__} or$SIG{__WARN__} handler can capture the error information held in those variables, if it is required to augment the error message, and if the code callingCarp left useful values there. Of course,Carp can't guarantee the latter.

You can also alter the way the output and logic ofCarp works, by changing some global variables in theCarp namespace. See the section onGLOBAL VARIABLES below.

Here is a more complete description of howcarp andcroak work. What they do is search the call-stack for a function call stack where they have not been told that there shouldn't be an error. If every call is marked safe, they give up and give a full stack backtrace instead. In other words they presume that the first likely looking potential suspect is guilty. Their rules for telling whether a call shouldn't generate errors work as follows:

1. Any call from a package to itself is safe.

2. Packages claim that there won't be errors on calls to or from packages explicitly marked as safe by inclusion in@CARP_NOT, or (if that array is empty)@ISA. The ability to override what @ISA says is new in 5.8.

3. The trust in item 2 is transitive. If A trusts B, and B trusts C, then A trusts C. So if you do not override@ISA with@CARP_NOT, then this trust relationship is identical to, "inherits from".

4. Any call from an internal Perl module is safe. (Nothing keeps user modules from marking themselves as internal to Perl, but this practice is discouraged.)

5. Any call to Perl's warning system (eg Carp itself) is safe. (This rule is what keeps it from reporting the error at the point where you callcarp orcroak.)

6. $Carp::CarpLevel can be set to skip a fixed number of additional call levels. Using this is not recommended because it is very difficult to get it to behave correctly.

Forcing a Stack Trace

As a debugging aid, you can force Carp to treat a croak as a confess and a carp as a cluck acrossall modules. In other words, force a detailed stack trace to be given. This can be very helpful when trying to understand why, or from where, a warning or error is being generated.

This feature is enabled by 'importing' the non-existent symbol 'verbose'. You would typically enable it by saying

perl -MCarp=verbose script.pl

or by including the string-MCarp=verbose in the PERL5OPT environment variable.

Alternately, you can set the global variable$Carp::Verbose to true. See theGLOBAL VARIABLES section below.

Stack Trace formatting

At each stack level, the subroutine's name is displayed along with its parameters. For simple scalars, this is sufficient. For complex data types, such as objects and other references, this can simply display'HASH(0x1ab36d8)'.

Carp gives two ways to control this.

1. For objects, a method,CARP_TRACE, will be called, if it exists. If this method doesn't exist, or it recurses intoCarp, or it otherwise throws an exception, this is skipped, and Carp moves on to the next option, otherwise checking stops and the string returned is used. It is recommended that the object's type is part of the string to make debugging easier.

2. For any type of reference,$Carp::RefArgFormatter is checked (see below). This variable is expected to be a code reference, and the current parameter is passed in. If this function doesn't exist (the variable is undef), or it recurses intoCarp, or it otherwise throws an exception, this is skipped, and Carp moves on to the next option, otherwise checking stops and the string returned is used.

3. Otherwise, if neitherCARP_TRACE nor$Carp::RefArgFormatter is available, stringify the value ignoring any overloading.

GLOBAL VARIABLES

$Carp::MaxEvalLen

This variable determines how many characters of a string-eval are to be shown in the output. Use a value of0 to show all text.

Defaults to0.

$Carp::MaxArgLen

This variable determines how many characters of each argument to a function to print. Use a value of0 to show the full length of the argument.

Defaults to64.

$Carp::MaxArgNums

This variable determines how many arguments to each function to show. Use a false value to show all arguments to a function call. To suppress all arguments, use-1 or'0 but true'.

Defaults to8.

$Carp::Verbose

This variable makescarp() andcroak() generate stack backtraces just likecluck() andconfess(). This is howuse Carp 'verbose' is implemented internally.

Defaults to0.

$Carp::RefArgFormatter

This variable sets a general argument formatter to display references. Plain scalars and objects that implementCARP_TRACE will not go through this formatter. CallingCarp from within this function is not supported.

local $Carp::RefArgFormatter = sub { require Data::Dumper; Data::Dumper::Dump($_[0]); # not necessarily safe };

@CARP_NOT

This variable,in your package, says which packages arenot to be considered as the location of an error. Thecarp() andcluck() functions will skip over callers when reporting where an error occurred.

NB: This variable must be in the package's symbol table, thus:

# These workour @CARP_NOT; # file scopeuse vars qw(@CARP_NOT); # package scope@My::Package::CARP_NOT = ... ; # explicit package variable# These don't worksub xyz { ... @CARP_NOT = ... } # w/o declarations abovemy @CARP_NOT; # even at top-level

Example of use:

package My::Carping::Package;use Carp;our @CARP_NOT;sub bar     { .... or _error('Wrong input') }sub _error  {    # temporary control of where'ness, __PACKAGE__ is implicit    local @CARP_NOT = qw(My::Friendly::Caller);    carp(@_)}

This would makeCarp report the error as coming from a caller not inMy::Carping::Package, nor fromMy::Friendly::Caller.

Also read the"DESCRIPTION" section above, about howCarp decides where the error is reported from.

Use@CARP_NOT, instead of$Carp::CarpLevel.

OverridesCarp's use of@ISA.

%Carp::Internal

This says what packages are internal to Perl.Carp will never report an error as being from a line in a package that is internal to Perl. For example:

$Carp::Internal{ (__PACKAGE__) }++;# time passes...sub foo { ... or confess("whatever") };

would give a full stack backtrace starting from the first caller outside of __PACKAGE__. (Unless that package was also internal to Perl.)

%Carp::CarpInternal

This says which packages are internal to Perl's warning system. For generating a full stack backtrace this is the same as being internal to Perl, the stack backtrace will not start inside packages that are listed in%Carp::CarpInternal. But it is slightly different for the summary message generated bycarp orcroak. There errors will not be reported on any lines that are calling packages in%Carp::CarpInternal.

For exampleCarp itself is listed in%Carp::CarpInternal. Therefore the full stack backtrace fromconfess will not start inside ofCarp, and the short message from callingcroak is not placed on the line wherecroak was called.

$Carp::CarpLevel

This variable determines how many additional call frames are to be skipped that would not otherwise be when reporting where an error occurred on a call to one ofCarp's functions. It is fairly easy to count these call frames on calls that generate a full stack backtrace. However it is much harder to do this accounting for calls that generate a short message. Usually people skip too many call frames. If they are lucky they skip enough thatCarp goes all of the way through the call stack, realizes that something is wrong, and then generates a full stack backtrace. If they are unlucky then the error is reported from somewhere misleading very high in the call stack.

Therefore it is best to avoid$Carp::CarpLevel. Instead use@CARP_NOT,%Carp::Internal and%Carp::CarpInternal.

Defaults to0.

BUGS

The Carp routines don't handle exception objects currently. If called with a first argument that is a reference, they simply call die() or warn(), as appropriate.

SEE ALSO

Carp::Always,Carp::Clan

CONTRIBUTING

Carp is maintained by the perl 5 porters as part of the core perl 5 version control repository. Please see theperlhack perldoc for how to submit patches and contribute to it.

AUTHOR

The Carp module first appeared in Larry Wall's perl 5.000 distribution. Since then it has been modified by several of the perl 5 porters. Andrew Main (Zefram) <zefram@fysh.org> divested Carp into an independent distribution.

COPYRIGHT

Copyright (C) 1994-2013 Larry Wall

Copyright (C) 2011, 2012, 2013 Andrew Main (Zefram) <zefram@fysh.org>

LICENSE

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