| Carp - alternative warn and die for modules |
Carp - alternative warn and die for modules
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 backtrace
confess "not implemented";
# cluck, longmess and shortmess not exported by default
use 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()" );
The Carp routines are useful in your own modules because
they act like die() or warn(), but with a message which is more
likely to be useful to a user of your module. In the case of
cluck() and confess(), 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 use carp() or croak() 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 calling Carp left useful values there.
Of course, Carp can't guarantee the latter.
You can also alter the way the output and logic of Carp works, by
changing some global variables in the Carp namespace. See the
section on GLOBAL VARIABLES below.
Here is a more complete description of how carp and croak 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:
@CARP_NOT, or
(if that array is empty) @ISA. The ability to override what
@ISA says is new in 5.8.
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''.
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.)
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 call carp or croak.)
$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.
As a debugging aid, you can force Carp to treat a croak as a confess and a carp as a cluck across all 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 the GLOBAL VARIABLES section below.
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.
CARP_TRACE, will be called, if it exists. If
this method doesn't exist, or it recurses into Carp, 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.
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 into Carp, 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.
Otherwise, if neither CARP_TRACE nor $Carp::RefArgFormatter is
available, stringify the value ignoring any overloading.
This variable determines how many characters of a string-eval are to
be shown in the output. Use a value of 0 to show all text.
Defaults to 0.
This variable determines how many characters of each argument to a
function to print. Use a value of 0 to show the full length of the
argument.
Defaults to 64.
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 to 8.
This variable makes carp() and croak() generate stack backtraces
just like cluck() and confess(). This is how use Carp 'verbose'
is implemented internally.
Defaults to 0.
This variable sets a general argument formatter to display references.
Plain scalars and objects that implement CARP_TRACE will not go through
this formatter. Calling Carp from within this function is not supported.
local $Carp::RefArgFormatter = sub { require Data::Dumper; Data::Dumper::Dump($_[0]); # not necessarily safe };
This variable, in your package, says which packages are not to be
considered as the location of an error. The carp() and cluck()
functions will skip over callers when reporting where an error occurred.
NB: This variable must be in the package's symbol table, thus:
# These work
our @CARP_NOT; # file scope
use vars qw(@CARP_NOT); # package scope
@My::Package::CARP_NOT = ... ; # explicit package variable
# These don't work
sub xyz { ... @CARP_NOT = ... } # w/o declarations above
my @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 make Carp report the error as coming from a caller not
in My::Carping::Package, nor from My::Friendly::Caller.
Also read the DESCRIPTION section above, about how Carp decides
where the error is reported from.
Use @CARP_NOT, instead of $Carp::CarpLevel.
Overrides Carp's use of @ISA.
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.)
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 by carp or croak. There errors
will not be reported on any lines that are calling packages in
%Carp::CarpInternal.
For example Carp itself is listed in %Carp::CarpInternal.
Therefore the full stack backtrace from confess will not start
inside of Carp, and the short message from calling croak is
not placed on the line where croak was called.
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 of Carp'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 that Carp 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 to 0.
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.
the Carp::Always manpage, the Carp::Clan manpage
the Carp manpage is maintained by the perl 5 porters as part of the core perl 5 version control repository. Please see the the perlhack manpage perldoc for how to submit patches and contribute to it.
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 (C) 1994-2013 Larry Wall
Copyright (C) 2011, 2012, 2013 Andrew Main (Zefram) <zefram@fysh.org>
This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
| Carp - alternative warn and die for modules |