splain
produce verbose warning diagnostics
Synopsis
Using the
"diagnostics" pragma:
use diagnostics;
use diagnostics -verbose;
enable diagnostics;
disable diagnostics;
Using the
"splain" standalone filter program:
perl program 2>diag.out
splain [-v] [-p] diag.out
Using
diagnostics to get stack traces from a misbehaving
script:
perl -Mdiagnostics=-traceonly my_script.pl
add an example, a script, a trick and tips
examples
The following file is certain to trigger a few errors at both
runtime and compiletime:
use diagnostics;
print NOWHERE "nothing\n";
print STDERR "\n\tThis message should be unadorned.\n";
warn "\tThis is a user warning";
print "\nDIAGNOSTIC TESTER: Please enter a <CR> here: ";
my $a, $b = scalar <STDIN>;
print "\n";
print $x/$y;
If you prefer to run your program first and look at its problem
afterwards, do this:
perl -w test.pl 2>test.out
./splain < test.out
Note that this is not in general possible in shells of more
dubious heritage, as the theoretical
(perl -w test.pl >/dev/tty) >& test.out
./splain < test.out
Because you just moved the existing stdout to somewhere
else.
If you don’t want to modify your source code, but still have
on-the-fly warnings, do this:
exec 3>&1; perl -w test.pl 2>&1 1>&3 3>&- | splain 1>&2 3>&-
Nifty, eh?
If you want to control warnings on the fly, do something like
this. Make sure you do the "use" first, or you won’t be
able to get at the enable() or disable() methods.
use diagnostics; # checks entire compilation phase
print "\ntime for 1st bogus diags: SQUAWKINGS\n";
print BOGUS1 'nada';
print "done with 1st bogus\n";
disable diagnostics; # only turns off runtime warnings
print "\ntime for 2nd bogus: (squelched)\n";
print BOGUS2 'nada';
print "done with 2nd bogus\n";
enable diagnostics; # turns back on runtime warnings
print "\ntime for 3rd bogus: SQUAWKINGS\n";
print BOGUS3 'nada';
print "done with 3rd bogus\n";
disable diagnostics;
print "\ntime for 4th bogus: (squelched)\n";
print BOGUS4 'nada';
print "done with 4th bogus\n";
description
The
"diagnostics" Pragma
This module extends the terse diagnostics normally emitted
by both the perl compiler and the perl interpreter (from
running perl with a -w switch or "use
warnings"), augmenting them with the more
explicative and endearing descriptions found in perldiag.
Like the other pragmata, it affects the compilation phase of
your program rather than merely the execution phase.
To use in your
program as a pragma, merely invoke
use diagnostics;
at the start
(or near the start) of your program. (Note that this
does enable perl’s -w flag.) Your
whole compilation will then be subject(ed :-) to the
enhanced diagnostics. These still go out
STDERR .
Due to the
interaction between runtime and compiletime issues, and
because it’s probably not a very good idea anyway, you
may not use "no diagnostics" to turn them
off at compiletime. However, you may control their behaviour
at runtime using the disable() and enable()
methods to turn them off and on respectively.
The
-verbose flag first prints out the perldiag
introduction before any other diagnostics. The
$diagnostics::PRETTY variable can generate nicer
escape sequences for pagers.
Warnings
dispatched from perl itself (or more accurately, those that
match descriptions found in perldiag) are only displayed
once (no duplicate descriptions). User code generated
warnings a la warn() are unaffected, allowing
duplicate user messages to be displayed.
This module
also adds a stack trace to the error message when perl dies.
This is useful for pinpointing what caused the death. The
-traceonly (or just -t) flag turns
off the explanations of warning messages leaving just the
stack traces. So if your script is dieing, run it again
with
perl -Mdiagnostics=-traceonly my_bad_script
to see the call
stack at the time of death. By supplying the
-warntrace (or just -w) flag, any
warnings emitted will also come with a stack trace.
The
splain Program
While apparently a whole nuther program, splain is
actually nothing more than a link to the (executable)
diagnostics.pm module, as well as a link to the
diagnostics.pod documentation. The -v
flag is like the "use diagnostics
-verbose" directive. The -p
flag is like the $diagnostics::PRETTY variable.
Since you’re post-processing with splain,
there’s no sense in being able to enable() or
disable() processing.
Output from
splain is directed to STDOUT ,
unlike the pragma.
internals
Diagnostic messages derive from the perldiag.pod file when
available at runtime. Otherwise, they may be embedded in the file
itself when the splain package is built. See the Makefile
for details.
If an extant $SIG{__WARN__} handler is discovered, it
will continue to be honored, but only after the
diagnostics::splainthis() function (the module’s
$SIG{__WARN__} interceptor) has had its way with your
warnings.
There is a $diagnostics::DEBUG variable you may set if
you’re desperately curious what sorts of things are being
intercepted.
BEGIN { $diagnostics::DEBUG = 1 }
bugs
Not being able
to say "no diagnostics" is annoying, but may not
be insurmountable.
The
"-pretty" directive is called too
late to affect matters. You have to do this instead, and
before you load the module.
BEGIN { $diagnostics::PRETTY = 1 }
I could start
up faster by delaying compilation until it should be needed,
but this gets a "panic: top_level" when using the
pragma form in Perl 5.001e.
While
it’s true that this documentation is somewhat
subserious, if you use a program named splain, you
should expect a bit of whimsy.
author
Tom
Christiansen <tchrist[:at:]mox.perl[:dot:]com>, 25 June
1995.