diagnostics, splain - produce verbose warning diagnostics


   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


   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()

   Output from splain is directed to STDOUT, unlike the pragma.


   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";


   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

   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 }


   Not being able to say "no diagnostics" is annoying, but may not be

   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.


   Tom Christiansen <tchrist@mox.perl.com>, 25 June 1995.


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