MirBSD manpage: splain(1)

SPLAIN(1)       Perl Programmers Reference Guide        SPLAIN(1)


     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

         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 compila-
     tion will then be subject(ed :-) to the enhanced diagnos-
     tics. These still go out STDERR.

     Due to the interaction between runtime and compiletime
     issues, and because it's probably not a very good idea any-
     way, 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.

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     Warnings dispatched from perl itself (or more accurately,
     those that match descriptions found in perldiag) are only
     displayed once (no duplicate descriptions).  User code gen-
     erated warnings a la warn() are unaffected, allowing dupli-
     cate 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 documenta-
     tion.  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.


     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

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         (perl -w test.pl >/dev/tty) >& test.out
         ./splain < test.out

     Because you just moved the existing stdout to somewhere

     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 warn-

     There is a $diagnostics::DEBUG variable you may set if
     you're desperately curious what sorts of things are being

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         BEGIN { $diagnostics::DEBUG = 1 }


     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 subseri-
     ous, 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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