[Chapter 7] 7.2.10 diagnostics - Force Verbose Warning Diagnostics

7.2.10 diagnostics - Force Verbose Warning Diagnostics

# As a pragma:
use diagnostics;
use diagnostics -verbose;

enable  diagnostics;
disable diagnostics;

# As a program:
$ perl program 2>diag.out
$ splain [-v] [-p] diag.out

The diagnostics module extends the terse diagnostics normally emitted by both the Perl compiler and the Perl interpreter, augmenting them with the more explicative and endearing descriptions found in Chapter 9, Diagnostic Messages. It affects the compilation phase of your program rather than merely the execution phase.

To use in your program as a pragma, merely say:

use diagnostics;

at the start (or near the start) of your program. (Note that this enables Perl's -w flag.) Your whole compilation will then be subject to the enhanced diagnostics. These are still issued to STDERR.

Due to the interaction between run-time and compile-time issues, and because it's probably not a very good idea anyway, you may not use:

no diagnostics

to turn diagnostics off at compile time. However, you can turn diagnostics on or off at run-time by invoking diagnostics::enable() and diagnostics::disable(), respectively.

The -verbose argument first prints out the perldiag(1) manpage introduction before any other diagnostics. The $diagnostics::PRETTY variable, if set in a BEGIN block, results in nicer escape sequences for pagers:

BEGIN { $diagnostics::PRETTY = 1 }

7.2.10.1 The standalone program

While apparently a whole other program, splain is actually nothing more than a link to the (executable) diagnostics.pm module. It acts upon the standard error output of a Perl program, which you may have treasured up in a file, or piped directly to splain.

The -v flag has the same effect as:

use diagnostics -verbose

The -p flag sets $diagnostics::PRETTY to true. Since you're post-processing with splain, there's no sense in being able to enable() or disable() diagnostics.

Output from splain (unlike the pragma) is directed to STDOUT.

7.2.10.2 Examples

The following file is certain to trigger a few errors at both run-time and compile-time:

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 problems afterward, do this while talking to a Bourne-like shell:

perl -w test.pl 2>test.out
./splain < test.out

If you don't want to modify your source code, but still want on-the-fly warnings, do this:

perl -w -Mdiagnostics test.pl

If you want to control warnings on the fly, do something like this. (Make sure the use comes 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 run-time warnings
print "\ntime for 2nd bogus: (squelched)\n";
print BOGUS2 'nada';
print "done with 2nd bogus\n";

enable diagnostics; # turns back on run-time 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";