splain Man page

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 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
ūüôā 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 here: “;
my $a, $b = scalar ;
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 , 25 June 1995.

perl v5.22.1 2016-03-13 SPLAIN(1)