diagnostics - Perl compiler pragma to force verbose warning diagnostics

splain - standalone program to do the same thing


As a pragma
use diagnostics;

use diagnostics -verbose;

enable diagnostics;

disable diagnostics;

Aa a program
perl program 2


The diagnostics Pragma

This 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 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 ala warn() are unaffected, allowing duplicate user messages to be displayed.

The splain Program

While apparently a whole nuther program, splain is actually nothing more than a link to the (executable) 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.


The following file is certain to trigger a few errors at

both runtime and compiletime
use diagnostics;


If you prefer to run your program first and look at its problem afterwards, do this
perl -w 2

Note that this is not in general possible in shells of more dubious heritage, as the theoretical

(perl -w

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

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


disable diagnostics; # only turns off runtime warnings


enable diagnostics; # turns back on runtime warnings


disable diagnostics;



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 }


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.


Tom Christiansen

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