diagnostics - Perl compiler pragma to force verbose warning diagnostics

- NAME
- SYNOPSIS
- DESCRIPTION
  - The diagnostics Pragma
  - The splain Program
- EXAMPLES
- INTERNALS
- BUGS
- AUTHOR

# NAME

diagnostics - Perl compiler pragma to force verbose warning diagnostics

splain - standalone program to do the same thing

# SYNOPSIS

As a pragma:

        use diagnostics;
        use diagnostics -verbose;

        enable  diagnostics;
        disable diagnostics;

Aa a program:

        perl program 2>diag.out
        splain [-v] [-p] diag.out

# DESCRIPTION

## The diagnostics Pragma

This module extends the terse diagnostics normally emitted by both the perl compiler and the perl interpeter, augmenting them with the more explicative and endearing descriptions found in the perldiag manpage. 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 there behaviour at runtime using the `disable()` and `enable()` methods to turn them off and on respectively.

The -verbose flag first prints out the the perldiag manpage introduction before any other diagnostics. The $diagnostics::PRETTY variable can generate nicer escape sequences for pagers.

## 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()` processing.

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

# EXAMPLES

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";

# INTERNALS

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 }

# BUGS

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.

# AUTHOR

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