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

6       diagnostics, splain - produce verbose warning diagnostics
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SYNOPSIS

9       Using the "diagnostics" pragma:
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11           use diagnostics;
12           use diagnostics -verbose;
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14           enable  diagnostics;
15           disable diagnostics;
16
17       Using the "splain" standalone filter program:
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19           perl program 2>diag.out
20           splain [-v] [-p] diag.out
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22       Using diagnostics to get stack traces from a misbehaving script:
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24           perl -Mdiagnostics=-traceonly my_script.pl
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DESCRIPTION

27   The "diagnostics" Pragma
28       This module extends the terse diagnostics normally emitted by both the
29       perl compiler and the perl interpreter (from running perl with a -w
30       switch or "use warnings"), augmenting them with the more explicative
31       and endearing descriptions found in perldiag.  Like the other pragmata,
32       it affects the compilation phase of your program rather than merely the
33       execution phase.
34
35       To use in your program as a pragma, merely invoke
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37           use diagnostics;
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39       at the start (or near the start) of your program.  (Note that this does
40       enable perl's -w flag.)  Your whole compilation will then be subject(ed
41       :-) to the enhanced diagnostics.  These still go out STDERR.
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43       Due to the interaction between runtime and compiletime issues, and
44       because it's probably not a very good idea anyway, you may not use "no
45       diagnostics" to turn them off at compiletime.  However, you may control
46       their behaviour at runtime using the disable() and enable() methods to
47       turn them off and on respectively.
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49       The -verbose flag first prints out the perldiag introduction before any
50       other diagnostics.  The $diagnostics::PRETTY variable can generate
51       nicer escape sequences for pagers.
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53       Warnings dispatched from perl itself (or more accurately, those that
54       match descriptions found in perldiag) are only displayed once (no
55       duplicate descriptions).  User code generated warnings a la warn() are
56       unaffected, allowing duplicate user messages to be displayed.
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58       This module also adds a stack trace to the error message when perl
59       dies.  This is useful for pinpointing what caused the death.  The
60       -traceonly (or just -t) flag turns off the explanations of warning
61       messages leaving just the stack traces.  So if your script is dieing,
62       run it again with
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64         perl -Mdiagnostics=-traceonly my_bad_script
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66       to see the call stack at the time of death.  By supplying the
67       -warntrace (or just -w) flag, any warnings emitted will also come with
68       a stack trace.
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70   The splain Program
71       While apparently a whole nuther program, splain is actually nothing
72       more than a link to the (executable) diagnostics.pm module, as well as
73       a link to the diagnostics.pod documentation.  The -v flag is like the
74       "use diagnostics -verbose" directive.  The -p flag is like the
75       $diagnostics::PRETTY variable.  Since you're post-processing with
76       splain, there's no sense in being able to enable() or disable()
77       processing.
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79       Output from splain is directed to STDOUT, unlike the pragma.
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EXAMPLES

82       The following file is certain to trigger a few errors at both runtime
83       and compiletime:
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85           use diagnostics;
86           print NOWHERE "nothing\n";
87           print STDERR "\n\tThis message should be unadorned.\n";
88           warn "\tThis is a user warning";
89           print "\nDIAGNOSTIC TESTER: Please enter a <CR> here: ";
90           my $a, $b = scalar <STDIN>;
91           print "\n";
92           print $x/$y;
93
94       If you prefer to run your program first and look at its problem
95       afterwards, do this:
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97           perl -w test.pl 2>test.out
98           ./splain < test.out
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100       Note that this is not in general possible in shells of more dubious
101       heritage, as the theoretical
102
103           (perl -w test.pl >/dev/tty) >& test.out
104           ./splain < test.out
105
106       Because you just moved the existing stdout to somewhere else.
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108       If you don't want to modify your source code, but still have on-the-fly
109       warnings, do this:
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111           exec 3>&1; perl -w test.pl 2>&1 1>&3 3>&- | splain 1>&2 3>&-
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113       Nifty, eh?
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115       If you want to control warnings on the fly, do something like this.
116       Make sure you do the "use" first, or you won't be able to get at the
117       enable() or disable() methods.
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119           use diagnostics; # checks entire compilation phase
120               print "\ntime for 1st bogus diags: SQUAWKINGS\n";
121               print BOGUS1 'nada';
122               print "done with 1st bogus\n";
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124           disable diagnostics; # only turns off runtime warnings
125               print "\ntime for 2nd bogus: (squelched)\n";
126               print BOGUS2 'nada';
127               print "done with 2nd bogus\n";
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129           enable diagnostics; # turns back on runtime warnings
130               print "\ntime for 3rd bogus: SQUAWKINGS\n";
131               print BOGUS3 'nada';
132               print "done with 3rd bogus\n";
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134           disable diagnostics;
135               print "\ntime for 4th bogus: (squelched)\n";
136               print BOGUS4 'nada';
137               print "done with 4th bogus\n";
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INTERNALS

140       Diagnostic messages derive from the perldiag.pod file when available at
141       runtime.  Otherwise, they may be embedded in the file itself when the
142       splain package is built.   See the Makefile for details.
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144       If an extant $SIG{__WARN__} handler is discovered, it will continue to
145       be honored, but only after the diagnostics::splainthis() function (the
146       module's $SIG{__WARN__} interceptor) has had its way with your
147       warnings.
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149       There is a $diagnostics::DEBUG variable you may set if you're
150       desperately curious what sorts of things are being intercepted.
151
152           BEGIN { $diagnostics::DEBUG = 1 }
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BUGS

155       Not being able to say "no diagnostics" is annoying, but may not be
156       insurmountable.
157
158       The "-pretty" directive is called too late to affect matters.  You have
159       to do this instead, and before you load the module.
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161           BEGIN { $diagnostics::PRETTY = 1 }
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163       I could start up faster by delaying compilation until it should be
164       needed, but this gets a "panic: top_level" when using the pragma form
165       in Perl 5.001e.
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167       While it's true that this documentation is somewhat subserious, if you
168       use a program named splain, you should expect a bit of whimsy.
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AUTHOR

171       Tom Christiansen <tchrist@mox.perl.com>, 25 June 1995.
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175perl v5.30.1                      2019-11-29                         SPLAIN(1)
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