[p5sagit/p5-mst-13.2.git] / pod / perllexwarn.pod

=head1 NAME

perllexwarn - Perl Lexical Warnings

=head1 DESCRIPTION

The C<use warnings> pragma is a replacement for both the command line
flag B<-w> and the equivalent Perl variable, C<$^W>.

The pragma works just like the existing "strict" pragma.
This means that the scope of the warning pragma is limited to the
enclosing block. It also means that that the pragma setting will not
leak across files (via C<use>, C<require> or C<do>). This allows
authors to independently define the degree of warning checks that will
be applied to their module.

By default, optional warnings are disabled, so any legacy code that
doesn't attempt to control the warnings will work unchanged.

All warnings are enabled in a block by either of these:
 
    use warnings ;
    use warnings 'all' ;
 
Similarly all warnings are disabled in a block by either of these:

    no warnings ;
    no warnings 'all' ;

For example, consider the code below:

    use warnings ;
    my $a ;
    my $b ;
    {
        no warnings ;
	$b = 2 if $a EQ 3 ;
    }
    $b = 1 if $a NE 3 ;

The code in the enclosing block has warnings enabled, but the inner
block has them disabled. In this case that means that the use of the C<EQ>
operator won't trip a C<"Use of EQ is deprecated"> warning, but the use of
C<NE> will produce a C<"Use of NE is deprecated"> warning.

=head2 Default Warnings and Optional Warnings

Before the introduction of lexical warnings, Perl had two classes of
warnings: mandatory and optional. 

As its name suggests, if your code tripped a mandatory warning, you
would get a warning whether you wanted it or not.
For example, the code below would always produce an C<"isn't numeric">
warning about the "2:".

    my $a = "2:" + 3;

though the result will be 5.

With the introduction of lexical warnings, mandatory warnings now become
I<default> warnings. The difference is that although the previously
mandatory warnings are still enabled by default, they can then be
subsequently enabled or disabled with the lexical warning pragma. For
example, in the code below, an C<"integer overflow"> warning will only
be reported for the C<$a> variable.

    my $a = "2:" + 3;
    no warnings ;
    my $b = "2:" + 3;

Note that neither the B<-w> flag or the C<$^W> can be used to
disable/enable default warnings. They are still mandatory in this case.

=head2 What's wrong with B<-w> and C<$^W>

Although very useful, the big problem with using B<-w> on the command
line to enable warnings is that it is all or nothing. Take the typical
scenario when you are writing a Perl program. Parts of the code you
will write yourself, but it's very likely that you will make use of
pre-written Perl modules. If you use the B<-w> flag in this case, you
end up enabling warnings in pieces of code that you haven't written.

Similarly, using C<$^W> to either disable or enable blocks of code is
fundamentally flawed. For a start, say you want to disable warnings in
a block of code. You might expect this to be enough to do the trick:

     {
         local ($^W) = 0 ;
	 my $a =+ 2 ;
	 my $b ; chop $b ;
     }

When this code is run with the B<-w> flag, a warning will be produced
for the C<$a> line -- C<"Reversed += operator">.

The problem is that Perl has both compile-time and run-time warnings. To
disable compile-time warnings you need to rewrite the code like this:

     {
         BEGIN { $^W = 0 }
	 my $a =+ 2 ;
	 my $b ; chop $b ;
     }

The other big problem with C<$^W> is that way you can inadvertently
change the warning setting in unexpected places in your code. For example,
when the code below is run (without the B<-w> flag), the second call
to C<doit> will trip a C<"Use of uninitialized value"> warning, whereas
the first will not.

    sub doit
    {
        my $b ; chop $b ;
    }

    doit() ;

    {
        local ($^W) = 1 ;
        doit()
    }

This is a side-effect of C<$^W> being dynamically scoped.

Lexical warnings get around these limitations by allowing finer control
over where warnings can or can't be tripped.

=head2 Controlling Warnings from the Command Line

There are three Command Line flags that can be used to control when
warnings are (or aren't) produced:

=over 5

=item B<-w>

This is  the existing flag. If the lexical warnings pragma is B<not>
used in any of you code, or any of the modules that you use, this flag
will enable warnings everywhere. See L<Backward Compatibility> for
details of how this flag interacts with lexical warnings.

=item B<-W>
 
If the B<-W> flag is used on the command line, it will enable all warnings
throughout the program regardless of whether warnings were disabled
locally using C<no warnings> or C<$^W =0>. This includes all files that get
included via C<use>, C<require> or C<do>.
Think of it as the Perl equivalent of the "lint" command.

=item B<-X>

Does the exact opposite to the B<-W> flag, i.e. it disables all warnings.

=back

=head2 Backward Compatibility

If you are used with working with a version of Perl prior to the
introduction of lexically scoped warnings, or have code that uses both
lexical warnings and C<$^W>, this section will describe how they interact.

How Lexical Warnings interact with B<-w>/C<$^W>:

=over 5

=item 1.

If none of the three command line flags (B<-w>, B<-W> or B<-X>) that
control warnings is used and neither C<$^W> or lexical warnings are used,
then default warnings will be enabled and optional warnings disabled.
This means that legacy code that doesn't attempt to control the warnings
will work unchanged.

=item 2.

The B<-w> flag just sets the global C<$^W> variable as in 5.005 -- this
means that any legacy code that currently relies on manipulating C<$^W>
to control warning behavior will still work as is. 

=item 3.
 
Apart from now being a boolean, the C<$^W> variable operates in exactly
the same horrible uncontrolled global way, except that it cannot
disable/enable default warnings.

=item 4.
 
If a piece of code is under the control of the lexical warning pragma,
both the C<$^W> variable and the B<-w> flag will be ignored for the
scope of the lexical warning.

=item 5.
 
The only way to override a lexical warnings setting is with the B<-W>
or B<-X> command line flags.

=back

The combined effect of 3 & 4 is that it will will allow code which uses
the lexical warnings pragma to control the warning behavior of $^W-type
code (using a C<local $^W=0>) if it really wants to, but not vice-versa.

=head1 EXPERIMENTAL FEATURES

The features described in this section are experimental, and so subject
to change.

=head2 Category Hierarchy
 
A B<tentative> hierarchy of "categories" have been defined to allow groups
of warnings to be enabled/disabled in isolation.  The current
hierarchy is:

    all - +--- unsafe -------+--- taint
          |                  |
          |                  +--- substr
          |                  |
          |                  +--- signal
          |                  |
          |                  +--- closure
          |                  |
          |                  +--- overflow
          |                  |
          |                  +--- portable
          |                  |
          |                  +--- untie
          |                  |
          |                  +--- utf8
          |                  
          +--- io   ---------+--- pipe
          |                  |
          |                  +--- unopened
          |                  |
          |                  +--- closed
          |                  |
          |                  +--- newline
          |                  |
          |                  +--- exec
          |
          +--- syntax    ----+--- ambiguous
          |                  |
          |                  +--- semicolon
          |                  |
          |                  +--- precedence
          |                  |
          |                  +--- reserved
          |                  |
          |                  +--- digit
          |                  |
          |                  +--- parenthesis
          |                  |
          |                  +--- deprecated
          |                  |
          |                  +--- printf
          |
          +--- severe    ----+--- inplace
          |                  |
          |                  +--- internal
          |                  |
          |                  +--- debugging
          |
          |--- uninitialized
          |
          +--- void
          |
          +--- recursion
          |
          +--- redefine
          |
          +--- numeric
          |
          +--- once
          |
          +--- misc


Just like the "strict" pragma any of these categories can be combined

    use warnings qw(void redefine) ;
    no warnings qw(io syntax untie) ;

Also like the "strict" pragma, if there is more than one instance of the
warnings pragma in a given scope the cumulative effect is additive. 

    use warnings qw(void) ; # only "void" warnings enabled
    ...
    use warnings qw(io) ;   # only "void" & "io" warnings enabled
    ...
    no warnings qw(void) ;  # only "io" warnings enabled


=head2 Fatal Warnings
 
The presence of the word "FATAL" in the category list will escalate any
warnings from the category/categories specified that are detected in
the lexical scope into fatal errors. In the code below, there are 3
places where a deprecated warning will be detected, the middle one will
produce a fatal error.


    use warnings ;
 
    $a = 1 if $a EQ $b ;
 
    {
        use warnings FATAL => qw(deprecated) ;
        $a = 1 if $a EQ $b ;
    }
 
    $a = 1 if $a EQ $b ;
 
=head1 TODO
 
The experimental features need bottomed out.

  perldiag.pod
    Need to add warning class information and notes on
    how to use the class info with the warnings pragma.

  perl5db.pl
    The debugger saves and restores C<$^W> at runtime. I haven't checked
    whether the debugger will still work with the lexical warnings
    patch applied.

  diagnostics.pm
    I *think* I've got diagnostics to work with the lexical warnings
    patch, but there were design decisions made in diagnostics to work
    around the limitations of C<$^W>. Now that those limitations are gone,
    the module should be revisited.

=head1 SEE ALSO

L<warnings>.
 
=head1 AUTHOR
 
Paul Marquess
Commit	Line	Data
0453d815	1	=head1 NAME
	2
	3	perllexwarn - Perl Lexical Warnings
	4
	5	=head1 DESCRIPTION
5a3e7812	6
4438c4b7	7	The C<use warnings> pragma is a replacement for both the command line
0453d815	8	flag B<-w> and the equivalent Perl variable, C<$^W>.
	9
	10	The pragma works just like the existing "strict" pragma.
	11	This means that the scope of the warning pragma is limited to the
	12	enclosing block. It also means that that the pragma setting will not
	13	leak across files (via C<use>, C<require> or C<do>). This allows
	14	authors to independently define the degree of warning checks that will
	15	be applied to their module.
	16
	17	By default, optional warnings are disabled, so any legacy code that
	18	doesn't attempt to control the warnings will work unchanged.
	19
	20	All warnings are enabled in a block by either of these:
	21
4438c4b7	22	use warnings ;
4438c4b7	23	use warnings 'all' ;
0453d815	24
	25	Similarly all warnings are disabled in a block by either of these:
	26
4438c4b7	27	no warnings ;
4438c4b7	28	no warnings 'all' ;
0453d815	29
	30	For example, consider the code below:
	31
4438c4b7	32	use warnings ;
0453d815	33	my $a ;
	34	my $b ;
	35	{
4438c4b7	36	no warnings ;
0453d815	37	$b = 2 if $a EQ 3 ;
	38	}
	39	$b = 1 if $a NE 3 ;
	40
	41	The code in the enclosing block has warnings enabled, but the inner
	42	block has them disabled. In this case that means that the use of the C<EQ>
	43	operator won't trip a C<"Use of EQ is deprecated"> warning, but the use of
	44	C<NE> will produce a C<"Use of NE is deprecated"> warning.
	45
	46	=head2 Default Warnings and Optional Warnings
	47
	48	Before the introduction of lexical warnings, Perl had two classes of
	49	warnings: mandatory and optional.
	50
	51	As its name suggests, if your code tripped a mandatory warning, you
	52	would get a warning whether you wanted it or not.
252aa082	53	For example, the code below would always produce an C<"isn't numeric">
252aa082	54	warning about the "2:".
0453d815	55
252aa082	56	my $a = "2:" + 3;
0453d815	57
252aa082	58	though the result will be 5.
0453d815	59
	60	With the introduction of lexical warnings, mandatory warnings now become
	61	I<default> warnings. The difference is that although the previously
	62	mandatory warnings are still enabled by default, they can then be
	63	subsequently enabled or disabled with the lexical warning pragma. For
	64	example, in the code below, an C<"integer overflow"> warning will only
	65	be reported for the C<$a> variable.
	66
252aa082	67	my $a = "2:" + 3;
4438c4b7	68	no warnings ;
252aa082	69	my $b = "2:" + 3;
0453d815	70
	71	Note that neither the B<-w> flag or the C<$^W> can be used to
	72	disable/enable default warnings. They are still mandatory in this case.
	73
	74	=head2 What's wrong with B<-w> and C<$^W>
	75
	76	Although very useful, the big problem with using B<-w> on the command
	77	line to enable warnings is that it is all or nothing. Take the typical
	78	scenario when you are writing a Perl program. Parts of the code you
	79	will write yourself, but it's very likely that you will make use of
	80	pre-written Perl modules. If you use the B<-w> flag in this case, you
	81	end up enabling warnings in pieces of code that you haven't written.
	82
	83	Similarly, using C<$^W> to either disable or enable blocks of code is
	84	fundamentally flawed. For a start, say you want to disable warnings in
	85	a block of code. You might expect this to be enough to do the trick:
	86
	87	{
	88	local ($^W) = 0 ;
	89	my $a =+ 2 ;
	90	my $b ; chop $b ;
	91	}
	92
	93	When this code is run with the B<-w> flag, a warning will be produced
	94	for the C<$a> line -- C<"Reversed += operator">.
	95
	96	The problem is that Perl has both compile-time and run-time warnings. To
	97	disable compile-time warnings you need to rewrite the code like this:
	98
	99	{
	100	BEGIN { $^W = 0 }
	101	my $a =+ 2 ;
	102	my $b ; chop $b ;
	103	}
	104
	105	The other big problem with C<$^W> is that way you can inadvertently
	106	change the warning setting in unexpected places in your code. For example,
	107	when the code below is run (without the B<-w> flag), the second call
	108	to C<doit> will trip a C<"Use of uninitialized value"> warning, whereas
	109	the first will not.
	110
	111	sub doit
	112	{
	113	my $b ; chop $b ;
	114	}
	115
	116	doit() ;
	117
	118	{
	119	local ($^W) = 1 ;
	120	doit()
	121	}
	122
	123	This is a side-effect of C<$^W> being dynamically scoped.
	124
	125	Lexical warnings get around these limitations by allowing finer control
	126	over where warnings can or can't be tripped.
	127
	128	=head2 Controlling Warnings from the Command Line
	129
	130	There are three Command Line flags that can be used to control when
	131	warnings are (or aren't) produced:
	132
	133	=over 5
134
135	=item B<-w>
136
137	This is the existing flag. If the lexical warnings pragma is B<not>
138	used in any of you code, or any of the modules that you use, this flag
139	will enable warnings everywhere. See L<Backward Compatibility> for
140	details of how this flag interacts with lexical warnings.
141
142	=item B<-W>
143
144	If the B<-W> flag is used on the command line, it will enable all warnings
145	throughout the program regardless of whether warnings were disabled
4438c4b7	146	locally using C<no warnings> or C<$^W =0>. This includes all files that get
0453d815	147	included via C<use>, C<require> or C<do>.
	148	Think of it as the Perl equivalent of the "lint" command.
	149
	150	=item B<-X>
	151
	152	Does the exact opposite to the B<-W> flag, i.e. it disables all warnings.
	153
	154	=back
	155
	156	=head2 Backward Compatibility
	157
	158	If you are used with working with a version of Perl prior to the
	159	introduction of lexically scoped warnings, or have code that uses both
	160	lexical warnings and C<$^W>, this section will describe how they interact.
	161
	162	How Lexical Warnings interact with B<-w>/C<$^W>:
5a3e7812	163
0453d815	164	=over 5
	165
	166	=item 1.
	167
	168	If none of the three command line flags (B<-w>, B<-W> or B<-X>) that
	169	control warnings is used and neither C<$^W> or lexical warnings are used,
	170	then default warnings will be enabled and optional warnings disabled.
	171	This means that legacy code that doesn't attempt to control the warnings
	172	will work unchanged.
	173
	174	=item 2.
	175
	176	The B<-w> flag just sets the global C<$^W> variable as in 5.005 -- this
	177	means that any legacy code that currently relies on manipulating C<$^W>
	178	to control warning behavior will still work as is.
	179
	180	=item 3.
	181
	182	Apart from now being a boolean, the C<$^W> variable operates in exactly
	183	the same horrible uncontrolled global way, except that it cannot
	184	disable/enable default warnings.
	185
	186	=item 4.
	187
	188	If a piece of code is under the control of the lexical warning pragma,
	189	both the C<$^W> variable and the B<-w> flag will be ignored for the
	190	scope of the lexical warning.
	191
	192	=item 5.
	193
	194	The only way to override a lexical warnings setting is with the B<-W>
	195	or B<-X> command line flags.
	196
	197	=back
	198
	199	The combined effect of 3 & 4 is that it will will allow code which uses
4438c4b7	200	the lexical warnings pragma to control the warning behavior of $^W-type
0453d815	201	code (using a C<local $^W=0>) if it really wants to, but not vice-versa.
	202
	203	=head1 EXPERIMENTAL FEATURES
	204
	205	The features described in this section are experimental, and so subject
	206	to change.
	207
	208	=head2 Category Hierarchy
	209
252aa082	210	A B<tentative> hierarchy of "categories" have been defined to allow groups
0453d815	211	of warnings to be enabled/disabled in isolation. The current
	212	hierarchy is:
	213
	214	all - +--- unsafe -------+--- taint
	215	\| \|
	216	\| +--- substr
	217	\| \|
	218	\| +--- signal
	219	\| \|
	220	\| +--- closure
	221	\| \|
627300f0	222	\| +--- overflow
	223	\| \|
	224	\| +--- portable
	225	\| \|
0453d815	226	\| +--- untie
	227	\| \|
	228	\| +--- utf8
	229	\|
	230	+--- io ---------+--- pipe
	231	\| \|
	232	\| +--- unopened
	233	\| \|
	234	\| +--- closed
	235	\| \|
	236	\| +--- newline
	237	\| \|
	238	\| +--- exec
	239	\|
	240	+--- syntax ----+--- ambiguous
	241	\| \|
	242	\| +--- semicolon
	243	\| \|
	244	\| +--- precedence
	245	\| \|
	246	\| +--- reserved
	247	\| \|
627300f0	248	\| +--- digit
0453d815	249	\| \|
	250	\| +--- parenthesis
	251	\| \|
	252	\| +--- deprecated
	253	\| \|
	254	\| +--- printf
	255	\|
	256	+--- severe ----+--- inplace
	257	\| \|
	258	\| +--- internal
	259	\| \|
	260	\| +--- debugging
	261	\|
	262	\|--- uninitialized
	263	\|
	264	+--- void
	265	\|
	266	+--- recursion
	267	\|
	268	+--- redefine
	269	\|
	270	+--- numeric
	271	\|
	272	+--- once
	273	\|
	274	+--- misc
	275
0453d815	276
4438c4b7	277	Just like the "strict" pragma any of these categories can be combined
	278
	279	use warnings qw(void redefine) ;
	280	no warnings qw(io syntax untie) ;
	281
	282	Also like the "strict" pragma, if there is more than one instance of the
	283	warnings pragma in a given scope the cumulative effect is additive.
	284
	285	use warnings qw(void) ; # only "void" warnings enabled
	286	...
	287	use warnings qw(io) ; # only "void" & "io" warnings enabled
	288	...
	289	no warnings qw(void) ; # only "io" warnings enabled
	290
0453d815	291
	292	=head2 Fatal Warnings
	293
0453d815	294	The presence of the word "FATAL" in the category list will escalate any
4438c4b7	295	warnings from the category/categories specified that are detected in
	296	the lexical scope into fatal errors. In the code below, there are 3
	297	places where a deprecated warning will be detected, the middle one will
	298	produce a fatal error.
	299
	300
	301	use warnings ;
0453d815	302
	303	$a = 1 if $a EQ $b ;
	304
	305	{
4438c4b7	306	use warnings FATAL => qw(deprecated) ;
0453d815	307	$a = 1 if $a EQ $b ;
	308	}
	309
	310	$a = 1 if $a EQ $b ;
	311
	312	=head1 TODO
	313
	314	The experimental features need bottomed out.
	315
6bc102ca	316	perldiag.pod
	317	Need to add warning class information and notes on
	318	how to use the class info with the warnings pragma.
	319
0453d815	320	perl5db.pl
	321	The debugger saves and restores C<$^W> at runtime. I haven't checked
	322	whether the debugger will still work with the lexical warnings
	323	patch applied.
	324
	325	diagnostics.pm
	326	I think I've got diagnostics to work with the lexical warnings
	327	patch, but there were design decisions made in diagnostics to work
	328	around the limitations of C<$^W>. Now that those limitations are gone,
	329	the module should be revisited.
	330
0453d815	331	=head1 SEE ALSO
0453d815	332
4438c4b7	333	L<warnings>.
0453d815	334
	335	=head1 AUTHOR
	336
	337	Paul Marquess