replace ckWARN macros with functions

[perl5.git] / pod / perlfaq7.pod

diff --git a/pod/perlfaq7.pod b/pod/perlfaq7.pod
index 19fa780..644b065 100644 (file)
--- a/pod/perlfaq7.pod
+++ b/pod/perlfaq7.pod
@@ -1,6 +1,6 @@
 =head1 NAME
 
 =head1 NAME
 

-perlfaq7 - General Perl Language Issues ($Revision: 1.21 $, $Date: 2005/01/21 12:10:22 $)
+perlfaq7 - General Perl Language Issues ($Revision: 1.23 $, $Date: 2005/04/07 21:39:34 $)

 
 =head1 DESCRIPTION
 
 
 =head1 DESCRIPTION
 


@@ -54,8 +54,8 @@ count as though they were quoted:
 
     This                    is like this
     ------------            ---------------
 
     This                    is like this
     ------------            ---------------

-    $foo{line}              $foo{"line"}
-    bar => stuff            "bar" => stuff
+    $foo{line}              $foo{'line'}
+    bar => stuff            'bar' => stuff

 
 The final semicolon in a block is optional, as is the final comma in a
 list.  Good style (see L<perlstyle>) says to put them in except for
 
 The final semicolon in a block is optional, as is the final comma in a
 list.  Good style (see L<perlstyle>) says to put them in except for


@@ -97,7 +97,7 @@ See L<perllexwarn> for more details.
        no warnings;          # temporarily turn off warnings
        $a = $b + $c;         # I know these might be undef
     }
        no warnings;          # temporarily turn off warnings
        $a = $b + $c;         # I know these might be undef
     }

-    
+

 Additionally, you can enable and disable categories of warnings.
 You turn off the categories you want to ignore and you can still
 get other categories of warnings.  See L<perllexwarn> for the
 Additionally, you can enable and disable categories of warnings.
 You turn off the categories you want to ignore and you can still
 get other categories of warnings.  See L<perllexwarn> for the


@@ -296,7 +296,7 @@ With the exception of regexes, you need to pass references to these
 objects.  See L<perlsub/"Pass by Reference"> for this particular
 question, and L<perlref> for information on references.
 
 objects.  See L<perlsub/"Pass by Reference"> for this particular
 question, and L<perlref> for information on references.
 

-See ``Passing Regexes'', below, for information on passing regular
+See "Passing Regexes", below, for information on passing regular

 expressions.
 
 =over 4
 expressions.
 
 =over 4


@@ -411,42 +411,62 @@ You could also investigate the can() method in the UNIVERSAL class
 
 =head2 How do I create a static variable?
 
 
 =head2 How do I create a static variable?
 

-As with most things in Perl, TMTOWTDI.  What is a "static variable" in
-other languages could be either a function-private variable (visible
-only within a single function, retaining its value between calls to
-that function), or a file-private variable (visible only to functions
-within the file it was declared in) in Perl.
+(contributed by brian d foy)

 
 

-Here's code to implement a function-private variable:
+Perl doesn't have "static" variables, which can only be accessed from
+the function in which they are declared. You can get the same effect
+with lexical variables, though.
+
+You can fake a static variable by using a lexical variable which goes
+of scope. In this example, you define the subroutine C<counter>, and
+it uses the lexical variable C<$count>. Since you wrap this in a BEGIN
+block, C<$count> is defined at compile-time, but also goes out of
+scope at the end of the BEGIN block. The BEGIN block also ensures that
+the subroutine and the value it uses is defined at compile-time so the
+subroutine is ready to use just like any other subroutine, and you can
+put this code in the same place as other subroutines in the program
+text (i.e. at the end of the code, typically). The subroutine
+C<counter> still has a reference to the data, and is the only way you
+can access the value (and each time you do, you increment the value).
+The data in chunk of memory defined by C<$count> is private to
+C<counter>.

 
     BEGIN {
 
     BEGIN {

-        my $counter = 42;
-        sub prev_counter { return --$counter }
-        sub next_counter { return $counter++ }
+        my $count = 1;
+        sub counter { $count++ }

     }
 
     }
 

-Now prev_counter() and next_counter() share a private variable $counter
-that was initialized at compile time.
+    my $start = count();

 
 

-To declare a file-private variable, you'll still use a my(), putting
-the declaration at the outer scope level at the top of the file.
-Assume this is in file Pax.pm:
+    .... # code that calls count();

 
 

-    package Pax;
-    my $started = scalar(localtime(time()));
+    my $end = count();

 
 

-    sub begun { return $started }
+In the previous example, you created a function-private variable
+because only one function remembered its reference. You could define
+multiple functions while the variable is in scope, and each function
+can share the "private" variable. It's not really "static" because you
+can access it outside the function while the lexical variable is in
+scope, and even create references to it. In this example,
+C<increment_count> and C<return_count> share the variable. One
+function adds to the value and the other simply returns the value.
+They can both access C<$count>, and since it has gone out of scope,
+there is no other way to access it.
+
+    BEGIN {
+        my $count = 1;
+        sub increment_count { $count++ }
+        sub return_count    { $count }
+    }

 
 

-When C<use Pax> or C<require Pax> loads this module, the variable will
-be initialized.  It won't get garbage-collected the way most variables
-going out of scope do, because the begun() function cares about it,
-but no one else can get it.  It is not called $Pax::started because
-its scope is unrelated to the package.  It's scoped to the file.  You
-could conceivably have several packages in that same file all
-accessing the same private variable, but another file with the same
-package couldn't get to it.
+To declare a file-private variable, you still use a lexical variable.
+A file is also a scope, so a lexical variable defined in the file
+cannot be seen from any other file.

 
 

-See L<perlsub/"Persistent Private Variables"> for details.
+See L<perlsub/"Persistent Private Variables"> for more information.
+The discussion of closures in L<perlref> may help you even though we
+did not use anonymous subroutines in this answer. See
+L<perlsub/"Persistent Private Variables"> for details.

 
 =head2 What's the difference between dynamic and lexical (static) scoping?  Between local() and my()?
 
 
 =head2 What's the difference between dynamic and lexical (static) scoping?  Between local() and my()?
 


@@ -757,7 +777,7 @@ with <=end>.
     by everyone
 
        =end comment
     by everyone
 
        =end comment

-       
+

     =cut
 
     # program continues
     =cut
 
     # program continues


@@ -896,16 +916,21 @@ you probably only want to use hard references.
 
 =head2 What does "bad interpreter" mean?
 
 
 =head2 What does "bad interpreter" mean?
 

+(contributed by brian d foy)
+

 The "bad interpreter" message comes from the shell, not perl.  The
 actual message may vary depending on your platform, shell, and locale
 settings.
 
 If you see "bad interpreter - no such file or directory", the first
 line in your perl script (the "shebang" line) does not contain the
 The "bad interpreter" message comes from the shell, not perl.  The
 actual message may vary depending on your platform, shell, and locale
 settings.
 
 If you see "bad interpreter - no such file or directory", the first
 line in your perl script (the "shebang" line) does not contain the

-right path to perl (or any other program capable of running scripts). 
+right path to perl (or any other program capable of running scripts).

 Sometimes this happens when you move the script from one machine to
 another and each machine has a different path to perl---/usr/bin/perl
 Sometimes this happens when you move the script from one machine to
 another and each machine has a different path to perl---/usr/bin/perl

-versus /usr/local/bin/perl for instance.
+versus /usr/local/bin/perl for instance. It may also indicate
+that the source machine has CRLF line terminators and the
+destination machine has LF only: the shell tries to find
+/usr/bin/perl<CR>, but can't.

 
 If you see "bad interpreter: Permission denied", you need to make your
 script executable.
 
 If you see "bad interpreter: Permission denied", you need to make your
 script executable.