perldelta for 2384afee9 / #123553

[perl5.git] / pod / perlpragma.pod

diff --git a/pod/perlpragma.pod b/pod/perlpragma.pod
index 51292a8..78dacbf 100644 (file)
--- a/pod/perlpragma.pod
+++ b/pod/perlpragma.pod
@@ -16,22 +16,22 @@ mathematical operators, and would like to provide your own pragma that
 functions much like C<use integer;> You'd like this code
 
     use MyMaths;
-    
+
     my $l = MyMaths->new(1.2);
     my $r = MyMaths->new(3.4);
-    
+
     print "A: ", $l + $r, "\n";
-    
+
     use myint;
     print "B: ", $l + $r, "\n";
-    
+
     {
         no myint;
         print "C: ", $l + $r, "\n";
     }
-    
+
     print "D: ", $l + $r, "\n";
-    
+
     no myint;
     print "E: ", $l + $r, "\n";
 
@@ -63,12 +63,12 @@ this:
             $$l + $$r;
         }
     };
-    
+
     sub new {
         my ($class, $value) = @_;
         bless \$value, $class;
     }
-    
+
     1;
 
 Note how we load the user pragma C<myint> with an empty list C<()> to
@@ -77,24 +77,24 @@ prevent its C<import> being called.
 The interaction with the Perl compilation happens inside package C<myint>:
 
     package myint;
-    
+
     use strict;
     use warnings;
-    
+
     sub import {
-        $^H{myint} = 1;
+        $^H{"myint/in_effect"} = 1;
     }
-    
+
     sub unimport {
-        $^H{myint} = 0;
+        $^H{"myint/in_effect"} = 0;
     }
-    
+
     sub in_effect {
         my $level = shift // 0;
         my $hinthash = (caller($level))[10];
-        return $hinthash->{myint};
+        return $hinthash->{"myint/in_effect"};
     }
-    
+
     1;
 
 As pragmata are implemented as modules, like any other module, C<use myint;>
@@ -117,15 +117,31 @@ for the user's code.
 
 User pragmata store their state by writing to the magical hash C<%^H>,
 hence these two routines manipulate it. The state information in C<%^H> is
-stored in the optree, and can be retrieved at runtime with C<caller()>, at
-index 10 of the list of returned results. In the example pragma, retrieval
+stored in the optree, and can be retrieved read-only at runtime with C<caller()>,
+at index 10 of the list of returned results. In the example pragma, retrieval
 is encapsulated into the routine C<in_effect()>, which takes as parameter
 the number of call frames to go up to find the value of the pragma in the
 user's script. This uses C<caller()> to determine the value of
-C<$^H{myint}> when each line of the user's script was called, and
+C<$^H{"myint/in_effect"}> when each line of the user's script was called, and
 therefore provide the correct semantics in the subroutine implementing the
 overloaded addition.
 
+=head1 Key naming
+
+There is only a single C<%^H>, but arbitrarily many modules that want
+to use its scoping semantics.  To avoid stepping on each other's toes,
+they need to be sure to use different keys in the hash.  It is therefore
+conventional for a module to use only keys that begin with the module's
+name (the name of its main package) and a "/" character.  After this
+module-identifying prefix, the rest of the key is entirely up to the
+module: it may include any characters whatsoever.  For example, a module
+C<Foo::Bar> should use keys such as C<Foo::Bar/baz> and C<Foo::Bar/$%/_!>.
+Modules following this convention all play nicely with each other.
+
+The Perl core uses a handful of keys in C<%^H> which do not follow this
+convention, because they predate it.  Keys that follow the convention
+won't conflict with the core's historical keys.
+
 =head1 Implementation details
 
 The optree is shared between threads.  This means there is a possibility that