no myint;
print "E: ", $l + $r, "\n";
-
+
to give the output
A: 4.6
1;
-Note how we load the user pragma C<myint> with C<()> to prevent its C<import>
-being called.
-
-The interaction with the Perl compile happens inside package C<myint>:
+Note how we load the user pragma C<myint> with an empty list C<()> to
+prevent its C<import> being called.
-package myint;
+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 $hinthash->{"myint/in_effect"};
}
1;
User pragmata store their state by writing to the magical hash C<%^H>,
hence these two routines manipulate it. The state information in C<%^H> is
-stored in the optree, and can be retrieved at runtime with C<caller>, at
-index 10 of the list of returned results. In the example pragma, retrieval
+stored in the optree, and can be retrieved read-only at runtime with C<caller()>,
+at index 10 of the list of returned results. In the example pragma, retrieval
is encapsulated into the routine C<in_effect()>, which takes as parameter
the number of call frames to go up to find the value of the pragma in the
user's script. This uses C<caller()> to determine the value of
-C<$^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
The deletion of a hash key from C<%^H> is recorded, and as ever can be
distinguished from the existence of a key with value C<undef> with
C<exists>.
+
+B<Don't> attempt to store references to data structures as integers which
+are retrieved via C<caller> and converted back, as this will not be threadsafe.
+Accesses would be to the structure without locking (which is not safe for
+Perl's scalars), and either the structure has to leak, or it has to be
+freed when its creating thread terminates, which may be before the optree
+referencing it is deleted, if other threads outlive it.