no myint;
print "E: ", $l + $r, "\n";
-
+
to give the output
A: 4.6
1;
-Note how we load the user pragma C<myint> with C<()> to prevent its C<import>
-being called.
-
-The interaction with the Perl compile happens inside package C<myint>:
+Note how we load the user pragma C<myint> with an empty list C<()> to
+prevent its C<import> being called.
-package myint;
+The interaction with the Perl compilation happens inside package C<myint>:
+ package myint;
+
use strict;
use warnings;
Hence the C<import> and C<unimport> routines are called at B<compile time>
for the user's code.
-User pragmata store their state by writing to C<%^H>, hence these two
-routines manipulate C<%^H>. The state information in C<%^H> is stored in the
-optree, and can be retrieved at runtime with C<caller>, at index 10 of the
-list of returned results. In the example pragma, retrieval is encapsulated
-into the routine C<in_effect()>. This uses C<caller(0)> to determine the
-state of C<$^H{myint}> when each line of the user's script was called, and
+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
+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
therefore provide the correct semantics in the subroutine implementing the
overloaded addition.
=head1 Implementation details
-The optree is shared between threads, which means there is a possibility that
-the optree will outlive the particular thread (and therefore interpreter
+The optree is shared between threads. This means there is a possibility that
+the optree will outlive the particular thread (and therefore the interpreter
instance) that created it, so true Perl scalars cannot be stored in the
-optree. Instead a compact form is used, which can only store values that are
+optree. Instead a compact form is used, which can only store values that are
integers (signed and unsigned), strings or C<undef> - references and
floating point values are stringified. If you need to store multiple values
or complex structures, you should serialise them, for example with C<pack>.
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.