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PERLPRAGMA(1)                         Perl Programmers Reference Guide                         PERLPRAGMA(1)



NAME
       perlpragma - how to write a user pragma

DESCRIPTION
       A pragma is a module which influences some aspect of the compile time or run time behaviour of Perl,
       such as "strict" or "warnings". With Perl 5.10 you are no longer limited to the built in pragmata;
       you can now create user pragmata that modify the behaviour of user functions within a lexical scope.

A basic example
       For example, say you need to create a class implementing overloaded mathematical operators, and would
       like to provide your own pragma that functions much like "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";

       to give the output

           A: 4.6
           B: 4
           C: 4.6
           D: 4
           E: 4.6

       i.e., where "use myint;" is in effect, addition operations are forced to integer, whereas by default
       they are not, with the default behaviour being restored via "no myint;"

       The minimal implementation of the package "MyMaths" would be something like this:

           package MyMaths;
           use warnings;
           use strict;
           use myint();
           use overload '+' => sub {
               my ($l, $r) = @_;
               # Pass 1 to check up one call level from here
               if (myint::in_effect(1)) {
                   int($$l) + int($$r);
               } else {
                   $$l + $$r;
               }
           };

           sub new {
               my ($class, $value) = @_;
               bless \$value, $class;
           }

           1;

       Note how we load the user pragma "myint" with an empty list "()" to prevent its "import" being
       called.

       The interaction with the Perl compilation happens inside package "myint":

           package myint;

           use strict;
           use warnings;

           sub import {
               $^H{"myint/in_effect"} = 1;
           }

           sub unimport {
               $^H{"myint/in_effect"} = 0;
           }

           sub in_effect {
               my $level = shift // 0;
               my $hinthash = (caller($level))[10];
               return $hinthash->{"myint/in_effect"};
           }

           1;

       As pragmata are implemented as modules, like any other module, "use myint;" becomes

           BEGIN {
               require myint;
               myint->import();
           }

       and "no myint;" is

           BEGIN {
               require myint;
               myint->unimport();
           }

       Hence the "import" and "unimport" routines are called at compile time for the user's code.

       User pragmata store their state by writing to the magical hash "%^H", hence these two routines
       manipulate it. The state information in "%^H" is stored in the optree, and can be retrieved read-only
       at runtime with "caller()", at index 10 of the list of returned results. In the example pragma,
       retrieval is encapsulated into the routine "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 "caller()" to
       determine the value of $^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.

Key naming
       There is only a single "%^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
       "Foo::Bar" should use keys such as "Foo::Bar/baz" and "Foo::Bar/$%/_!".  Modules following this
       convention all play nicely with each other.

       The Perl core uses a handful of keys in "%^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.

Implementation details
       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
       integers (signed and unsigned), strings or "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 "pack".  The deletion of a hash key from "%^H" is recorded, and as ever can be
       distinguished from the existence of a key with value "undef" with "exists".

       Don't attempt to store references to data structures as integers which are retrieved via "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.



perl v5.16.2                                     2012-10-11                                    PERLPRAGMA(1)