perlpragma(1)

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 v5.30.3                      2020-06-07                     PERLPRAGMA(1)