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

6       perlpragma - how to write a user pragma
7

DESCRIPTION

9       A pragma is a module which influences some aspect of the compile time
10       or run time behaviour of Perl, such as "strict" or "warnings". With
11       Perl 5.10 you are no longer limited to the built in pragmata; you can
12       now create user pragmata that modify the behaviour of user functions
13       within a lexical scope.
14

A basic example

16       For example, say you need to create a class implementing overloaded
17       mathematical operators, and would like to provide your own pragma that
18       functions much like "use integer;" You'd like this code
19
20           use MyMaths;
21
22           my $l = MyMaths->new(1.2);
23           my $r = MyMaths->new(3.4);
24
25           print "A: ", $l + $r, "\n";
26
27           use myint;
28           print "B: ", $l + $r, "\n";
29
30           {
31               no myint;
32               print "C: ", $l + $r, "\n";
33           }
34
35           print "D: ", $l + $r, "\n";
36
37           no myint;
38           print "E: ", $l + $r, "\n";
39
40       to give the output
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42           A: 4.6
43           B: 4
44           C: 4.6
45           D: 4
46           E: 4.6
47
48       i.e., where "use myint;" is in effect, addition operations are forced
49       to integer, whereas by default they are not, with the default behaviour
50       being restored via "no myint;"
51
52       The minimal implementation of the package "MyMaths" would be something
53       like this:
54
55           package MyMaths;
56           use warnings;
57           use strict;
58           use myint();
59           use overload '+' => sub {
60               my ($l, $r) = @_;
61               # Pass 1 to check up one call level from here
62               if (myint::in_effect(1)) {
63                   int($$l) + int($$r);
64               } else {
65                   $$l + $$r;
66               }
67           };
68
69           sub new {
70               my ($class, $value) = @_;
71               bless \$value, $class;
72           }
73
74           1;
75
76       Note how we load the user pragma "myint" with an empty list "()" to
77       prevent its "import" being called.
78
79       The interaction with the Perl compilation happens inside package
80       "myint":
81
82           package myint;
83
84           use strict;
85           use warnings;
86
87           sub import {
88               $^H{"myint/in_effect"} = 1;
89           }
90
91           sub unimport {
92               $^H{"myint/in_effect"} = 0;
93           }
94
95           sub in_effect {
96               my $level = shift // 0;
97               my $hinthash = (caller($level))[10];
98               return $hinthash->{"myint/in_effect"};
99           }
100
101           1;
102
103       As pragmata are implemented as modules, like any other module, "use
104       myint;" becomes
105
106           BEGIN {
107               require myint;
108               myint->import();
109           }
110
111       and "no myint;" is
112
113           BEGIN {
114               require myint;
115               myint->unimport();
116           }
117
118       Hence the "import" and "unimport" routines are called at compile time
119       for the user's code.
120
121       User pragmata store their state by writing to the magical hash "%^H",
122       hence these two routines manipulate it. The state information in "%^H"
123       is stored in the optree, and can be retrieved read-only at runtime with
124       "caller()", at index 10 of the list of returned results. In the example
125       pragma, retrieval is encapsulated into the routine "in_effect()", which
126       takes as parameter the number of call frames to go up to find the value
127       of the pragma in the user's script. This uses "caller()" to determine
128       the value of $^H{"myint/in_effect"} when each line of the user's script
129       was called, and therefore provide the correct semantics in the
130       subroutine implementing the overloaded addition.
131

Key naming

133       There is only a single "%^H", but arbitrarily many modules that want to
134       use its scoping semantics.  To avoid stepping on each other's toes,
135       they need to be sure to use different keys in the hash.  It is
136       therefore conventional for a module to use only keys that begin with
137       the module's name (the name of its main package) and a "/" character.
138       After this module-identifying prefix, the rest of the key is entirely
139       up to the module: it may include any characters whatsoever.  For
140       example, a module "Foo::Bar" should use keys such as "Foo::Bar/baz" and
141       "Foo::Bar/$%/_!".  Modules following this convention all play nicely
142       with each other.
143
144       The Perl core uses a handful of keys in "%^H" which do not follow this
145       convention, because they predate it.  Keys that follow the convention
146       won't conflict with the core's historical keys.
147

Implementation details

149       The optree is shared between threads.  This means there is a
150       possibility that the optree will outlive the particular thread (and
151       therefore the interpreter instance) that created it, so true Perl
152       scalars cannot be stored in the optree.  Instead a compact form is
153       used, which can only store values that are integers (signed and
154       unsigned), strings or "undef" - references and floating point values
155       are stringified.  If you need to store multiple values or complex
156       structures, you should serialise them, for example with "pack".  The
157       deletion of a hash key from "%^H" is recorded, and as ever can be
158       distinguished from the existence of a key with value "undef" with
159       "exists".
160
161       Don't attempt to store references to data structures as integers which
162       are retrieved via "caller" and converted back, as this will not be
163       threadsafe.  Accesses would be to the structure without locking (which
164       is not safe for Perl's scalars), and either the structure has to leak,
165       or it has to be freed when its creating thread terminates, which may be
166       before the optree referencing it is deleted, if other threads outlive
167       it.
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171perl v5.34.0                      2021-10-18                     PERLPRAGMA(1)
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