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.
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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} = 1;
89           }
90
91           sub unimport {
92               $^H{myint} = 0;
93           }
94
95           sub in_effect {
96               my $level = shift // 0;
97               my $hinthash = (caller($level))[10];
98               return $hinthash->{myint};
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 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} when each line of the user's script was called,
129       and therefore provide the correct semantics in the subroutine
130       implementing the overloaded addition.
131

Implementation details

133       The optree is shared between threads.  This means there is a
134       possibility that the optree will outlive the particular thread (and
135       therefore the interpreter instance) that created it, so true Perl
136       scalars cannot be stored in the optree.  Instead a compact form is
137       used, which can only store values that are integers (signed and
138       unsigned), strings or "undef" - references and floating point values
139       are stringified.  If you need to store multiple values or complex
140       structures, you should serialise them, for example with "pack".  The
141       deletion of a hash key from "%^H" is recorded, and as ever can be
142       distinguished from the existence of a key with value "undef" with
143       "exists".
144
145       Don't attempt to store references to data structures as integers which
146       are retrieved via "caller" and converted back, as this will not be
147       threadsafe.  Accesses would be to the structure without locking (which
148       is not safe for Perl's scalars), and either the structure has to leak,
149       or it has to be freed when its creating thread terminates, which may be
150       before the optree referencing it is deleted, if other threads outlive
151       it.
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155perl v5.10.1                      2009-02-12                     PERLPRAGMA(1)
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