1=head1 NAME 2 3perlpragma - how to write a user pragma 4 5=head1 DESCRIPTION 6 7A pragma is a module which influences some aspect of the compile time or run 8time behaviour of Perl, such as C<strict> or C<warnings>. With Perl 5.10 you 9are no longer limited to the built in pragmata; you can now create user 10pragmata that modify the behaviour of user functions within a lexical scope. 11 12=head1 A basic example 13 14For example, say you need to create a class implementing overloaded 15mathematical operators, and would like to provide your own pragma that 16functions much like C<use integer;> You'd like this code 17 18 use MyMaths; 19 20 my $l = MyMaths->new(1.2); 21 my $r = MyMaths->new(3.4); 22 23 print "A: ", $l + $r, "\n"; 24 25 use myint; 26 print "B: ", $l + $r, "\n"; 27 28 { 29 no myint; 30 print "C: ", $l + $r, "\n"; 31 } 32 33 print "D: ", $l + $r, "\n"; 34 35 no myint; 36 print "E: ", $l + $r, "\n"; 37 38to give the output 39 40 A: 4.6 41 B: 4 42 C: 4.6 43 D: 4 44 E: 4.6 45 46I<i.e.>, where C<use myint;> is in effect, addition operations are forced 47to integer, whereas by default they are not, with the default behaviour being 48restored via C<no myint;> 49 50The minimal implementation of the package C<MyMaths> would be something like 51this: 52 53 package MyMaths; 54 use v5.36; 55 use myint(); 56 use overload '+' => sub { 57 my ($l, $r) = @_; 58 # Pass 1 to check up one call level from here 59 if (myint::in_effect(1)) { 60 int($$l) + int($$r); 61 } else { 62 $$l + $$r; 63 } 64 }; 65 66 sub new { 67 my ($class, $value) = @_; 68 bless \$value, $class; 69 } 70 71 1; 72 73Note how we load the user pragma C<myint> with an empty list C<()> to 74prevent its C<import> being called. 75 76The interaction with the Perl compilation happens inside package C<myint>: 77 78 package myint; 79 80 use v5.36; 81 82 sub import { 83 $^H{"myint/in_effect"} = 1; 84 } 85 86 sub unimport { 87 $^H{"myint/in_effect"} = 0; 88 } 89 90 sub in_effect { 91 my $level = shift // 0; 92 my $hinthash = (caller($level))[10]; 93 return $hinthash->{"myint/in_effect"}; 94 } 95 96 1; 97 98As pragmata are implemented as modules, like any other module, C<use myint;> 99becomes 100 101 BEGIN { 102 require myint; 103 myint->import(); 104 } 105 106and C<no myint;> is 107 108 BEGIN { 109 require myint; 110 myint->unimport(); 111 } 112 113Hence the C<import> and C<unimport> routines are called at B<compile time> 114for the user's code. 115 116User pragmata store their state by writing to the magical hash C<%^H>, 117hence these two routines manipulate it. The state information in C<%^H> is 118stored in the optree, and can be retrieved read-only at runtime with C<caller()>, 119at index 10 of the list of returned results. In the example pragma, retrieval 120is encapsulated into the routine C<in_effect()>, which takes as parameter 121the number of call frames to go up to find the value of the pragma in the 122user's script. This uses C<caller()> to determine the value of 123C<$^H{"myint/in_effect"}> when each line of the user's script was called, and 124therefore provide the correct semantics in the subroutine implementing the 125overloaded addition. 126 127=head1 Key naming 128 129There is only a single C<%^H>, but arbitrarily many modules that want 130to use its scoping semantics. To avoid stepping on each other's toes, 131they need to be sure to use different keys in the hash. It is therefore 132conventional for a module to use only keys that begin with the module's 133name (the name of its main package) and a "/" character. After this 134module-identifying prefix, the rest of the key is entirely up to the 135module: it may include any characters whatsoever. For example, a module 136C<Foo::Bar> should use keys such as C<Foo::Bar/baz> and C<Foo::Bar/$%/_!>. 137Modules following this convention all play nicely with each other. 138 139The Perl core uses a handful of keys in C<%^H> which do not follow this 140convention, because they predate it. Keys that follow the convention 141won't conflict with the core's historical keys. 142 143=head1 Implementation details 144 145The optree is shared between threads. This means there is a possibility that 146the optree will outlive the particular thread (and therefore the interpreter 147instance) that created it, so true Perl scalars cannot be stored in the 148optree. Instead a compact form is used, which can only store values that are 149integers (signed and unsigned), strings or C<undef> - references and 150floating point values are stringified. If you need to store multiple values 151or complex structures, you should serialise them, for example with C<pack>. 152The deletion of a hash key from C<%^H> is recorded, and as ever can be 153distinguished from the existence of a key with value C<undef> with 154C<exists>. 155 156B<Don't> attempt to store references to data structures as integers which 157are retrieved via C<caller> and converted back, as this will not be threadsafe. 158Accesses would be to the structure without locking (which is not safe for 159Perl's scalars), and either the structure has to leak, or it has to be 160freed when its creating thread terminates, which may be before the optree 161referencing it is deleted, if other threads outlive it. 162