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