perlpragma.pdf

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Perl version 5.10.0 documentation - perlpragma
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 + $r, "\n";
to give the output
A:
B:
C:
D:
E:
4.6
4
4.6
4
4.6
i.e.,
where
use myint;
is in effect, addition operations are forced to integer, whereas by default they
are not, with the default behaviour being restored via
no myint;
The minimal implementation of the package
MyMaths
would be something like this:
package MyMaths;
use warnings;
use strict;
use myint();
use overload '+' => sub {
my ($l, $r) = @_;
# Pass 1 to check up one call level from here
if (myint::in_effect(1)) {
int($$l) + int($$r);
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Perl version 5.10.0 documentation - perlpragma
} 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} = 1;
}
sub unimport {
$^H{myint} = 0;
}
sub in_effect {
my $level = shift // 0;
my $hinthash = (caller($level))[10];
return $hinthash->{myint};
}
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
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Perl version 5.10.0 documentation - perlpragma
manipulate it. The state information in
%^H
is stored in the optree, and can be retrieved 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}
when each line of the user's script was called, and therefore provide the correct
semantics in the subroutine implementing the overloaded addition.
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.
http://perldoc.perl.org
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