perlpragma
(1)
Name
perlpragma - how to write a user pragma
Synopsis
Please see following description for synopsis
Description
Perl Programmers Reference Guide PERLPRAGMA(1)
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 + $r, "\n";
to give the output
A: 4.6
B: 4
C: 4.6
D: 4
E: 4.6
i.e., where "use myint;" is in effect, addition operations
are forced to integer, whereas by default they are not, with
the default behaviour being restored via "no myint;"
The minimal implementation of the package "MyMaths" would be
something like this:
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package MyMaths;
use warnings;
use strict;
use myint();
use overload '+' => sub {
my ($l, $r) = @_;
# Pass 1 to check up one call level from here
if (myint::in_effect(1)) {
int($$l) + int($$r);
} 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
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Perl Programmers Reference Guide PERLPRAGMA(1)
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 manipulate it. The
state information in "%^H" is stored in the optree, and can
be retrieved read-only 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.
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ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
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