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 + $r, "\n";
to give the output
A: 4.6
B: 4
C: 4.6
D: 4
E: 4.6
i.e., where "use myint;" is in effect, addition operations are forced
to integer, whereas by default they are not, with the default behaviour
being restored via "no myint;"
The minimal implementation of the package "MyMaths" would be something
like this:
package MyMaths;
use warnings;
use strict;
use myint();
use overload '+' => sub {
my ($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/in_effect"} = 1;
}
sub unimport {
$^H{"myint/in_effect"} = 0;
}
sub in_effect {
my $level = shift // 0;
my $hinthash = (caller($level))[10];
return $hinthash->{"myint/in_effect"};
}
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 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 $^H{"myint/in_effect"} when each line of the user's script
was called, and therefore provide the correct semantics in the
subroutine implementing the overloaded addition.
Key naming
There is only a single "%^H", but arbitrarily many modules that want to use its scoping semantics. To avoid stepping on each other's toes, they need to be sure to use different keys in the hash. It is therefore conventional for a module to use only keys that begin with the module's name (the name of its main package) and a "/" character. After this module-identifying prefix, the rest of the key is entirely up to the module: it may include any characters whatsoever. For example, a module "Foo::Bar" should use keys such as "Foo::Bar/baz" and "Foo::Bar/$%/_!". Modules following this convention all play nicely with each other. The Perl core uses a handful of keys in "%^H" which do not follow this convention, because they predate it. Keys that follow the convention won't conflict with the core's historical keys.
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.
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
