Perl6::Placeholders - Perl 6 implicitly declared parameters for Perl 5
This document describes version 0.05 of Perl6::Placeholders, released May 29, 2002.
use Perl6::Placeholders;
my $add = { $^a + $^b }; # Create a sub that adds its two args
print $add->(1,2), "\n"; # Call it
# Use as map, grep, and sort blocks
print join ",", sort { $^y <=> $^x } 1..10;
print join "\n", map { $^value**2 } 1..10;
print join "\n", map { $data{$_-1}.$^value**2 } 1..10;
print join "\n", grep { $data{$^value} } 1..10;
my $div = { $^x / $^y }; # Create a HOF that divides its two args
print $div->(1,2), "\n"; # Do a division
The Perl6::Placeholders module lets you try out the new Perl 6 implicit parameter specification syntax in Perl 5.
Perl 6 reserves all variables of the form $^name or @^name or %^name as "placeholders" that can be used to turn regular blocks into subroutine references.
Any block containing one or more such placeholders is treated as a reference to a subroutine in which the placeholders are replaced by the appropriate number and sequence of arguments.
That is, the expression:
# Perl 6 code
$check = { $^a == $^b**2 * $^c or die $^err_msg };
is equivalent to:
# Perl 6 code
$check = sub ($a, $b, $c, $err_msg) {
$a == $b**2 * $c or die $err_msg
};
This could then be invoked:
# Perl 6 code
$check.($i,$j,$k,$msg);
It is also be possible to interpolate an argument list into a static expression like so:
# Perl 6 code
{ $^a == $^b**2 * $^c or die $^err_msg }.($i,$j,$k,$msg);
The placeholders are sorted UTF8-abetically before they are used to create the subroutine's parameter list. Hence the following:
# Perl 6 code
@reverse_sorted = sort {$^b <=> $^a} @list;
works as expected. That is, it's equivalent to:
@reverse_sorted = sort sub($a,$b){$b <=> $a}, @list;
The Perl6::Placeholders module allows you to use (almost) the same syntax in Perl 5.
That is, the expression:
# Perl 5 code
use Perl6::Placeholders;
$check = { $^a == $^b**2 * $^c or die $^err_msg };
is equivalent to:
# Perl 5 code
$check = sub ($a, $b, $c, $err_msg) {
my ($a, $b, $c, $err_msg) = @_;
$a == $b**2 * $c or die $err_msg;
};
This could then be invoked:
# Perl 5 code
$check->($i,$j,$k,$msg);
It is also be possible to interpolate an argument list into a static expression like so:
# Perl 5 code
use Perl6::Placeholders;
{ $^a == $^b**2 * $^c or die $^err_msg }->($i,$j,$k,$msg);
Note that the placeholders are restricted to scalars (though a future release may support array and hash parameters too).
The placeholders are sorted ASCIIbetically before they are used to create the subroutine's parameter list. Hence the following:
# Perl 5 code
use Perl6::Placeholders;
@reverse_sorted = sort {$^b <=> $^a} @list;
works as expected (even in earlier perls that don't support sub refs as sort specifiers!)
The module is implemented using Filter::Simple and requires that module to be installed.
Damian Conway (damian@conway.org)
This module is not designed for serious implementation work.
It uses some relatively sophisticated heuristics to translate Perl 6 syntax back to Perl 5. It will make mistakes if your code gets even moderately tricky.
Nevertheless, bug reports are most welcome.
Copyright (c) 2002, Damian Conway. All Rights Reserved. This module is free software. It may be used, redistributed and/or modified under the terms of the Perl Artistic License (see http://www.perl.com/perl/misc/Artistic.html)
