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package Scalar::Lazy; |
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use warnings; |
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use strict; |
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our $VERSION = sprintf "%d.%02d", q$Revision: 0.3 $ =~ /(\d+)/g; |
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use base 'Exporter'; |
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our @EXPORT = qw/ delay lazy /; |
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sub new($&;$) { |
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my ($pkg, $code, $init) = @_; |
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if ($init){ |
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my $val = $code->(); |
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$code = sub { $val }; |
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} |
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bless $code, $pkg; |
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} |
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sub lazy(&;$) { __PACKAGE__->new(@_) } |
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*delay = \&lazy; |
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sub force($){ |
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my $pkg = ref $_[0]; |
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bless $_[0], $pkg . '::FORCE'; |
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my $val = $_[0]->(); |
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bless $_[0], $pkg; |
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$val; |
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} |
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use overload ( |
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fallback => 1, |
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map { $_ => \&force } qw( bool "" 0+ ${} @{} %{} &{} *{} ) |
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); |
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1; # End of Scalar::Lazy |
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=head1 NAME |
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Scalar::Lazy - Yet another lazy evaluation in Perl |
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=head1 VERSION |
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$Id: Lazy.pm,v 0.3 2008/06/01 17:09:08 dankogai Exp dankogai $ |
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=head1 SYNOPSIS |
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use Scalar::Lazy; |
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my $scalar = lazy { 1 }; |
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print $scalar; # you don't have to force |
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49
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# Y-combinator made easy |
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my $zm = sub { my $f = shift; |
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sub { my $x = shift; |
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lazy { $f->($x->($x)) } |
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}->(sub { my $x = shift; |
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lazy { $f->($x->($x)) } |
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})}; |
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my $fact = $zm->(sub { my $f = shift; |
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sub { my $n = shift; |
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$n < 2 ? 1 : $n * $f->($n - 1) } }); |
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print $fact->(10); # 3628800 |
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61
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=head1 DISCUSSION |
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The classical way to implement lazy evaluation in an eager-evaluating |
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languages (including perl, of course) is to wrap the value with a closure: |
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66
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sub delay{ |
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my $value = shift; |
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sub { $value } |
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} |
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my $l = delay(42); |
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Then evaluate the closure whenever you need it. |
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my $v = $l->(); |
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Marking the variable lazy can be easier with prototypes: |
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sub delay(&){ $_[0] } |
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my $l = delay { 42 } |
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81
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But forcing the value is pain in the neck. |
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83
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This module makes it easier by making the value auto-forcing. |
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=head2 HOW IT WORKS |
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Check the source. That's what the source is for. |
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There are various CPAN modules that does what this does. But I found |
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others too complicated. Hey, the whole code is only 25 lines long! |
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(Well, was until 0.03) Nicely fits in a good-old terminal screen. |
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The closest module is L, a brainchild of Audrey Tang. |
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But I didn't like the way it (ab)?uses namespace. |
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L depends too many modules. |
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And L is overkill. |
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100
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All I needed was auto-forcing and this module does just that. |
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102
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=head1 EXPORT |
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104
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C and C. |
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106
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=head1 FUNCTIONS |
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108
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=head2 lazy |
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110
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lazy { value } |
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112
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is really: |
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114
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Scalar::Lazy->new(sub { value }); |
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116
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You can optionally set the second parameter. If set, the value |
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becomes constant. The folloing example illustrates the difference. |
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119
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my $x = 0; |
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my $once = lazy { ++$x } 'init'; # $once is always 1 |
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is $once, 1, 'once'; |
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is $once, 1, 'once'; |
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my $succ = lazy { ++$x }; # $succ always increments $x |
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isnt $succ, 1, 'succ'; |
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is $succ, 3, 'succ'; |
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127
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=head2 delay |
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129
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an alias to L. |
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131
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=head1 METHODS |
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133
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=head2 new |
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135
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Makes a lazy variable which auto-forces on demand. |
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137
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=head2 force |
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139
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You don't really need to call this method (that's the whole point of this |
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module!) but if you want, you can |
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142
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my $l = lazy { 1 }; |
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my $v = $l->force; |
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145
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=head1 AUTHOR |
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147
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Dan Kogai, C<< >> |
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149
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=head1 BUGS |
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151
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Please report any bugs or feature requests to C, or through |
152
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the web interface at L. I will be notified, and then you'll |
153
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automatically be notified of progress on your bug as I make changes. |
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155
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=head1 SUPPORT |
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157
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You can find documentation for this module with the perldoc command. |
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159
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perldoc Scalar::Lazy |
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161
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You can also look for information at: |
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163
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=over 4 |
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165
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=item * RT: CPAN's request tracker |
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167
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L |
168
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169
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=item * AnnoCPAN: Annotated CPAN documentation |
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171
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L |
172
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173
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=item * CPAN Ratings |
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175
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L |
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177
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=item * Search CPAN |
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179
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L |
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181
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=back |
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183
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=head1 ACKNOWLEDGEMENTS |
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185
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Highly inspired by L by Audrey Tang. |
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187
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=head1 COPYRIGHT & LICENSE |
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189
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Copyright 2008 Dan Kogai, all rights reserved. |
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191
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This program is free software; you can redistribute it and/or modify it |
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under the same terms as Perl itself. |
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194
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195
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=cut |