Perl中的闭包（closure）

dickrong

　　什么是闭包，“This is a notion out of the Lisp world that says if you define an anonymous function in a particular lexical context, it pretends to run in that context even when it's called outside of the context.”【2】。在面向对象的语言里面，“A closure is a callable object thatretains information from the scope in which it was created. From this definition, you can see that an inner class is an object-oriented closure, because it doesn’t just contain each piece of information from the outer-class object ("the scope in which it wascreated"), but it automatically holds a reference back to the whole outer-class object, where it has permission to manipulate all the members, even private ones.”【3】

　　先看这个例子：
#!/usr/bin/perl -wuse strict;{my $inc = 10;sub incr {print "$inc\n";$inc++;}}incr();incr();#prints:#10#11
这个例子说明命名函数默认是全局的，即使是定义在一个block里面。我们不能引用变量$inc，但是却可以调用函数。#!/usr/bin/perl -wuse strict;sub make_incr {my $inc = shift;return sub { print "$inc\n"; $inc++ };}my $c1 = make_incr(10);my $c2 = make_incr(20);$c1->();$c2->();$c1->();$c2->();#prints:#10#20#11#21这个例子我们看到了，Perl的函数返回其实是一个匿名函数引用，这个就是magic所在了。这个也是Perl如何实现闭包的。
#!/usr/bin/perl -wuse strict;sub exclaim {my $prefix = shift;return sub { print "$prefix $_[0]!\n" };}my $batman = exclaim('Indeed');my $robin  = exclaim('Holy');$robin->('Mackerel');    # prints: Holy Mackerel!$batman->('Robin');      # prints: Indeed Robin!　　那么闭包有什么作用呢？以下摘自“Learning Perl Objects, References & Modules”的第6章【1】：
　　用法一 在subroutine中返回subroutine的引用，通常作为回调函数：
use File::Find;sub create_find_callbacks_that_sum_the_size {my $total_size = 0;return ( sub { $total_size += -s if -f }, sub { return $total_size } );}my ( $count_em, $get_results ) = create_find_callbacks_that_sum_the_size();find( $count_em, "bin" );my $total_size = &$get_results();print "total size of bin is $total_size \n";这段代码用于计算某个目录下所包含的所有文件的大小之和.　　用法二 使用闭环变量作为输入，用作函数生成器，来生成不同的函数指针：
#!/usr/bin/perl -wuse strict;sub print_bigger_than {my $minimum_size = shift;return sub { print "$File::Find::name/n" if -f and -s >= $minimum_size };}my $bigger_than_1024 = print_bigger_than(1024);find( $bigger_than_1024, "bin" );print_bigger_than在这里相当于一个函数生成器，不同的输入变量可以生成不同的函数指针.这里生成了一个可以打印出文件大小大于1024字节文件名的回调函数.　　用法三 作为静态局部变量使用，提供了c语言静态局部变量的功能:
BEGIN {my $countdown = 10;sub count_down { $countdown-- }sub count_remaining { $countdown }}这里用到了关键字BEGIN. BEGIN的作用就是，当perl编译完这段代码之后，停止当前编译，然后直接进入运行阶段，执行BEGIN块内部的代码.然后再回到编译状态， 继续编译剩余的代码. 这就保证了无论BEGIN块位于程序中的哪个位置，在调用count_down之前，$countdown被确保初始化为10.　　最后附上一个相当cool的例子，来在“Perl Best Practices”：
# Generate a new sorting routine whose name is the string in $sub_name# and which sorts on keys extracted by the subroutine referred to by $key_sub_refsub make_sorter {my ( $sub_name, $key_sub_ref ) = @_;# Create a new anonymous subroutine that implements the sort...my $sort_sub_ref = sub {# Sort using the Schwartzian transform...return map { $_->[0] }                # 3. Return original valuesort     { $a->[1] cmp $b->[1] }    # 2. Compare keysmap { [ $_, $key_sub_ref->() ] }    # 1. Extract key, cache with value@_;                                 # 0. Perform sort on full arg list};# Install the new anonymous sub into the caller's namespaceuse Sub::Installer;caller->install_sub( $sub_name, $sort_sub_ref );return;}# and then...make_sorter( sort_sha => sub { sha512($_) } );make_sorter( sort_ids => sub { /^ID:(\d+)/ } );make_sorter( sort_len => sub { length } );# and later...@names_shortest_first = sort_len(@names);@names_digested_first = sort_sha(@names);@names_identity_first = sort_ids(@names);
　　参考：

• http://blog.csdn.net/mac_philips/article/details/6058946
  
• http://unlser1.unl.csi.cuny.edu/faqs/perl-faq/Q3.14.html
  
• Think in Java, 4th
  
• http://www.itworld.com/nl/perl/08302001/
  
• http://docstore.mik.ua/orelly/perl/advprog/ch04_03.htm