Perl中的闭包（closure）

1、perl中的闭包（closure）什么是闭包，“this is a notion out of the lisp world that says if you define an anonymous fution in a particular lical context, it pretends to run in that context even when its call outse of the context.”【2】。在面对对象的语言里面，“a clore is a callable object that retains infoation from the scope in

2、it was created. from this definition, you can see that an inner class is an object-oriented closure, because it doesnt just contain each piece of information from the outer-class object ( the scope in which it was created ), but it automatically holds a reference back to the le outer-class object, w

3、here it has permission to manipulate all the members, even private ones.”【3】 先看这个例子： !/usr/bin/perl -w use sict; my $inc = 10; sub incr print $incn $inc+; incr(); incr(); prints: 11 这个例子解释命名函数默认是全局的，即使是定义在一个block里面。我们不能引用变量$inc，但是却可以调用函数。 !/usr/bin/perl -w use strict; sub make_incr my $inc = shift;

4、return sub print $incn $inc+ ; my $c1 = make_incr(10); my $c2 = make_incr(20); $c1- $c2- $c1- $c2- prints: 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 -w use strict; sub exclaim my $prefix = shift; return sub print $prefix $_0!n ; my $batman = exclaim(indeed); my $r

5、obin = 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 moles”的第6章【1】： 使用一 在subroutine中返回subroutine的引用，通常作为回调函数： use file:find; sub create_callbacks_that_sum_the_size my $total_size = 0; return ( sub $

6、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名目下所包含的全部文件的大小之和. 使用二 用法闭环变量作为输入，用作函数生成器，来生成不同的函数指针： !/usr/bin/pe

7、rl -w use 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. 使用三 作

8、为静态局部变量用法，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 $countdown 这里用到了关键字begin. begin的作用就是，当perl编译完这段代码之后，停止当前编译，然后挺直进入运行阶段，执行begin块内部的代码.然后再回到编译状态， 继续编译剩余的代码. 这就保证了无论begin块位于程序中的哪个位置，在调用count_down之前，$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 prac

9、tices”： generate a new ing routine whose name is the string in $sub_name and which sorts on keys extracted by the subroutine referred to by $key_sub_ref sub make_sorter my ( $sub_name, $key_sub_ref ) = _; create a new anonymous subroutine that implements the sort. my $sort_sub_ref = sub sort using t

10、he schwartzian transform. return map $_- 0 3. return original value sort $a- 1 $b- 1 2. compare keys map $_, $key_sub_ref- () 1. extract key, cache with value _; 0. perform sort on full arg list install the new anonymous sub into the callers namespace use 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 len