深入学习Perl：构建更大程序与对象导向编程

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"Intermediate Perl.pdf" 是一本针对已经掌握基础Perl编程的读者进阶学习的书籍。这本书由Randal Schwartz等人编写，旨在帮助读者提升Perl编程技能，将Perl应用于更复杂的程序设计。Perl语言自初版Learning Perl发布以来，发展迅速，从主要用于Unix系统管理的脚本语言进化为一种跨平台的强类型面向对象编程语言。本书不是简单地扩展第一版的内容，而是以"Learning More Perl"为主题，涵盖了如何使用Perl编写更大规模的程序。书中每个章节设计得相对紧凑，适合在一个小时内阅读完成，并在每章末尾提供了练习题以巩固所学。内容适用于Unix、Windows ActivePerl等不同环境，无需高级Unix知识，只要熟悉Learning Perl中的基础内容即可。本书结构清晰，逐章递进，建议按照顺序阅读并完成练习。具体章节包括： 1. 引言：介绍书中的主题和内容。 2. 中级基础：奠定后续章节所需的基础Perl技巧。 3. 使用模块：介绍如何利用Perl的核心模块及第三方模块。 4. 引用入门：引入数据的间接操作，允许代码处理不同数据集。 5. 引用与作用域：理解Perl如何跟踪数据指针，以及匿名数据结构和自动活化（autovivification）的概念。 6. 操作复杂数据结构：创建、访问和打印多层次嵌套的数据结构。 7. 子程序引用：动态创建和延迟执行的匿名子程序。 8. 文件句柄引用：将文件句柄存储在变量中以便于在程序中传递或存入数据结构。 9. 实用引用技巧：解决复杂操作，如Schwartzian变换和递归数据处理。 10. 构建大型程序：通过将代码分离到单独的文件和命名空间来构建更大的程序。 11. 对象基础：介绍类、方法调用、继承和覆盖等面向对象概念。 12. 带有数据的对象：添加实例数据，包括构造函数、获取器等。 "Intermediate Perl"是Perl程序员进一步提升技能的必备读物，它将帮助读者深入理解Perl的高级特性，实现更高效、更专业的编程实践。

Intermediate Perl

You can see that map is pretty versatile; we can produce any number of output items

for each input item. And we don't always need to produce the same number of output

items. Let's see what happens when we break apart the digits:

my @result = map { split //, $_ } @input_numbers;

The inline block of code splits each number into its individual digits. For 1, 2,

4, and 8, we get a single result. For 16, 32, and 64, we get two results per number.

When map concatenates the results lists, we end up with 1, 2, 4, 8, 1, 6, 3, 2, 6,

and 4.

If a particular invocation results in an empty list, map concatenates that empty result

into the larger list, contributing nothing to the list. We can use this feature to

select and reject items. For example, suppose we want only the split digits of numbers

ending in 4:

my @result = map {

my @digits = split //, $_;

if ($digits[-1] = = 4) {

@digits;

} else {

( );

}

} @input_numbers;

If the last digit is 4, we return the digits themselves by evaluating @digits (which

is in list context). If the last digit is not 4, we return an empty list, effectively

removing results for that particular item. Thus, we can always use a map in place

of a grep, but not vice versa.

Of course, everything we can do with map and grep, we can also do with explicit foreach

loops. But then again, we can also code in assembler or by toggling bits into a front

panel.

[*]

The point is that proper application of grep and map can help reduce the

complexity of the program, allowing us to concentrate on high-level issues rather

than details.

[*]

If you're old enough to remember those front panels.

2.2. Trapping Errors with eval

Many lines of ordinary code have the potential to terminate a program prematurely

if something goes wrong.

my $average = $total / $count; # divide by zero?

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Intermediate Perl

print "okay\n" unless /$match/; # illegal pattern?

open MINNOW, '>ship.txt'

or die "Can't create 'ship.txt': $!"; # user-defined die?

&implement($_) foreach @rescue_scheme; # die inside sub?

But just because something has gone wrong with one part of our code, that doesn't

mean that we want everything to crash. Perl uses the eval operator as its

error-trapping mechanism.

eval { $average = $total / $count } ;

If an error happens while running code inside an eval block, the block is done executing.

But even though the code inside the block is finished, Perl continues running the

code just after the eval. It's most common after an eval to immediately check $@,

which will either be empty (meaning that there was no error) or the dying words Perl

had from the code that failed, perhaps something like "divide by zero" or a longer

error message.

eval { $average = $total / $count } ;

print "Continuing after error: $@" if $@;

eval { &rescue_scheme_42 } ;

print "Continuing after error: $@" if $@;

The semicolon is needed after the eval block because eval is a function (not a control

structure, such as if or while). But the block is a true block and may include lexical

variables ("my" variables) and any other arbitrary statements. As a function, eval

has a return value much like a subroutine's (the last expression evaluated, or a value

returned early by the return keyword). Of course, if the code in the block fails,

no value is returned; this gives undef in a scalar context, or an empty list in a

list context. Thus, another way to calculate an average safely looks like this:

my $average = eval { $total / $count } ;

Now $average is either the quotient or undef, depending upon whether the operation

completed successfully or not.

Perl even supports nested eval blocks. The power of an eval block to trap errors extends

for as long as it's executing, so it catches errors deep within nested subroutine

calls. eval can't trap the most serious of errors, though: the ones in which Perl

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Intermediate Perl

Chapter 3. Using Modules

Modules are the building blocks for our programs. They provide reusable subroutines,

variables, and even object-oriented classes. On our way to building our own modules ,

we'll show you some of those you might be interested in. We'll also look at the basics

of using modules that others have already written.

3.1. The Standard Distribution

Perl comes with many of the popular modules already. Indeed, most of the 50+ MB of

the most recent distribution are from modules. In October 1996, Perl 5.003_07 had

98 modules. Today, at the beginning of 2006, Perl 5.8.8 has 359.

[*]

Indeed, this is

one of the advantages of Perl: it already comes with a lot of stuff that you need

to make useful and complex programs without doing a lot of work yourself.

[*]

After you make it through this book, you should be able to use Module::CoreList

to discover that count for yourself. That's what we did to get those numbers, after

all.

Throughout this book, we'll try to identify which modules comes with Perl (and in

most cases, with which version they started coming with Perl). We'll call these "core

modules " or note that they're in "the standard distribution ." If you have Perl,

you should have these modules. Since we're using Perl 5.8.7 as we write this, we'll

assume that's the current version of Perl.

As you develop your code, you may want to consider if you want to use only core modules,

so that you can be sure that anyone with Perl will have that module as long as they

have at least the same version as you.

[ ]

We'll avoid that debate here, mostly

because we love CPAN too much to do without it.

[]

Although we don't go into here, the Module::CoreList module has the lists of

which modules came with which versions of Perl, along with other historical data.

3.2. Using Modules

Almost every Perl module comes with documentation, and even though we might not know

how all of the behind-the-scenes magic works, we really don't have to worry about

that stuff if we know how to use the interface. That's why the interface is there,

after all: to hide the details.

On our local machine, we can read the module documentation with the perldoc command.

We give it the module name we're interested in, and it prints out its documentation.

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