Perl编程新视野：Modern Perl第四版

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"Modern Perl, Fourth Edition (英文，第四版）" 本书《Modern Perl, Fourth Edition》专注于介绍Perl编程语言的最新特性，截止到Perl 5.22版本。这是一本非扫描PDF格式的书籍，拥有完整的目录结构，便于读者查阅和学习。 Perl是一种功能强大的脚本语言，自1987年发布以来，经历了多次更新和改进。第四版的《Modern Perl》旨在帮助开发者理解和掌握Perl 5.22及其后续版本中的新特性和最佳实践。作者chromatic以其对Perl深入的理解，展示了如何编写清晰、结构化和表达力强的代码，打破了Perl代码难以阅读和理解的传统印象。书中的内容不仅涵盖了Perl的基本语法和核心概念，如变量、数据类型、控制结构、正则表达式等，还深入探讨了模块化编程、错误处理、对象导向编程以及现代Perl中的高级主题。作者强调了Perl的idioms，即符合Perl语言风格的习惯用法，这对于写出高效且易于理解的Perl代码至关重要。《Modern Perl》在教导读者“如何”编写Perl代码的同时，也解释了“为什么”要这样写。书中通过丰富的示例代码和详尽的解释，让读者理解这些设计决策背后的思考过程。此外，书中的案例涵盖了从简单的文本处理到复杂的网络编程，全面展示了Perl在实际开发中的应用。读者评价高度赞扬了这本书。David Golden提到，如果要推荐一本教授Perl 5的书，他会首选《Modern Perl》，因为它像经典的K&R一样，能够全面教会读者如何编写高质量的Perl 5代码。Belden Lyman表示，他甚至愿意用第四版替代第一版来培训新员工，因为新版更新了内容，更适合现代Perl的学习。《Modern Perl, Fourth Edition》是一本针对Perl 5.22及以后版本的权威指南，无论你是初学者还是有经验的Perl开发者，都能从中受益匪浅，提升你的Perl编程技能。通过深入学习本书，你可以更好地适应Perl语言的发展，掌握最新的编程技术和最佳实践。

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good taste with experience. Express yourself, but be mindful of readability

and maintainability, especially for those who come after you.

Perl novices often find certain syntactic constructs opaque. These idioms (

Idioms on page 213) offer great (if subtle) power to experienced programmers,

but it’s okay to avoid them until you’re comfortable with them.

As another design goal, Perl tries to avoid surprising experienced (Perl) pro-

grammers. For example, adding two variables (

$first_num + $second_num

) is obvi-

ously a numeric operation (Numeric Operators on page 87). You’ve expressed

your intent to treat the values of those variables as numbers by using a

numeric operator. Perl happily does so. No matter the contents of

$first_num

and

$second_num

, Perl will coerce them to numeric values (Numeric Coercion

on page 67).

Perl adepts often call this principle DWIM, or do what I mean. You could just

as well call this the principle of least astonishment. Given a cursory under-

standing of Perl (especially context; Context on page 5), it should be possible

to understand the intent of an unfamiliar Perl expression. You will develop

this skill as you learn Perl.

Perl’s expressivity allows novices to write useful programs without having to

understand the entire language. This is by design! Experienced developers

often call the results baby Perl as a term of endearment. Everyone begins as

a novice. Through practice and learning from more experienced programmers,

you’ll understand and adopt more powerful idioms and techniques. It’s okay

for you to write simple code that you understand. Keep practicing and you’ll

become a native speaker.

A novice Perl hacker might triple a list of numbers with this:

my @tripled;

for (my $i = 0; $i < scalar @numbers; $i++) {

$tripled[$i] = $numbers[$i] * 3;

}

And a Perl adept might write the following:

my @tripled;

for my $num (@numbers) {

push @tripled, $num * 3;

}

While an experienced Perl hacker could write this:

my @tripled = map { $_ * 3 } @numbers;

Chapter 1. The Perl Philosophy • 4

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Every one of these three programs generates the same result. Each uses Perl

in a different way.

As you get more comfortable with Perl, you can let the language do more for

you. With experience, you can focus on what you want to do rather than how

to do it. Perl doesn’t care if you write baby or expert code. Design and refine

your programs for clarity, expressivity, reuse, and maintainability, in part or

in whole. Take advantage of this flexibility and pragmatism: it’s far better to

accomplish your task effectively now than to write a conceptually pure and

beautiful program next year.

Context

In spoken languages, the meaning of a word or phrase depends on how you

use it; the local context of other grammatical constructs helps clarify the

intent. For example, the inappropriate pluralization of “Please give me one

hamburgers!” sounds wrong (the pluralization of the noun differs from the

amount), just as the incorrect gender of “la gato” (the article is feminine, but

the noun is masculine) makes native speakers chuckle. Some words do double

duty; one sheep is a sheep just as two sheep are also sheep and you program

a program.

Perl uses context to express how to treat a piece of data. This governs the

amount of data as well as the kind of data. For example, several Perl operations

produce different behaviors when you expect zero, one, or many results. A

specific construct in Perl may do something different if you write “Do this,

but I don’t care about any results” compared to “Do this and give me multiple

results.” Other operations allow you to specify whether you expect to work

with numeric, textual, or true or false data.

You must keep context in mind when you read Perl code. Every expression

is part of a larger context. You may find yourself slapping your forehead after

a long debugging session when you discover that your assumptions about

context were incorrect. If instead you’re aware of context, your code will be

more correct—and cleaner, flexible, and more concise.

Void, Scalar, and List Context

Amount context governs how many items you expect an operation to produce.

Think of subject-verb number agreement in English. Even without knowing

the formal description of this principle, you probably understand the error

in the sentence “Perl are a fun language.” (In terms of amount context, you

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Context • 5

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could say that the verb are expects a plural noun or noun phrase.) In Perl,

the number of items you request influences how many you receive.

Suppose the function (Declaring Functions on page 91) called

find_chores()

sorts

your household todo list in order of priority. The number of chores you expect

to read from your list influences what the function produces. If you expect

nothing, you’re just pretending to be busy. If you expect one task, you have

something to do for the next fifteen minutes. If you have a burst of energy on

a free weekend, you could get all of your chores.

Why does context matter? A context-aware function can examine its calling

context and decide how much work it must do. When you call a function and

never use its return value, you’ve used void context:

find_chores();

Assigning the function’s return value to a single item (Scalars on page 50)

enforces scalar context:

my $single_result = find_chores();

Assigning the results of calling the function to an array (Arrays on page 52)

or a list, or using it in a list, evaluates the function in list context:

my @all_results = find_chores();

my ($single_element, @rest) = find_chores();

# list of results passed to a function

process_list_of_results( find_chores() );

The parentheses in the second line of the previous example group the two

variable declarations (Lexical Scope on page 104) into a single unit so that

assignment assigns to both of the variables. A single-item list is still a list,

though. You could also correctly write this:

my ($single_element) = find_chores();

In this case the parentheses tell the Perl parser that you intend list context

for the single variable

$single_element

. This is subtle, but now that you know

about it, the difference of amount context between these two statements

should be obvious:

my $scalar_context = find_chores();

my ($list_context) = find_chores();

Lists propagate list context to the expressions they contain. This often con-

fuses novices until they understand it. Both of these calls to

find_chores()

occur

in list context:

Chapter 1. The Perl Philosophy • 6

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Prepared exclusively for alex chen

In rare circumstances, you may not be able to use the appropriate operator

to enforce value context. To force a numeric context, add zero to a variable.

To force a string context, concatenate a variable with the empty string. To

force a boolean context, double up the negation operator:

my $numeric_x = 0 + $x; # forces numeric context

my $stringy_x = '' . $x; # forces string context

my $boolean_x = !!$x; # forces boolean context

Value contexts are easier to identify than amount contexts. Once you know

which operators provide which contexts (Operator Types on page 87), you’ll

rarely make mistakes.

Implicit Ideas

Perl code can seem dense at first, but it’s full of linguistic shortcuts. These

allow experienced programmers to glance at code and understand its important

implications. Context is one shortcut. Another is default variables—the pro-

gramming equivalent of pronouns.

The Default Scalar Variable

The default scalar variable (or topic variable),

, is most notable in its absence:

many of Perl’s built-in operations work on the contents of

in the absence

of an explicit variable. You can still type

if it makes your code clearer to

you, but it’s often unnecessary.

Many of Perl’s scalar operators (including

chr

ord

length

reverse

, and

) work

on the default scalar variable if you don’t provide an alternative. For example,

the

chomp

built-in removes any trailing newline sequence (technically the

contents of

; see

perldoc -f chomp

) from its operand:

my $uncle = "Bob\n";

chomp $uncle;

say "'$uncle'";

behaves the same way in Perl as the pronoun it does in English. Without

an explicit variable,

chomp

removes the trailing newline sequence from

When you write

chomp;

, Perl will always chomp it. These two lines of code are

equivalent:

chomp $_;

chomp;

say

and

also operate on

in the absence of other arguments:

print; # prints $_ to the current filehandle

say; # prints $_ and a newline to the current filehandle

Chapter 1. The Perl Philosophy • 8

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