Emacs Lisp编程入门指南

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“emacs-lisp-intro.pdf - 一本介绍Emacs Lisp编程的书籍，适合非程序员阅读。由Robert J. Chassell编写，第三版修订版，2009年10月28日发布。出版商是GNUPress，属于Free Software Foundation。该书遵循GNU Free Documentation License 1.3或更高版本的许可协议。” 《An Introduction to Programming in Emacs Lisp》是一本专为非程序员设计的Emacs Lisp编程入门指南。作者Robert J. Chassell在书中详细介绍了Emacs Lisp的基础知识，使得初学者即使没有编程背景也能理解并开始编写Emacs扩展。 Emacs Lisp是Emacs编辑器的核心语言，用于定制和扩展这个强大的文本编辑器。通过学习Emacs Lisp，用户可以自定义Emacs的工作流程，创建快捷键、宏、模式以及更复杂的插件，以适应个人的需求和喜好。本书的第三版修订版（Edition 3.10）包含了从1990年至2009年的更新和改进，反映了Emacs Lisp语言的发展和Emacs编辑器的进步。书中可能涵盖了以下内容： 1. **基础语法**：讲解Emacs Lisp的基本数据类型，如符号、字符串、列表和数字，以及基本操作，如函数调用、赋值和条件判断。 2. **函数和宏**：介绍如何定义和使用函数，以及Emacs Lisp中的宏，它们允许程序员编写元代码来生成其他代码。 3. **交互式编程**：强调Emacs的交互式特性，如何在Emacs中测试和调试代码，以及使用Emacs内置的REPL（Read-Eval-Print Loop）进行动态开发。 4. **面向对象编程**：Emacs Lisp支持面向对象编程，包括类、对象和方法的概念，以及继承和多态性。 5. **Emacs接口**：介绍如何访问和控制Emacs的内部状态，包括缓冲区、窗口、进程和其他Emacs对象。 6. **包系统**：讲解Emacs的包管理机制，如ELPA（Emacs Lisp Package Archive），以及如何安装和使用他人编写的Emacs Lisp库。 7. **调试和优化**：介绍调试Emacs Lisp代码的技巧，以及如何优化性能和减少内存消耗。 8. **实践项目**：可能包含实际的项目示例，帮助读者将所学知识应用到实践中，如创建自定义的编辑模式或工具栏。这本书遵循了GNU Free Documentation License，意味着你可以自由地复制、分发和修改文档，而且没有不变章节的要求。FSF的后封面文本强调了读者拥有自由地使用、分享和改进文档的权利，这与开源软件的精神相吻合。《An Introduction to Programming in Emacs Lisp》是学习Emacs Lisp和增强Emacs编辑器功能的宝贵资源，尤其适合那些希望通过编程提高工作效率和生产力的Emacs用户。

4 Preface

Thank You

My thanks to all who helped me with this boo k. My especial thanks to

Jim Blandy, Noah Friedman, Jim Kingdon, Roland McGrath, Frank Ritter,

Randy Smith, Richard M. Stallman, and Meliss a Weisshaus. My tha nks also go

to both Philip Johnson and David Stamp e fo r their patient encouragement. My

mistakes are my own.

Robert J. Chassell

bob@gnu.org

Section 1.1: Lisp Lists 1

1 List Processing

To the untutored eye, Lisp is a strange programming language. In Lisp code there

are parentheses everywhere. Some people even claim that the name stands for

‘Lots of Isolated Silly Parentheses’. But the claim is unwarranted. Lisp stands for

LISt Processing, and the programming language handles lists (and lis ts of lists) by

putting them between parentheses. The parentheses mark the boundaries of the

list. Sometimes a list is preceded by a single apostrophe or quo tation ma rk, ‘’’

Lists are the basis of Lisp.

1.1 Lisp Lists

In Lisp, a list loo ks like this: ’(rose violet daisy buttercup). This list is pr e -

ceded by a single apostrophe. It could just as well be written as follows, which looks

more like the kind of list you are likely to be familiar with:

’(rose

violet

daisy

buttercup)

The elements of this list a re the na mes of the four diﬀerent ﬂowers, separated from

each other by whitespace and sur rounded by parentheses, like ﬂowers in a ﬁeld with

a stone wa ll around them.

Lists can also have numbe rs in them, as in this lis t: (+ 2 2). This list has a

plus-sign, ‘+’, followed by two ‘2’s, each separated by whitespace.

In Lisp, both data and programs are represented the same way; that is, they a re

both lists of words, numbers, or other lists, separated by whitespa c e and surrounded

by parentheses. (Since a program looks like data, one program may easily serve

as data for another; this is a very p owerful feature of Lisp.) (Incidentally, thes e

two parenthetical remarks are not Lisp lists, because they contain ‘;’ and ‘.’ as

punctuation marks.)

Here is another list, this time with a list inside of it:

’(this list has (a list inside of it))

The components of this list are the words ‘this’, ‘list’, ‘has’, and the list ‘(a

list inside of it)’. The interior list is made up of the words ‘a’, ‘list’, ‘inside’,

‘of’, ‘it’.

1.1.1 Lisp Atoms

In Lisp, what we have been calling words are called atoms. This term comes from

the historical meaning of the word atom, which means ‘indivisible’. As far as Lisp

is concerned, the words we have been using in the lists cannot be divided into

any smalle r parts and still mean the same thing as part of a program; likewise with

numbers and single character symbols like ‘+’. On the other hand, unlike an ancient

The single apostrophe or qu otation mark is an abbreviation for the function quote; you

need not think about functions now; functions are deﬁ ned in Section 1.3 “Generate

an Error Message”, page 4.

2 Chapter 1: List Processing

atom, a list can be split into parts. (See Cha pter 7 “car cdr & cons Fundamental

Functio ns”, page 72.)

In a list, atoms ar e separated from each other by whitespace. They can be right

next to a parenthesis.

Te chnically speaking, a list in Lisp consists of par e ntheses surrounding atoms

separated by whitespace or surrounding other lists or surrounding both atoms and

other lists. A list c an have just one atom in it or have nothing in it at all. A list

with nothing in it looks like this: (), and is called the empty list. Unlike anything

else, an empty list is considered both an atom and a list at the same time.

The printed representation of both a toms and lists are called symbolic expres-

sions or, more concisely, s-expressions. The word expr ession by itself can r efer to

either the printed re presentation, or to the atom or list as it is held internally in the

computer. Often, people use the term expression indiscriminately. (Also, in many

texts, the word form is used as a syno nym for expression.)

Incidentally, the atoms that make up our universe were named such when they

were thought to be indivisible; but it has been found that physical atoms are not

indivisible. Parts can split oﬀ an atom or it can ﬁssion into two parts of roughly

equal size. Physical atoms were named pre maturely, b efore their truer nature was

found. In Lisp, certain kinds of atom, such as an array, can be separated into parts;

but the mechanism for doing this is diﬀerent from the mechanism for splitting a

list. As far as list operations are concerned, the atoms of a list are unsplittable.

As in English, the mea nings of the component letters of a Lisp atom are diﬀerent

from the meaning the letters make as a word. For example, the word for the South

American sloth, the ‘ai’, is completely diﬀerent from the two words, ‘a’, and ‘i’.

There are many kinds of atom in nature but only a few in Lisp: for ex-

ample, numbers, s uch as 37, 51 1, or 1729 , and symbols, such as ‘+’, ‘foo’, or

‘forward-line’. The words we have listed in the examples above are all symbols.

In everyday Lisp convers ation, the wor d “atom” is not often used, because program-

mers usually try to be more speciﬁc about what kind of atom they are dea ling with.

Lisp prog ramming is mostly about symbols (and sometimes numbers) within lists.

(Incidentally, the preceding three word parenthetical remark is a proper list in Lis p,

since it c onsists of ato ms , which in this case are symbols, separ ated by whitespace

and enclosed by pa rentheses, without any non-Lisp punctuation.)

Te xt between double quotation marks—even sentences or paragraphs—is also

an atom. Here is an example:

’(this list includes "text between quotation marks.")

In Lisp, all of the quoted text including the punctuation mark and the blank spaces

is a single atom. This kind of atom is called a string (for ‘string of characters’) and

is the sort of thing that is used for messages that a computer can print for a human

to read. Strings are a diﬀerent kind of atom than numbers or sy mbols a nd are used

diﬀerently.

Section 1.2: Run a Program 3

1.1.2 Whitespace in Lists

The amount of whitespace in a list does not matter. From the point of view of

the Lisp language,

’(this list

looks like this)

is exactly the same as this:

’(this list looks like this)

Both examples show what to Lisp is the same list, the list made up of the

symbols ‘this’, ‘list’, ‘looks’, ‘like’, and ‘this’ in that or der.

Extra whitespace and newlines are designed to make a list more readable by

humans . When Lisp reads the expression, it gets rid of all the extra whitespa ce

(but it needs to have at least one space between atoms in order to tell them apart.)

Odd as it seems, the examples we have seen cover almost all of what Lisp lists

look like! Every other list in Lisp looks more or less like one of these examples,

except that the list may be longer and more complex. In brief, a list is between

parentheses, a string is between quo tation marks, a symbol looks like a word, and

a number looks like a number. (For cer tain situations, square brackets, dots and

a few other special characters may be used; however, we will go quite far without

them.)

1.1.3 GNU Emacs Helps You Type Lists

When you type a Lisp ex pression in GNU Ema c s using either Lisp Interactio n mode

or Emacs Lisp mode, you have available to you several co mmands to format the Lisp

expression so it is easy to read. For example, pressing the TAB key automatically

indents the line the cursor is on by the right amount. A co mmand to properly

indent the c ode in a regio n is customarily bound to M-C-\. Indentation is designed

so that you can see which elements of a list b elong to which list—elements of a

sub-list are indented more than the elements of the enclosing list.

In addition, when you type a closing parenthesis, Emacs momentarily jumps the

cursor back to the matching o pening parenthesis, so you can see which one it is.

This is very useful, since every list you type in Lisp must have its closing pare nthesis

match its opening parenthesis. (See Section “Major Modes” in The GNU Emacs

Manual, for mor e infor mation about Emacs’ modes.)

1.2 Run a Progr am

A list in Lisp—any list—is a program ready to run. If you run it (for which the Lisp

jargon is evaluate), the computer will do one of three things: do nothing except

return to you the list itself; send you an error message; or, treat the ﬁrst symbol in

the list as a command to do something. (Usually, of cours e, it is the last of these

three things that you really want!)

The single apostrophe, ’, that I put in front of some of the example lists in

preceding sections is called a quote; w hen it precedes a list, it tells Lisp to do

nothing with the list, other than take it as it is written. But if there is no quote

preceding a list, the ﬁrst item of the list is special: it is a command for the computer

4 Chapter 1: List Processing

to obey. (In Lisp, these commands are called functions.) The list (+ 2 2) shown

above did not have a quote in front of it, so Lisp understands that the + is an

instruction to do something with the res t of the list: add the numbers that follow.

If you ar e reading this inside of GNU Emacs in Info, here is how you can evaluate

such a list: place your cursor immediately after the right hand parenthesis of the

following list and then type C-x C-e:

(+ 2 2)

You will see the number 4 appear in the echo area. (In the jargon, what you have

just done is “evaluate the list.” The echo area is the line at the bottom of the screen

that displays or “echoes” text.) Now try the same thing with a quoted list: place

the cursor right a fter the following list and type C-x C-e:

’(this is a quoted list)

You will see (this is a quoted list) appear in the echo area.

In both cases, what you are doing is giving a command to the program inside

of GNU Emacs called the Lisp interpreter—giving the interpreter a command to

evaluate the express ion. The name of the Lisp interpreter comes from the word for

the task done by a human who comes up with the meaning of an expression—who

“interprets” it.

You can also evaluate an atom that is not part of a list—one that is not sur-

rounded by parentheses; again, the Lisp interpreter translates from the humanly

readable expression to the language of the computer. But before discussing this

(see Section 1.7 “Variables”, page 9), we will discuss what the Lisp interpreter

does when you make an err or.

1.3 Generate an Error Message

Partly so you won’t worry if you do it accidentally, we will now give a command to

the Lisp interpreter that gener ates an error message. This is a harmless a c tiv ity;

and indeed, we will often try to generate error messa ges intentionally. Onc e you

understand the jargon, error messages can be informa tive. Instead of being called

“error” messages, they should be called “help” messages. They are like signposts to

a traveller in a strange country; deciphering them can be hard, but once understood,

they can point the way.

The error message is generated by a built-in GNU E macs debugger. We will

‘enter the debugger’. You get out of the debugger by typing q.

What we will do is e valuate a list that is not quoted and does not have a

meaningful command as its ﬁrs t element. Here is a list almost exactly the same as

the one we just used, but without the single-quote in front of it. Position the cursor

right after it and type C-x C-e:

(this is an unquoted list)

What you see depends on which version of Emacs you are running. GNU Emacs

version 22 provides more information than version 20 and before. First, the more

recent result of g enerating an error; then the earlier, version 20 result.

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