This is an introduction to Programming in Emacs Lisp, for people who are not programmers.
This master menu first lists each chapter and index; then it lists every node in every chapter.
defun.
.emacs file.
--- The Detailed Node Listing ---
Preface
List Processing
Lisp Lists
The Lisp Interpreter
Evaluation
Variables
Arguments
Setting the Value of a Variable
setq to count.
Practicing Evaluation
How To Write Function Definitions
defun special form.
interactive.
Install a Function Definition
Make a Function Interactive
let
The if Special Form
if expression.
Truth and Falsehood in Emacs Lisp
nil has two meanings.
save-excursion
A Few Buffer--Related Functions
goto-char,
point-min, and push-mark.
beginning-of-buffer.
save-excursion and
insert-buffer-substring.
The Definition of mark-whole-buffer
The Definition of append-to-buffer
let expression.
save-excursion works.
A Few More Complex Functions
set-buffer, get-buffer-create.
or.
goto-char,
point-min, and push-mark.
The Definition of insert-buffer
or and a let.
if instead of an or.
or expression works.
save-excursion expressions.
The Interactive Expression in insert-buffer
Complete Definition of beginning-of-buffer
beginning-of-buffer with an Argument
Narrowing and Widening
save-restriction special form.
car, cdr, cons: Fundamental Functions
cdr repeatedly.
cons
Cutting and Storing Text
zap-to-char
progn special form.
point and search-forward.
kill-region
Initializing a Variable with defvar
copy-region-as-kill
copy-region-as-kill.
The Body of copy-region-as-kill
How Lists are Implemented
Yanking Text Back
kill-ring-yank-pointer variable.
Loops and Recursion
while
while loop that uses a list.
while, car, cdr.
A Loop with an Incrementing Counter
Loop with a Decrementing Counter
Save your time: dolist and dotimes
Recursion
Recursion in Place of a Counter
Recursive Patterns
Regular Expression Searches
sentence-end.
search-forward.
TAGS table.
forward-sentence
while loops.
forward-paragraph: a Goldmine of Functions
let* expression.
while loop.
forward-paragraph code.
Counting: Repetition and Regexps
The count-words-region Function
while loop.
count-words-region.
Counting Words in a defun
count-words.
Count Words in defuns in Different Files
Prepare the Data for Display in a Graph
Readying a Graph
Your .emacs File
.emacs file.
Debugging
Handling the Kill Ring
The rotate-yank-pointer Function
rotate-yank-pointer.
The Body of rotate-yank-pointer
if expression.
%, function.
% in rotate-yank-pointer.
yank
rotate-yank-pointer.
A Graph with Labelled Axes
let expression in print-graph.
The print-Y-axis Function
The print-X-axis Function
print-Y-axis, but not exactly.
Printing the Whole Graph
Most of the GNU Emacs integrated environment is written in the programming language called Emacs Lisp. The code written in this programming language is the software--the sets of instructions--that tell the computer what to do when you give it commands. Emacs is designed so that you can write new code in Emacs Lisp and easily install it as an extension to the editor.
(GNU Emacs is sometimes called an "extensible editor", but it does much more than provide editing capabilities. It is better to refer to Emacs as an "extensible computing environment". However, that phrase is quite a mouthful. It is easier to refer to Emacs simply as an editor. Moreover, everything you do in Emacs--find the Mayan date and phases of the moon, simplify polynomials, debug code, manage files, read letters, write books--all these activities are kinds of editing in the most general sense of the word.)
Although Emacs Lisp is usually thought of in association only with Emacs, it is a full computer programming language. You can use Emacs Lisp as you would any other programming language.
Perhaps you want to understand programming; perhaps you want to extend Emacs; or perhaps you want to become a programmer. This introduction to Emacs Lisp is designed to get you started: to guide you in learning the fundamentals of programming, and more importantly, to show you how you can teach yourself to go further.
All through this document, you will see little sample programs you can run inside of Emacs. If you read this document in Info inside of GNU Emacs, you can run the programs as they appear. (This is easy to do and is explained when the examples are presented.) Alternatively, you can read this introduction as a printed book while sitting beside a computer running Emacs. (This is what I like to do; I like printed books.) If you don't have a running Emacs beside you, you can still read this book, but in this case, it is best to treat it as a novel or as a travel guide to a country not yet visited: interesting, but not the same as being there.
Much of this introduction is dedicated to walk-throughs or guided tours of code used in GNU Emacs. These tours are designed for two purposes: first, to give you familiarity with real, working code (code you use every day); and, second, to give you familiarity with the way Emacs works. It is interesting to see how a working environment is implemented. Also, I hope that you will pick up the habit of browsing through source code. You can learn from it and mine it for ideas. Having GNU Emacs is like having a dragon's cave of treasures.
In addition to learning about Emacs as an editor and Emacs Lisp as a
programming language, the examples and guided tours will give you an
opportunity to get acquainted with Emacs as a Lisp programming
environment. GNU Emacs supports programming and provides tools that
you will want to become comfortable using, such as M-. (the key
which invokes the find-tag command). You will also learn about
buffers and other objects that are part of the environment.
Learning about these features of Emacs is like learning new routes
around your home town.
Finally, I hope to convey some of the skills for using Emacs to learn aspects of programming that you don't know. You can often use Emacs to help you understand what puzzles you or to find out how to do something new. This self-reliance is not only a pleasure, but an advantage.
This text is written as an elementary introduction for people who are not programmers. If you are a programmer, you may not be satisfied with this primer. The reason is that you may have become expert at reading reference manuals and be put off by the way this text is organized.
An expert programmer who reviewed this text said to me:
I prefer to learn from reference manuals. I "dive into" each paragraph, and "come up for air" between paragraphs.When I get to the end of a paragraph, I assume that that subject is done, finished, that I know everything I need (with the possible exception of the case when the next paragraph starts talking about it in more detail). I expect that a well written reference manual will not have a lot of redundancy, and that it will have excellent pointers to the (one) place where the information I want is.
This introduction is not written for this person!
Firstly, I try to say everything at least three times: first, to introduce it; second, to show it in context; and third, to show it in a different context, or to review it.
Secondly, I hardly ever put all the information about a subject in one place, much less in one paragraph. To my way of thinking, that imposes too heavy a burden on the reader. Instead I try to explain only what you need to know at the time. (Sometimes I include a little extra information so you won't be surprised later when the additional information is formally introduced.)
When you read this text, you are not expected to learn everything the first time. Frequently, you need only make, as it were, a `nodding acquaintance' with some of the items mentioned. My hope is that I have structured the text and given you enough hints that you will be alert to what is important, and concentrate on it.
You will need to "dive into" some paragraphs; there is no other way to read them. But I have tried to keep down the number of such paragraphs. This book is intended as an approachable hill, rather than as a daunting mountain.
This introduction to Programming in Emacs Lisp has a companion document, Top. The reference manual has more detail than this introduction. In the reference manual, all the information about one topic is concentrated in one place. You should turn to it if you are like the programmer quoted above. And, of course, after you have read this Introduction, you will find the Reference Manual useful when you are writing your own programs.
Lisp was first developed in the late 1950s at the Massachusetts Institute of Technology for research in artificial intelligence. The great power of the Lisp language makes it superior for other purposes as well, such as writing editor commands and integrated environments.
GNU Emacs Lisp is largely inspired by Maclisp, which was written at MIT
in the 1960s. It is somewhat inspired by Common Lisp, which became a
standard in the 1980s. However, Emacs Lisp is much simpler than Common
Lisp. (The standard Emacs distribution contains an optional extensions
file, cl.el, that adds many Common Lisp features to Emacs Lisp.)
If you don't know GNU Emacs, you can still read this document profitably. However, I recommend you learn Emacs, if only to learn to move around your computer screen. You can teach yourself how to use Emacs with the on-line tutorial. To use it, type C-h t. (This means you press and release the <CTRL> key and the h at the same time, and then press and release t.)
Also, I often refer to one of Emacs' standard commands by listing the
keys which you press to invoke the command and then giving the name of
the command in parentheses, like this: M-C-\
(indent-region). What this means is that the
indent-region command is customarily invoked by typing
M-C-\. (You can, if you wish, change the keys that are typed to
invoke the command; this is called rebinding. See Keymaps.) The abbreviation M-C-\ means that you type your
<META> key, <CTRL> key and <\> key all at the same time.
(On many modern keyboards the <META> key is labelled
<ALT>.)
Sometimes a combination like this is called a keychord, since it is
similar to the way you play a chord on a piano. If your keyboard does
not have a <META> key, the <ESC> key prefix is used in place
of it. In this case, M-C-\ means that you press and release your
<ESC> key and then type the <CTRL> key and the <\> key at
the same time. But usually M-C-\ means press the <CTRL> key
along with the key that is labelled <ALT> and, at the same time,
press the <\> key.
In addition to typing a lone keychord, you can prefix what you type with C-u, which is called the `universal argument'. The C-u keychord passes an argument to the subsequent command. Thus, to indent a region of plain text by 6 spaces, mark the region, and then type C-u 6 M-C-\. (If you do not specify a number, Emacs either passes the number 4 to the command or otherwise runs the command differently than it would otherwise.) See Arguments.
If you are reading this in Info using GNU Emacs, you can read through this whole document just by pressing the space bar, <SPC>. (To learn about Info, type C-h i and then select Info.)
A note on terminology: when I use the word Lisp alone, I often am referring to the various dialects of Lisp in general, but when I speak of Emacs Lisp, I am referring to GNU Emacs Lisp in particular.
My thanks to all who helped me with this book. My especial thanks to Jim Blandy, Noah Friedman, Jim Kingdon, Roland McGrath, Frank Ritter, Randy Smith, Richard M. Stallman, and Melissa Weisshaus. My thanks also go to both Philip Johnson and David Stampe for their patient encouragement. My mistakes are my own.
Robert J. Chassell
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 lists 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 quotation mark,
'. Lists are the basis of Lisp.
In Lisp, a list looks like this: '(rose violet daisy buttercup).
This list is preceded 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 are the names of the four different flowers, separated from each other by whitespace and surrounded by parentheses, like flowers in a field with a stone wall around them.
Lists can also have numbers in them, as in this list: (+ 2 2).
This list has a plus-sign, +, followed by two 2s, each
separated by whitespace.
In Lisp, both data and programs are represented the same way; that is,
they are both lists of words, numbers, or other lists, separated by
whitespace and surrounded by parentheses. (Since a program looks like
data, one program may easily serve as data for another; this is a very
powerful feature of Lisp.) (Incidentally, these 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.
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 smaller 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
atom, a list can be split into parts. (See car cdr & cons Fundamental Functions.)
In a list, atoms are separated from each other by whitespace. They can be right next to a parenthesis.
Technically speaking, a list in Lisp consists of parentheses surrounding
atoms separated by whitespace or surrounding other lists or surrounding
both atoms and other lists. A list can 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 atoms and lists are called symbolic expressions or, more concisely, s-expressions. The word expression by itself can refer to either the printed representation, 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 synonym 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 prematurely, before 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 meanings 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
example, numbers, such as 37, 511, 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
conversation, the word "atom" is not often used, because programmers
usually try to be more specific about what kind of atom they are dealing
with. Lisp programming is mostly about symbols (and sometimes numbers)
within lists. (Incidentally, the preceding three word parenthetical
remark is a proper list in Lisp, since it consists of atoms, which in
this case are symbols, separated by whitespace and enclosed by
parentheses, without any non-Lisp punctuation.)
In addition, text between double quotation marks--even sentences or paragraphs--is 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 symbols and are used differently.
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 order.
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 whitespace (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 quotation marks, a symbol looks like a word, and a number looks like a number. (For certain situations, square brackets, dots and a few other special characters may be used; however, we will go quite far without them.)
When you type a Lisp expression in GNU Emacs using either Lisp Interaction mode or Emacs Lisp mode, you have available to you several commands 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 command to properly indent the code in a region is customarily bound to M-C-\. Indentation is designed so that you can see which elements of a list belongs 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 opening parenthesis, so you can see which one it is. This is very useful, since every list you type in Lisp must have its closing parenthesis match its opening parenthesis. (See Major Modes, for more information about Emacs' modes.)
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 course, 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; when 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
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
rest of the list: add the numbers that follow.
If you are 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 after 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 expression. 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 surrounded by parentheses; again, the Lisp interpreter translates from the humanly readable expression to the language of the computer. But before discussing this (see Variables), we will discuss what the Lisp interpreter does when you make an error.
Partly so you won't worry if you do it accidentally, we will now give a command to the Lisp interpreter that generates an error message. This is a harmless activity; and indeed, we will often try to generate error messages intentionally. Once you understand the jargon, error messages can be informative. 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 Emacs debugger. We
will `enter the debugger'. You get out of the debugger by typing q.
What we will do is evaluate a list that is not quoted and does not have a meaningful command as its first 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 21 provides more information than version 20 and before. First, the more recent result of generating an error; then the earlier, version 20 result.
In GNU Emacs version 21, a *Backtrace* window will open up and
you will see the following in it:
---------- Buffer: *Backtrace* ---------- Debugger entered--Lisp error: (void-function this) (this is an unquoted list) eval((this is an unquoted list)) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ---------- Buffer: *Backtrace* ----------
Your cursor will be in this window (you may have to wait a few seconds before it becomes visible). To quit the debugger and make the debugger window go away, type:
q
Please type q right now, so you become confident that you can get out of the debugger. Then, type C-x C-e again to re-enter it.
Based on what we already know, we can almost read this error message.
You read the *Backtrace* buffer from the bottom up; it tells
you what Emacs did. When you typed C-x C-e, you made an
interactive call to the command eval-last-sexp. eval is
an abbreviation for `evaluate' and sexp is an abbreviation for
`symbolic expression'. The command means `evaluate last symbolic
expression', which is the expression just before your cursor.
Each line above tells you what the Lisp interpreter evaluated next.
The most recent action is at the top. The buffer is called the
*Backtrace* buffer because it enables you to track Emacs
backwards.
At the top of the *Backtrace* buffer, you see the line:
Debugger entered--Lisp error: (void-function this)
The Lisp interpreter tried to evaluate the first atom of the list, the
word this. It is this action that generated the error message
void-function this.
The message contains the words void-function and this.
The word function was mentioned once before. It is a very
important word. For our purposes, we can define it by saying that a
function is a set of instructions to the computer that tell the
computer to do something.
Now we can begin to understand the error message: void-function
this. The function (that is, the word this) does not have a
definition of any set of instructions for the computer to carry out.
The slightly odd word, void-function, is designed to cover the
way Emacs Lisp is implemented, which is that when a symbol does not
have a function definition attached to it, the place that should
contain the instructions is `void'.
On the other hand, since we were able to add 2 plus 2 successfully, by
evaluating (+ 2 2), we can infer that the symbol + must
have a set of instructions for the computer to obey and those
instructions must be to add the numbers that follow the +.
In GNU Emacs version 20, and in earlier versions, you will see only one line of error message; it will appear in the echo area and look like this:
Symbol's function definition is void: this
(Also, your terminal may beep at you--some do, some don't; and others blink. This is just a device to get your attention.) The message goes away as soon as you type another key, even just to move the cursor.
We know the meaning of the word Symbol. It refers to the first
atom of the list, the word this. The word function
refers to the instructions that tell the computer what to do.
(Technically, the symbol tells the computer where to find the
instructions, but this is a complication we can ignore for the
moment.)
The error message can be understood: Symbol's function
definition is void: this. The symbol (that is, the word
this) lacks instructions for the computer to carry out.
We can articulate another characteristic of Lisp based on what we have
discussed so far--an important characteristic: a symbol, like
+, is not itself the set of instructions for the computer to
carry out. Instead, the symbol is used, perhaps temporarily, as a way
of locating the definition or set of instructions. What we see is the
name through which the instructions can be found. Names of people
work the same way. I can be referred to as Bob; however, I am
not the letters B, o, b but am the consciousness
consistently associated with a particular life-form. The name is not
me, but it can be used to refer to me.
In Lisp, one set of instructions can be attached to several names.
For example, the computer instructions for adding numbers can be
linked to the symbol plus as well as to the symbol +
(and are in some dialects of Lisp). Among humans, I can be referred
to as Robert as well as Bob and by other words as well.
On the other hand, a symbol can have only one function definition
attached to it at a time. Otherwise, the computer would be confused as
to which definition to use. If this were the case among people, only
one person in the world could be named Bob. However, the function
definition to which the name refers can be changed readily.
(See Install a Function Definition.)
Since Emacs Lisp is large, it is customary to name symbols in a way
that identifies the part of Emacs to which the function belongs.
Thus, all the names for functions that deal with Texinfo start with
texinfo- and those for functions that deal with reading mail
start with rmail-.
Based on what we have seen, we can now start to figure out what the Lisp interpreter does when we command it to evaluate a list. First, it looks to see whether there is a quote before the list; if there is, the interpreter just gives us the list. On the other hand, if there is no quote, the interpreter looks at the first element in the list and sees whether it has a function definition. If it does, the interpreter carries out the instructions in the function definition. Otherwise, the interpreter prints an error message.
This is how Lisp works. Simple. There are added complications which we will get to in a minute, but these are the fundamentals. Of course, to write Lisp programs, you need to know how to write function definitions and attach them to names, and how to do this without confusing either yourself or the computer.
Now, for the first complication. In addition to lists, the Lisp interpreter can evaluate a symbol that is not quoted and does not have parentheses around it. The Lisp interpreter will attempt to determine the symbol's value as a variable. This situation is described in the section on variables. (See Variables.)
The second complication occurs because some functions are unusual and do not work in the usual manner. Those that don't are called special forms. They are used for special jobs, like defining a function, and there are not many of them. In the next few chapters, you will be introduced to several of the more important special forms.
The third and final complication is this: if the function that the Lisp interpreter is looking at is not a special form, and if it is part of a list, the Lisp interpreter looks to see whether the list has a list inside of it. If there is an inner list, the Lisp interpreter first figures out what it should do with the inside list, and then it works on the outside list. If there is yet another list embedded inside the inner list, it works on that one first, and so on. It always works on the innermost list first. The interpreter works on the innermost list first, to evaluate the result of that list. The result may be used by the enclosing expression.
Otherwise, the interpreter works left to right, from one expression to the next.
One other aspect of interpreting: the Lisp interpreter is able to interpret two kinds of entity: humanly readable code, on which we will focus exclusively, and specially processed code, called byte compiled code, which is not humanly readable. Byte compiled code runs faster than humanly readable code.
You can transform humanly readable code into byte compiled code by
running one of the compile commands such as byte-compile-file.
Byte compiled code is usually stored in a file that ends with a
.elc extension rather than a .el extension. You will
see both kinds of file in the emacs/lisp directory; the files
to read are those with .el extensions.
As a practical matter, for most things you might do to customize or extend Emacs, you do not need to byte compile; and I will not discuss the topic here. See Byte Compilation, for a full description of byte compilation.
When the Lisp interpreter works on an expression, the term for the activity is called evaluation. We say that the interpreter `evaluates the expression'. I've used this term several times before. The word comes from its use in everyday language, `to ascertain the value or amount of; to appraise', according to Webster's New Collegiate Dictionary.
After evaluating an expression, the Lisp interpreter will most likely return the value that the computer produces by carrying out the instructions it found in the function definition, or perhaps it will give up on that function and produce an error message. (The interpreter may also find itself tossed, so to speak, to a different function or it may attempt to repeat continually what it is doing for ever and ever in what is called an `infinite loop'. These actions are less common; and we can ignore them.) Most frequently, the interpreter returns a value.
At the same time the interpreter returns a value, it may do something else as well, such as move a cursor or copy a file; this other kind of action is called a side effect. Actions that we humans think are important, such as printing results, are often "side effects" to the Lisp interpreter. The jargon can sound peculiar, but it turns out that it is fairly easy to learn to use side effects.
In summary, evaluating a symbolic expression most commonly causes the Lisp interpreter to return a value and perhaps carry out a side effect; or else produce an error.
If evaluation applies to a list that is inside another list, the outer list may use the value returned by the first evaluation as information when the outer list is evaluated. This explains why inner expressions are evaluated first: the values they return are used by the outer expressions.
We can investigate this process by evaluating another addition example. Place your cursor after the following expression and type C-x C-e:
(+ 2 (+ 3 3))
The number 8 will appear in the echo area.
What happens is that the Lisp interpreter first evaluates the inner
expression, (+ 3 3), for which the value 6 is returned; then it
evaluates the outer expression as if it were written (+ 2 6), which
returns the value 8. Since there are no more enclosing expressions to
evaluate, the interpreter prints that value in the echo area.
Now it is easy to understand the name of the command invoked by the
keystrokes C-x C-e: the name is eval-last-sexp. The
letters sexp are an abbreviation for `symbolic expression', and
eval is an abbreviation for `evaluate'. The command means
`evaluate last symbolic expression'.
As an experiment, you can try evaluating the expression by putting the cursor at the beginning of the next line immediately following the expression, or inside the expression.
Here is another copy of the expression:
(+ 2 (+ 3 3))
If you place the cursor at the beginning of the blank line that
immediately follows the expression and type C-x C-e, you will
still get the value 8 printed in the echo area. Now try putting the
cursor inside the expression. If you put it right after the next to
last parenthesis (so it appears to sit on top of the last parenthesis),
you will get a 6 printed in the echo area! This is because the command
evaluates the expression (+ 3 3).
Now put the cursor immediately after a number. Type C-x C-e and
you will get the number itself. In Lisp, if you evaluate a number, you
get the number itself--this is how numbers differ from symbols. If you
evaluate a list starting with a symbol like +, you will get a
value returned that is the result of the computer carrying out the
instructions in the function definition attached to that name. If a
symbol by itself is evaluated, something different happens, as we will
see in the next section.
In Emacs Lisp, a symbol can have a value attached to it just as it can have a function definition attached to it. The two are different. The function definition is a set of instructions that a computer will obey. A value, on the other hand, is something, such as number or a name, that can vary (which is why such a symbol is called a variable). The value of a symbol can be any expression in Lisp, such as a symbol, number, list, or string. A symbol that has a value is often called a variable.
A symbol can have both a function definition and a value attached to it at the same time. Or it can have just one or the other. The two are separate. This is somewhat similar to the way the name Cambridge can refer to the city in Massachusetts and have some information attached to the name as well, such as "great programming center".
Another way to think about this is to imagine a symbol as being a chest of drawers. The function definition is put in one drawer, the value in another, and so on. What is put in the drawer holding the value can be changed without affecting the contents of the drawer holding the function definition, and vice-versa.
fill-column, an Example VariableThe variable fill-column illustrates a symbol with a value
attached to it: in every GNU Emacs buffer, this symbol is set to some
value, usually 72 or 70, but sometimes to some other value. To find the
value of this symbol, evaluate it by itself. If you are reading this in
Info inside of GNU Emacs, you can do this by putting the cursor after
the symbol and typing C-x C-e:
fill-column
After I typed C-x C-e, Emacs printed the number 72 in my echo
area. This is the value for which fill-column is set for me as I
write this. It may be different for you in your Info buffer. Notice
that the value returned as a variable is printed in exactly the same way
as the value returned by a function carrying out its instructions. From
the point of view of the Lisp interpreter, a value returned is a value
returned. What kind of expression it came from ceases to matter once
the value is known.
A symbol can have any value attached to it or, to use the jargon, we can
bind the variable to a value: to a number, such as 72; to a
string, "such as this"; to a list, such as (spruce pine
oak); we can even bind a variable to a function definition.
A symbol can be bound to a value in several ways. See Setting the Value of a Variable, for information about one way to do this.
When we evaluated fill-column to find its value as a variable,
we did not place parentheses around the word. This is because we did
not intend to use it as a function name.
If fill-column were the first or only element of a list, the
Lisp interpreter would attempt to find the function definition
attached to it. But fill-column has no function definition.
Try evaluating this:
(fill-column)
In GNU Emacs version 21, you will create a *Backtrace* buffer
that says:
---------- Buffer: *Backtrace* ---------- Debugger entered--Lisp error: (void-function fill-column) (fill-column) eval((fill-column)) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ---------- Buffer: *Backtrace* ----------
(Remember, to quit the debugger and make the debugger window go away,
type q in the *Backtrace* buffer.)
In GNU Emacs 20 and before, you will produce an error message that says:
Symbol's function definition is void: fill-column
(The message will go away away as soon as you move the cursor or type another key.)
If you attempt to evaluate a symbol that does not have a value bound to
it, you will receive an error message. You can see this by
experimenting with our 2 plus 2 addition. In the following expression,
put your cursor right after the +, before the first number 2,
type C-x C-e:
(+ 2 2)
In GNU Emacs 21, you will create a *Backtrace* buffer that
says:
---------- Buffer: *Backtrace* ---------- Debugger entered--Lisp error: (void-variable +) eval(+) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ---------- Buffer: *Backtrace* ----------
(As with the other times we entered the debugger, you can quit by
typing q in the *Backtrace* buffer.)
This backtrace is different from the very first error message we saw,
which said, Debugger entered--Lisp error: (void-function this).
In this case, the function does not have a value as a variable; while
in the other error message, the function (the word `this') did not
have a definition.
In this experiment with the +, what we did was cause the Lisp
interpreter to evaluate the + and look for the value of the
variable instead of the function definition. We did this by placing the
cursor right after the symbol rather than after the parenthesis of the
enclosing list as we did before. As a consequence, the Lisp interpreter
evaluated the preceding s-expression, which in this case was the
+ by itself.
Since + does not have a value bound to it, just the function
definition, the error message reported that the symbol's value as a
variable was void.
In GNU Emacs version 20 and before, your error message will say:
Symbol's value as variable is void: +
The meaning is the same as in GNU Emacs 21.
To see how information is passed to functions, let's look again at our old standby, the addition of two plus two. In Lisp, this is written as follows:
(+ 2 2)
If you evaluate this expression, the number 4 will appear in your echo
area. What the Lisp interpreter does is add the numbers that follow
the +.
The numbers added by + are called the arguments of the
function +. These numbers are the information that is given to
or passed to the function.
The word `argument' comes from the way it is used in mathematics and
does not refer to a disputation between two people; instead it refers to
the information presented to the function, in this case, to the
+. In Lisp, the arguments to a function are the atoms or lists
that follow the function. The values returned by the evaluation of
these atoms or lists are passed to the function. Different functions
require different numbers of arguments; some functions require none at
all.1
The type of data that should be passed to a function depends on what
kind of information it uses. The arguments to a function such as
+ must have values that are numbers, since + adds numbers.
Other functions use different kinds of data for their arguments.
For example, the concat function links together or unites two or
more strings of text to produce a string. The arguments are strings.
Concatenating the two character strings abc, def produces
the single string abcdef. This can be seen by evaluating the
following:
(concat "abc" "def")
The value produced by evaluating this expression is "abcdef".
A function such as substring uses both a string and numbers as
arguments. The function returns a part of the string, a substring of
the first argument. This function takes three arguments. Its first
argument is the string of characters, the second and third arguments are
numbers that indicate the beginning and end of the substring. The
numbers are a count of the number of characters (including spaces and
punctuations) from the beginning of the string.
For example, if you evaluate the following:
(substring "The quick brown fox jumped." 16 19)
you will see "fox" appear in the echo area. The arguments are the
string and the two numbers.
Note that the string passed to substring is a single atom even
though it is made up of several words separated by spaces. Lisp counts
everything between the two quotation marks as part of the string,
including the spaces. You can think of the substring function as
a kind of `atom smasher' since it takes an otherwise indivisible atom
and extracts a part. However, substring is only able to extract
a substring from an argument that is a string, not from another type of
atom such as a number or symbol.
An argument can be a symbol that returns a value when it is evaluated.
For example, when the symbol fill-column by itself is evaluated,
it returns a number. This number can be used in an addition.
Position the cursor after the following expression and type C-x C-e:
(+ 2 fill-column)
The value will be a number two more than what you get by evaluating
fill-column alone. For me, this is 74, because the value of
fill-column is 72.
As we have just seen, an argument can be a symbol that returns a value
when evaluated. In addition, an argument can be a list that returns a
value when it is evaluated. For example, in the following expression,
the arguments to the function concat are the strings
"The " and " red foxes." and the list
(number-to-string (+ 2 fill-column)).
(concat "The " (number-to-string (+ 2 fill-column)) " red foxes.")
If you evaluate this expression--and if, as with my Emacs,
fill-column evaluates to 72--"The 74 red foxes." will
appear in the echo area. (Note that you must put spaces after the
word The and before the word red so they will appear in
the final string. The function number-to-string converts the
integer that the addition function returns to a string.
number-to-string is also known as int-to-string.)
Some functions, such as concat, + or *, take any
number of arguments. (The * is the symbol for multiplication.)
This can be seen by evaluating each of the following expressions in
the usual way. What you will see in the echo area is printed in this
text after =>, which you may read as `evaluates to'.
In the first set, the functions have no arguments:
(+) => 0 (*) => 1
In this set, the functions have one argument each:
(+ 3) => 3 (* 3) => 3
In this set, the functions have three arguments each:
(+ 3 4 5) => 12 (* 3 4 5) => 60
When a function is passed an argument of the wrong type, the Lisp
interpreter produces an error message. For example, the +
function expects the values of its arguments to be numbers. As an
experiment we can pass it the quoted symbol hello instead of a
number. Position the cursor after the following expression and type
C-x C-e:
(+ 2 'hello)
When you do this you will generate an error message. What has happened
is that + has tried to add the 2 to the value returned by
'hello, but the value returned by 'hello is the symbol
hello, not a number. Only numbers can be added. So +
could not carry out its addition.
In GNU Emacs version 21, you will create and enter a
*Backtrace* buffer that says:
---------- Buffer: *Backtrace* ----------
Debugger entered--Lisp error:
(wrong-type-argument number-or-marker-p hello)
+(2 hello)
eval((+ 2 (quote hello)))
eval-last-sexp-1(nil)
eval-last-sexp(nil)
call-interactively(eval-last-sexp)
---------- Buffer: *Backtrace* ----------
As usual, the error message tries to be helpful and makes sense after you learn how to read it.
The first part of the error message is straightforward; it says
wrong type argument. Next comes the mysterious jargon word
number-or-marker-p. This word is trying to tell you what
kind of argument the + expected.
The symbol number-or-marker-p says that the Lisp interpreter is
trying to determine whether the information presented it (the value of
the argument) is a number or a marker (a special object representing a
buffer position). What it does is test to see whether the + is
being given numbers to add. It also tests to see whether the
argument is something called a marker, which is a specific feature of
Emacs Lisp. (In Emacs, locations in a buffer are recorded as markers.
When the mark is set with the C-@ or C-<SPC> command,
its position is kept as a marker. The mark can be considered a
number--the number of characters the location is from the beginning
of the buffer.) In Emacs Lisp, + can be used to add the
numeric value of marker positions as numbers.
The p of number-or-marker-p is the embodiment of a
practice started in the early days of Lisp programming. The p
stands for `predicate'. In the jargon used by the early Lisp
researchers, a predicate refers to a function to determine whether some
property is true or false. So the p tells us that
number-or-marker-p is the name of a function that determines
whether it is true or false that the argument supplied is a number or
a marker. Other Lisp symbols that end in p include zerop,
a function that tests whether its argument has the value of zero, and
listp, a function that tests whether its argument is a list.
Finally, the last part of the error message is the symbol hello.
This is the value of the argument that was passed to +. If the
addition had been passed the correct type of object, the value passed
would have been a number, such as 37, rather than a symbol like
hello. But then you would not have got the error message.
In GNU Emacs version 20 and before, the echo area displays an error message that says:
Wrong type argument: number-or-marker-p, hello
This says, in different words, the same as the top line of the
*Backtrace* buffer.
message FunctionLike +, the message function takes a variable number of
arguments. It is used to send messages to the user and is so useful
that we will describe it here.
A message is printed in the echo area. For example, you can print a message in your echo area by evaluating the following list:
(message "This message appears in the echo area!")
The whole string between double quotation marks is a single argument
and is printed in toto. (Note that in this example, the message
itself will appear in the echo area within double quotes; that is
because you see the value returned by the message function. In
most uses of message in programs that you write, the text will
be printed in the echo area as a side-effect, without the quotes.
See multiply-by-seven in detail, for an example of this.)
However, if there is a %s in the quoted string of characters, the
message function does not print the %s as such, but looks
to the argument that follows the string. It evaluates the second
argument and prints the value at the location in the string where the
%s is.
You can see this by positioning the cursor after the following expression and typing C-x C-e:
(message "The name of this buffer is: %s." (buffer-name))
In Info, "The name of this buffer is: *info*." will appear in the
echo area. The function buffer-name returns the name of the
buffer as a string, which the message function inserts in place
of %s.
To print a value as an integer, use %d in the same way as
%s. For example, to print a message in the echo area that
states the value of the fill-column, evaluate the following:
(message "The value of fill-column is %d." fill-column)
On my system, when I evaluate this list, "The value of
fill-column is 72." appears in my echo area2.
If there is more than one %s in the quoted string, the value of
the first argument following the quoted string is printed at the
location of the first %s and the value of the second argument is
printed at the location of the second %s, and so on.
For example, if you evaluate the following,
(message "There are %d %s in the office!"
(- fill-column 14) "pink elephants")
a rather whimsical message will appear in your echo area. On my system
it says, "There are 58 pink elephants in the office!".
The expression (- fill-column 14) is evaluated and the resulting
number is inserted in place of the %d; and the string in double
quotes, "pink elephants", is treated as a single argument and
inserted in place of the %s. (That is to say, a string between
double quotes evaluates to itself, like a number.)
Finally, here is a somewhat complex example that not only illustrates
the computation of a number, but also shows how you can use an
expression within an expression to generate the text that is substituted
for %s:
(message "He saw %d %s"
(- fill-column 34)
(concat "red "
(substring
"The quick brown foxes jumped." 16 21)
" leaping."))
In this example, message has three arguments: the string,
"He saw %d %s", the expression, (- fill-column 32), and
the expression beginning with the function concat. The value
resulting from the evaluation of (- fill-column 32) is inserted
in place of the %d; and the value returned by the expression
beginning with concat is inserted in place of the %s.
When I evaluate the expression, the message "He saw 38 red
foxes leaping." appears in my echo area.
There are several ways by which a variable can be given a value. One of
the ways is to use either the function set or the function
setq. Another way is to use let (see let). (The
jargon for this process is to bind a variable to a value.)
The following sections not only describe how set and setq
work but also illustrate how arguments are passed.
setq to count.
setTo set the value of the symbol flowers to the list '(rose
violet daisy buttercup), evaluate the following expression by
positioning the cursor after the expression and typing C-x C-e.
(set 'flowers '(rose violet daisy buttercup))
The list (rose violet daisy buttercup) will appear in the echo
area. This is what is returned by the set function. As a
side effect, the symbol flowers is bound to the list ; that is,
the symbol flowers, which can be viewed as a variable, is given
the list as its value. (This process, by the way, illustrates how a
side effect to the Lisp interpreter, setting the value, can be the
primary effect that we humans are interested in. This is because every
Lisp function must return a value if it does not get an error, but it
will only have a side effect if it is designed to have one.)
After evaluating the set expression, you can evaluate the symbol
flowers and it will return the value you just set. Here is the
symbol. Place your cursor after it and type C-x C-e.
flowers
When you evaluate flowers, the list
(rose violet daisy buttercup) appears in the echo area.
Incidentally, if you evaluate 'flowers, the variable with a quote
in front of it, what you will see in the echo area is the symbol itself,
flowers. Here is the quoted symbol, so you can try this:
'flowers
Note also, that when you use set, you need to quote both
arguments to set, unless you want them evaluated. Since we do
not want either argument evaluated, neither the variable
flowers nor the list (rose violet daisy buttercup), both
are quoted. (When you use set without quoting its first
argument, the first argument is evaluated before anything else is
done. If you did this and flowers did not have a value
already, you would get an error message that the Symbol's value
as variable is void; on the other hand, if flowers did return
a value after it was evaluated, the set would attempt to set
the value that was returned. There are situations where this is the
right thing for the function to do; but such situations are rare.)
setqAs a practical matter, you almost always quote the first argument to
set. The combination of set and a quoted first argument
is so common that it has its own name: the special form setq.
This special form is just like set except that the first argument
is quoted automatically, so you don't need to type the quote mark
yourself. Also, as an added convenience, setq permits you to set
several different variables to different values, all in one expression.
To set the value of the variable carnivores to the list
'(lion tiger leopard) using setq, the following expression
is used:
(setq carnivores '(lion tiger leopard))
This is exactly the same as using set except the first argument
is automatically quoted by setq. (The q in setq
means quote.)
With set, the expression would look like this:
(set 'carnivores '(lion tiger leopard))
Also, setq can be used to assign different values to
different variables. The first argument is bound to the value
of the second argument, the third argument is bound to the value of the
fourth argument, and so on. For example, you could use the following to
assign a list of trees to the symbol trees and a list of herbivores
to the symbol herbivores:
(setq trees '(pine fir oak maple)
herbivores '(gazelle antelope zebra))
(The expression could just as well have been on one line, but it might not have fit on a page; and humans find it easier to read nicely formatted lists.)
Although I have been using the term `assign', there is another way of
thinking about the workings of set and setq; and that is to
say that set and setq make the symbol point to the
list. This latter way of thinking is very common and in forthcoming
chapters we shall come upon at least one symbol that has `pointer' as
part of its name. The name is chosen because the symbol has a value,
specifically a list, attached to it; or, expressed another way,
the symbol is set to "point" to the list.
Here is an example that shows how to use setq in a counter. You
might use this to count how many times a part of your program repeats
itself. First set a variable to zero; then add one to the number each
time the program repeats itself. To do this, you need a variable that
serves as a counter, and two expressions: an initial setq
expression that sets the counter variable to zero; and a second
setq expression that increments the counter each time it is
evaluated.
(setq counter 0) ; Let's call this the initializer. (setq counter (+ counter 1)) ; This is the incrementer. counter ; This is the counter.
(The text following the ; are comments. See Change a Function Definition.)
If you evaluate the first of these expressions, the initializer,
(setq counter 0), and then evaluate the third expression,
counter, the number 0 will appear in the echo area. If
you then evaluate the second expression, the incrementer, (setq
counter (+ counter 1)), the counter will get the value 1. So if you
again evaluate counter, the number 1 will appear in the
echo area. Each time you evaluate the second expression, the value of
the counter will be incremented.
When you evaluate the incrementer, (setq counter (+ counter 1)),
the Lisp interpreter first evaluates the innermost list; this is the
addition. In order to evaluate this list, it must evaluate the variable
counter and the number 1. When it evaluates the variable
counter, it receives its current value. It passes this value and
the number 1 to the + which adds them together. The sum
is then returned as the value of the inner list and passed to the
setq which sets the variable counter to this new value.
Thus, the value of the variable, counter, is changed.
Learning Lisp is like climbing a hill in which the first part is the steepest. You have now climbed the most difficult part; what remains becomes easier as you progress onwards.
In summary,
forward-paragraph, single
character symbols like +, strings of characters between double
quotation marks, or numbers.
', tells the Lisp interpreter that it should
return the following expression as written, and not evaluate it as it
would if the quote were not there.
A few simple exercises:
Before learning how to write a function definition in Emacs Lisp, it is useful to spend a little time evaluating various expressions that have already been written. These expressions will be lists with the functions as their first (and often only) element. Since some of the functions associated with buffers are both simple and interesting, we will start with those. In this section, we will evaluate a few of these. In another section, we will study the code of several other buffer-related functions, to see how they were written.
Whenever you give an editing command to Emacs Lisp, such as the command to move the cursor or to scroll the screen, you are evaluating an expression, the first element of which is a function. This is how Emacs works.
When you type keys, you cause the Lisp interpreter to evaluate an
expression and that is how you get your results. Even typing plain text
involves evaluating an Emacs Lisp function, in this case, one that uses
self-insert-command, which simply inserts the character you
typed. The functions you evaluate by typing keystrokes are called
interactive functions, or commands; how you make a function
interactive will be illustrated in the chapter on how to write function
definitions. See Making a Function Interactive.
In addition to typing keyboard commands, we have seen a second way to evaluate an expression: by positioning the cursor after a list and typing C-x C-e. This is what we will do in the rest of this section. There are other ways to evaluate an expression as well; these will be described as we come to them.
Besides being used for practicing evaluation, the functions shown in the next few sections are important in their own right. A study of these functions makes clear the distinction between buffers and files, how to switch to a buffer, and how to determine a location within it.
The two functions, buffer-name and buffer-file-name, show
the difference between a file and a buffer. When you evaluate the
following expression, (buffer-name), the name of the buffer
appears in the echo area. When you evaluate (buffer-file-name),
the name of the file to which the buffer refers appears in the echo
area. Usually, the name returned by (buffer-name) is the same as
the name of the file to which it refers, and the name returned by
(buffer-file-name) is the full path-name of the file.
A file and a buffer are two different entities. A file is information recorded permanently in the computer (unless you delete it). A buffer, on the other hand, is information inside of Emacs that will vanish at the end of the editing session (or when you kill the buffer). Usually, a buffer contains information that you have copied from a file; we say the buffer is visiting that file. This copy is what you work on and modify. Changes to the buffer do not change the file, until you save the buffer. When you save the buffer, the buffer is copied to the file and is thus saved permanently.
If you are reading this in Info inside of GNU Emacs, you can evaluate each of the following expressions by positioning the cursor after it and typing C-x C-e.
(buffer-name) (buffer-file-name)
When I do this, "introduction.texinfo" is the value returned by
evaluating (buffer-name), and
"/gnu/work/intro/introduction.texinfo" is the value returned by
evaluating (buffer-file-name). The former is the name of the
buffer and the latter is the name of the file. (In the expressions, the
parentheses tell the Lisp interpreter to treat buffer-name and
buffer-file-name as functions; without the parentheses, the
interpreter would attempt to evaluate the symbols as variables.
See Variables.)
In spite of the distinction between files and buffers, you will often find that people refer to a file when they mean a buffer and vice-versa. Indeed, most people say, "I am editing a file," rather than saying, "I am editing a buffer which I will soon save to a file." It is almost always clear from context what people mean. When dealing with computer programs, however, it is important to keep the distinction in mind, since the computer is not as smart as a person.
The word `buffer', by the way, comes from the meaning of the word as a cushion that deadens the force of a collision. In early computers, a buffer cushioned the interaction between files and the computer's central processing unit. The drums or tapes that held a file and the central processing unit were pieces of equipment that were very different from each other, working at their own speeds, in spurts. The buffer made it possible for them to work together effectively. Eventually, the buffer grew from being an intermediary, a temporary holding place, to being the place where work is done. This transformation is rather like that of a small seaport that grew into a great city: once it was merely the place where cargo was warehoused temporarily before being loaded onto ships; then it became a business and cultural center in its own right.
Not all buffers are associated with files. For example, when you start
an Emacs session by typing the command emacs alone, without
naming any files, Emacs will start with the *scratch* buffer on
the screen. This buffer is not visiting any file. Similarly, a
*Help* buffer is not associated with any file.
If you switch to the *scratch* buffer, type (buffer-name),
position the cursor after it, and type C-x C-e to evaluate the
expression, the name "*scratch*" is returned and will appear in
the echo area. "*scratch*" is the name of the buffer. However,
if you type (buffer-file-name) in the *scratch* buffer and
evaluate that, nil will appear in the echo area. nil is
from the Latin word for `nothing'; in this case, it means that the
*scratch* buffer is not associated with any file. (In Lisp,
nil is also used to mean `false' and is a synonym for the empty
list, ().)
Incidentally, if you are in the *scratch* buffer and want the
value returned by an expression to appear in the *scratch*
buffer itself rather than in the echo area, type C-u C-x C-e
instead of C-x C-e. This causes the value returned to appear
after the expression. The buffer will look like this:
(buffer-name)"*scratch*"
You cannot do this in Info since Info is read-only and it will not allow you to change the contents of the buffer. But you can do this in any buffer you can edit; and when you write code or documentation (such as this book), this feature is very useful.
The buffer-name function returns the name of the buffer;
to get the buffer itself, a different function is needed: the
current-buffer function. If you use this function in code, what
you get is the buffer itself.
A name and the object or entity to which the name refers are different
from each other. You are not your name. You are a person to whom
others refer by name. If you ask to speak to George and someone hands you
a card with the letters G, e, o, r,
g, and e written on it, you might be amused, but you would
not be satisfied. You do not want to speak to the name, but to the
person to whom the name refers. A buffer is similar: the name of the
scratch buffer is *scratch*, but the name is not the buffer. To
get a buffer itself, you need to use a function such as
current-buffer.
However, there is a slight complication: if you evaluate
current-buffer in an expression on its own, as we will do here,
what you see is a printed representation of the name of the buffer
without the contents of the buffer. Emacs works this way for two
reasons: the buffer may be thousands of lines long--too long to be
conveniently displayed; and, another buffer may have the same contents
but a different name, and it is important to distinguish between them.
Here is an expression containing the function:
(current-buffer)
If you evaluate the expression in the usual way, #<buffer *info*>
appears in the echo area. The special format indicates that the
buffer itself is being returned, rather than just its name.
Incidentally, while you can type a number or symbol into a program, you
cannot do that with the printed representation of a buffer: the only way
to get a buffer itself is with a function such as current-buffer.
A related function is other-buffer. This returns the most
recently selected buffer other than the one you are in currently. If
you have recently switched back and forth from the *scratch*
buffer, other-buffer will return that buffer.
You can see this by evaluating the expression:
(other-buffer)
You should see #<buffer *scratch*> appear in the echo area, or
the name of whatever other buffer you switched back from most
recently3.
The other-buffer function actually provides a buffer when it is
used as an argument to a function that requires one. We can see this
by using other-buffer and switch-to-buffer to switch to a
different buffer.
But first, a brief introduction to the switch-to-buffer
function. When you switched back and forth from Info to the
*scratch* buffer to evaluate (buffer-name), you most
likely typed C-x b and then typed *scratch*4 when
prompted in the minibuffer for the name of the buffer to which you
wanted to switch. The keystrokes, C-x b, cause the Lisp
interpreter to evaluate the interactive function
switch-to-buffer. As we said before, this is how Emacs works:
different keystrokes call or run different functions. For example,
C-f calls forward-char, M-e calls
forward-sentence, and so on.
By writing switch-to-buffer in an expression, and giving it a
buffer to switch to, we can switch buffers just the way C-x b
does.
Here is the Lisp expression:
(switch-to-buffer (other-buffer))
The symbol switch-to-buffer is the first element of the list,
so the Lisp interpreter will treat it as a function and carry out the
instructions that are attached to it. But before doing that, the
interpreter will note that other-buffer is inside parentheses
and work on that symbol first. other-buffer is the first (and
in this case, the only) element of this list, so the Lisp interpreter
calls or runs the function. It returns another buffer. Next, the
interpreter runs switch-to-buffer, passing to it, as an
argument, the other buffer, which is what Emacs will switch to. If
you are reading this in Info, try this now. Evaluate the expression.
(To get back, type C-x b <RET>.)5
In the programming examples in later sections of this document, you will
see the function set-buffer more often than
switch-to-buffer. This is because of a difference between
computer programs and humans: humans have eyes and expect to see the
buffer on which they are working on their computer terminals. This is
so obvious, it almost goes without saying. However, programs do not
have eyes. When a computer program works on a buffer, that buffer does
not need to be visible on the screen.
switch-to-buffer is designed for humans and does two different
things: it switches the buffer to which Emacs' attention is directed; and
it switches the buffer displayed in the window to the new buffer.
set-buffer, on the other hand, does only one thing: it switches
the attention of the computer program to a different buffer. The buffer
on the screen remains unchanged (of course, normally nothing happens
there until the command finishes running).
Also, we have just introduced another jargon term, the word call. When you evaluate a list in which the first symbol is a function, you are calling that function. The use of the term comes from the notion of the function as an entity that can do something for you if you `call' it--just as a plumber is an entity who can fix a leak if you call him or her.
Finally, let's look at several rather simple functions,
buffer-size, point, point-min, and
point-max. These give information about the size of a buffer and
the location of point within it.
The function buffer-size tells you the size of the current
buffer; that is, the function returns a count of the number of
characters in the buffer.
(buffer-size)
You can evaluate this in the usual way, by positioning the cursor after the expression and typing C-x C-e.
In Emacs, the current position of the cursor is called point.
The expression (point) returns a number that tells you where the
cursor is located as a count of the number of characters from the
beginning of the buffer up to point.
You can see the character count for point in this buffer by evaluating the following expression in the usual way:
(point)
As I write this, the value of point is 65724. The point
function is frequently used in some of the examples later in this
book.
The value of point depends, of course, on its location within the buffer. If you evaluate point in this spot, the number will be larger:
(point)
For me, the value of point in this location is 66043, which means that there are 319 characters (including spaces) between the two expressions.
The function point-min is somewhat similar to point, but
it returns the value of the minimum permissible value of point in the
current buffer. This is the number 1 unless narrowing is in
effect. (Narrowing is a mechanism whereby you can restrict yourself,
or a program, to operations on just a part of a buffer.
See Narrowing and Widening.) Likewise, the
function point-max returns the value of the maximum permissible
value of point in the current buffer.
Find a file with which you are working and move towards its middle. Find its buffer name, file name, length, and your position in the file.
When the Lisp interpreter evaluates a list, it looks to see whether the first symbol on the list has a function definition attached to it; or, put another way, whether the symbol points to a function definition. If it does, the computer carries out the instructions in the definition. A symbol that has a function definition is called, simply, a function (although, properly speaking, the definition is the function and the symbol refers to it.)
defun special form.
interactive.
All functions are defined in terms of other functions, except for a few primitive functions that are written in the C programming language. When you write functions' definitions, you will write them in Emacs Lisp and use other functions as your building blocks. Some of the functions you will use will themselves be written in Emacs Lisp (perhaps by you) and some will be primitives written in C. The primitive functions are used exactly like those written in Emacs Lisp and behave like them. They are written in C so we can easily run GNU Emacs on any computer that has sufficient power and can run C.
Let me re-emphasize this: when you write code in Emacs Lisp, you do not distinguish between the use of functions written in C and the use of functions written in Emacs Lisp. The difference is irrelevant. I mention the distinction only because it is interesting to know. Indeed, unless you investigate, you won't know whether an already-written function is written in Emacs Lisp or C.
defun Special FormIn Lisp, a symbol such as mark-whole-buffer has code attached to
it that tells the computer what to do when the function is called.
This code is called the function definition and is created by
evaluating a Lisp expression that starts with the symbol defun
(which is an abbreviation for define function). Because
defun does not evaluate its arguments in the usual way, it is
called a special form.
In subsequent sections, we will look at function definitions from the
Emacs source code, such as mark-whole-buffer. In this section,
we will describe a simple function definition so you can see how it
looks. This function definition uses arithmetic because it makes for a
simple example. Some people dislike examples using arithmetic; however,
if you are such a person, do not despair. Hardly any of the code we
will study in the remainder of this introduction involves arithmetic or
mathematics. The examples mostly involve text in one way or another.
A function definition has up to five parts following the word
defun:
().
It is helpful to think of the five parts of a function definition as being organized in a template, with slots for each part:
(defun function-name (arguments...) "optional-documentation..." (interactive argument-passing-info)