| 1 | @c -*-texinfo-*- |
| 2 | @c This is part of the GNU Emacs Lisp Reference Manual. |
| 3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. |
| 4 | @c See the file elisp.texi for copying conditions. |
| 5 | @setfilename ../info/objects |
| 6 | @node Types of Lisp Object, Numbers, Introduction, Top |
| 7 | @chapter Lisp Data Types |
| 8 | @cindex object |
| 9 | @cindex Lisp object |
| 10 | @cindex type |
| 11 | @cindex data type |
| 12 | |
| 13 | A Lisp @dfn{object} is a piece of data used and manipulated by Lisp |
| 14 | programs. For our purposes, a @dfn{type} or @dfn{data type} is a set of |
| 15 | possible objects. |
| 16 | |
| 17 | Every object belongs to at least one type. Objects of the same type |
| 18 | have similar structures and may usually be used in the same contexts. |
| 19 | Types can overlap, and objects can belong to two or more types. |
| 20 | Consequently, we can ask whether an object belongs to a particular type, |
| 21 | but not for ``the'' type of an object. |
| 22 | |
| 23 | @cindex primitive type |
| 24 | A few fundamental object types are built into Emacs. These, from |
| 25 | which all other types are constructed, are called @dfn{primitive |
| 26 | types}. Each object belongs to one and only one primitive type. These |
| 27 | types include @dfn{integer}, @dfn{float}, @dfn{cons}, @dfn{symbol}, |
| 28 | @dfn{string}, @dfn{vector}, @dfn{subr}, @dfn{byte-code function}, and |
| 29 | several special types, such as @dfn{buffer}, that are related to |
| 30 | editing. (@xref{Editing Types}.) |
| 31 | |
| 32 | Each primitive type has a corresponding Lisp function that checks |
| 33 | whether an object is a member of that type. |
| 34 | |
| 35 | Note that Lisp is unlike many other languages in that Lisp objects are |
| 36 | @dfn{self-typing}: the primitive type of the object is implicit in the |
| 37 | object itself. For example, if an object is a vector, nothing can treat |
| 38 | it as a number; Lisp knows it is a vector, not a number. |
| 39 | |
| 40 | In most languages, the programmer must declare the data type of each |
| 41 | variable, and the type is known by the compiler but not represented in |
| 42 | the data. Such type declarations do not exist in Emacs Lisp. A Lisp |
| 43 | variable can have any type of value, and remembers the type of any value |
| 44 | you store in it. |
| 45 | |
| 46 | This chapter describes the purpose, printed representation, and read |
| 47 | syntax of each of the standard types in GNU Emacs Lisp. Details on how |
| 48 | to use these types can be found in later chapters. |
| 49 | |
| 50 | @menu |
| 51 | * Printed Representation:: How Lisp objects are represented as text. |
| 52 | * Comments:: Comments and their formatting conventions. |
| 53 | * Programming Types:: Types found in all Lisp systems. |
| 54 | * Editing Types:: Types specific to Emacs. |
| 55 | * Type Predicates:: Tests related to types. |
| 56 | * Equality Predicates:: Tests of equality between any two objects. |
| 57 | @end menu |
| 58 | |
| 59 | @node Printed Representation |
| 60 | @comment node-name, next, previous, up |
| 61 | @section Printed Representation and Read Syntax |
| 62 | @cindex printed representation |
| 63 | @cindex read syntax |
| 64 | |
| 65 | The @dfn{printed representation} of an object is the format of the |
| 66 | output generated by the Lisp printer (the function @code{prin1}) for |
| 67 | that object. The @dfn{read syntax} of an object is the format of the |
| 68 | input accepted by the Lisp reader (the function @code{read}) for that |
| 69 | object. Most objects have more than one possible read syntax. Some |
| 70 | types of object have no read syntax; except for these cases, the printed |
| 71 | representation of an object is also a read syntax for it. |
| 72 | |
| 73 | In other languages, an expression is text; it has no other form. In |
| 74 | Lisp, an expression is primarily a Lisp object and only secondarily the |
| 75 | text that is the object's read syntax. Often there is no need to |
| 76 | emphasize this distinction, but you must keep it in the back of your |
| 77 | mind, or you will occasionally be very confused. |
| 78 | |
| 79 | @cindex hash notation |
| 80 | Every type has a printed representation. Some types have no read |
| 81 | syntax, since it may not make sense to enter objects of these types |
| 82 | directly in a Lisp program. For example, the buffer type does not have |
| 83 | a read syntax. Objects of these types are printed in @dfn{hash |
| 84 | notation}: the characters @samp{#<} followed by a descriptive string |
| 85 | (typically the type name followed by the name of the object), and closed |
| 86 | with a matching @samp{>}. Hash notation cannot be read at all, so the |
| 87 | Lisp reader signals the error @code{invalid-read-syntax} whenever it |
| 88 | encounters @samp{#<}. |
| 89 | @kindex invalid-read-syntax |
| 90 | |
| 91 | @example |
| 92 | (current-buffer) |
| 93 | @result{} #<buffer objects.texi> |
| 94 | @end example |
| 95 | |
| 96 | When you evaluate an expression interactively, the Lisp interpreter |
| 97 | first reads the textual representation of it, producing a Lisp object, |
| 98 | and then evaluates that object (@pxref{Evaluation}). However, |
| 99 | evaluation and reading are separate activities. Reading returns the |
| 100 | Lisp object represented by the text that is read; the object may or may |
| 101 | not be evaluated later. @xref{Input Functions}, for a description of |
| 102 | @code{read}, the basic function for reading objects. |
| 103 | |
| 104 | @node Comments |
| 105 | @comment node-name, next, previous, up |
| 106 | @section Comments |
| 107 | @cindex comments |
| 108 | @cindex @samp{;} in comment |
| 109 | |
| 110 | A @dfn{comment} is text that is written in a program only for the sake |
| 111 | of humans that read the program, and that has no effect on the meaning |
| 112 | of the program. In Lisp, a semicolon (@samp{;}) starts a comment if it |
| 113 | is not within a string or character constant. The comment continues to |
| 114 | the end of line. The Lisp reader discards comments; they do not become |
| 115 | part of the Lisp objects which represent the program within the Lisp |
| 116 | system. |
| 117 | |
| 118 | @xref{Comment Tips}, for conventions for formatting comments. |
| 119 | |
| 120 | @node Programming Types |
| 121 | @section Programming Types |
| 122 | @cindex programming types |
| 123 | |
| 124 | There are two general categories of types in Emacs Lisp: those having |
| 125 | to do with Lisp programming, and those having to do with editing. The |
| 126 | former exist in many Lisp implementations, in one form or another. The |
| 127 | latter are unique to Emacs Lisp. |
| 128 | |
| 129 | @menu |
| 130 | * Integer Type:: Numbers without fractional parts. |
| 131 | * Floating Point Type:: Numbers with fractional parts and with a large range. |
| 132 | * Character Type:: The representation of letters, numbers and |
| 133 | control characters. |
| 134 | * Sequence Type:: Both lists and arrays are classified as sequences. |
| 135 | * List Type:: Lists gave Lisp its name (not to mention reputation). |
| 136 | * Array Type:: Arrays include strings and vectors. |
| 137 | * String Type:: An (efficient) array of characters. |
| 138 | * Vector Type:: One-dimensional arrays. |
| 139 | * Symbol Type:: A multi-use object that refers to a function, |
| 140 | variable, property list, or itself. |
| 141 | * Lisp Function Type:: A piece of executable code you can call from elsewhere. |
| 142 | * Lisp Macro Type:: A method of expanding an expression into another |
| 143 | expression, more fundamental but less pretty. |
| 144 | * Primitive Function Type:: A function written in C, callable from Lisp. |
| 145 | * Byte-Code Type:: A function written in Lisp, then compiled. |
| 146 | * Autoload Type:: A type used for automatically loading seldom-used |
| 147 | functions. |
| 148 | @end menu |
| 149 | |
| 150 | @node Integer Type |
| 151 | @subsection Integer Type |
| 152 | |
| 153 | Integers were the only kind of number in Emacs version 18. The range |
| 154 | of values for integers is @minus{}8388608 to 8388607 (24 bits; i.e., |
| 155 | @ifinfo |
| 156 | -2**23 |
| 157 | @end ifinfo |
| 158 | @tex |
| 159 | $-2^{23}$ |
| 160 | @end tex |
| 161 | to |
| 162 | @ifinfo |
| 163 | 2**23 - 1) |
| 164 | @end ifinfo |
| 165 | @tex |
| 166 | $2^{23}-1$) |
| 167 | @end tex |
| 168 | on most machines, but is 25 or 26 bits on some systems. It is important |
| 169 | to note that the Emacs Lisp arithmetic functions do not check for |
| 170 | overflow. Thus @code{(1+ 8388607)} is @minus{}8388608 on 24-bit |
| 171 | implementations.@refill |
| 172 | |
| 173 | The read syntax for numbers is a sequence of (base ten) digits with an |
| 174 | optional sign at the beginning and an optional period at the end. The |
| 175 | printed representation produced by the Lisp interpreter never has a |
| 176 | leading @samp{+} or a final @samp{.}. |
| 177 | |
| 178 | @example |
| 179 | @group |
| 180 | -1 ; @r{The integer -1.} |
| 181 | 1 ; @r{The integer 1.} |
| 182 | 1. ; @r{Also The integer 1.} |
| 183 | +1 ; @r{Also the integer 1.} |
| 184 | 16777217 ; @r{Also the integer 1!} |
| 185 | ; @r{ (on a 24-bit or 25-bit implementation)} |
| 186 | @end group |
| 187 | @end example |
| 188 | |
| 189 | @xref{Numbers}, for more information. |
| 190 | |
| 191 | @node Floating Point Type |
| 192 | @subsection Floating Point Type |
| 193 | |
| 194 | Emacs version 19 supports floating point numbers (though there is a |
| 195 | compilation option to disable them). The precise range of floating |
| 196 | point numbers is machine-specific. |
| 197 | |
| 198 | The printed representation for floating point numbers requires either |
| 199 | a decimal point (with at least one digit following), an exponent, or |
| 200 | both. For example, @samp{1500.0}, @samp{15e2}, @samp{15.0e2}, |
| 201 | @samp{1.5e3}, and @samp{.15e4} are five ways of writing a floating point |
| 202 | number whose value is 1500. They are all equivalent. |
| 203 | |
| 204 | @xref{Numbers}, for more information. |
| 205 | |
| 206 | @node Character Type |
| 207 | @subsection Character Type |
| 208 | @cindex @sc{ASCII} character codes |
| 209 | |
| 210 | A @dfn{character} in Emacs Lisp is nothing more than an integer. In |
| 211 | other words, characters are represented by their character codes. For |
| 212 | example, the character @kbd{A} is represented as the @w{integer 65}. |
| 213 | |
| 214 | Individual characters are not often used in programs. It is far more |
| 215 | common to work with @emph{strings}, which are sequences composed of |
| 216 | characters. @xref{String Type}. |
| 217 | |
| 218 | Characters in strings, buffers, and files are currently limited to the |
| 219 | range of 0 to 255---eight bits. If you store a larger integer into a |
| 220 | string, buffer or file, it is truncated to that range. Characters that |
| 221 | represent keyboard input have a much wider range. |
| 222 | |
| 223 | @cindex read syntax for characters |
| 224 | @cindex printed representation for characters |
| 225 | @cindex syntax for characters |
| 226 | Since characters are really integers, the printed representation of a |
| 227 | character is a decimal number. This is also a possible read syntax for |
| 228 | a character, but writing characters that way in Lisp programs is a very |
| 229 | bad idea. You should @emph{always} use the special read syntax formats |
| 230 | that Emacs Lisp provides for characters. These syntax formats start |
| 231 | with a question mark. |
| 232 | |
| 233 | The usual read syntax for alphanumeric characters is a question mark |
| 234 | followed by the character; thus, @samp{?A} for the character |
| 235 | @kbd{A}, @samp{?B} for the character @kbd{B}, and @samp{?a} for the |
| 236 | character @kbd{a}. |
| 237 | |
| 238 | For example: |
| 239 | |
| 240 | @example |
| 241 | ?Q @result{} 81 ?q @result{} 113 |
| 242 | @end example |
| 243 | |
| 244 | You can use the same syntax for punctuation characters, but it is |
| 245 | often a good idea to add a @samp{\} to prevent Lisp mode from getting |
| 246 | confused. For example, @samp{?\ } is the way to write the space |
| 247 | character. If the character is @samp{\}, you @emph{must} use a second |
| 248 | @samp{\} to quote it: @samp{?\\}. |
| 249 | |
| 250 | @cindex whitespace |
| 251 | @cindex bell character |
| 252 | @cindex @samp{\a} |
| 253 | @cindex backspace |
| 254 | @cindex @samp{\b} |
| 255 | @cindex tab |
| 256 | @cindex @samp{\t} |
| 257 | @cindex vertical tab |
| 258 | @cindex @samp{\v} |
| 259 | @cindex formfeed |
| 260 | @cindex @samp{\f} |
| 261 | @cindex newline |
| 262 | @cindex @samp{\n} |
| 263 | @cindex return |
| 264 | @cindex @samp{\r} |
| 265 | @cindex escape |
| 266 | @cindex @samp{\e} |
| 267 | You can express the characters Control-g, backspace, tab, newline, |
| 268 | vertical tab, formfeed, return, and escape as @samp{?\a}, @samp{?\b}, |
| 269 | @samp{?\t}, @samp{?\n}, @samp{?\v}, @samp{?\f}, @samp{?\r}, @samp{?\e}, |
| 270 | respectively. Those values are 7, 8, 9, 10, 11, 12, 13, and 27 in |
| 271 | decimal. Thus, |
| 272 | |
| 273 | @example |
| 274 | ?\a @result{} 7 ; @r{@kbd{C-g}} |
| 275 | ?\b @result{} 8 ; @r{backspace, @key{BS}, @kbd{C-h}} |
| 276 | ?\t @result{} 9 ; @r{tab, @key{TAB}, @kbd{C-i}} |
| 277 | ?\n @result{} 10 ; @r{newline, @key{LFD}, @kbd{C-j}} |
| 278 | ?\v @result{} 11 ; @r{vertical tab, @kbd{C-k}} |
| 279 | ?\f @result{} 12 ; @r{formfeed character, @kbd{C-l}} |
| 280 | ?\r @result{} 13 ; @r{carriage return, @key{RET}, @kbd{C-m}} |
| 281 | ?\e @result{} 27 ; @r{escape character, @key{ESC}, @kbd{C-[}} |
| 282 | ?\\ @result{} 92 ; @r{backslash character, @kbd{\}} |
| 283 | @end example |
| 284 | |
| 285 | @cindex escape sequence |
| 286 | These sequences which start with backslash are also known as |
| 287 | @dfn{escape sequences}, because backslash plays the role of an escape |
| 288 | character; this usage has nothing to do with the character @key{ESC}. |
| 289 | |
| 290 | @cindex control characters |
| 291 | Control characters may be represented using yet another read syntax. |
| 292 | This consists of a question mark followed by a backslash, caret, and the |
| 293 | corresponding non-control character, in either upper or lower case. For |
| 294 | example, both @samp{?\^I} and @samp{?\^i} are valid read syntax for the |
| 295 | character @kbd{C-i}, the character whose value is 9. |
| 296 | |
| 297 | Instead of the @samp{^}, you can use @samp{C-}; thus, @samp{?\C-i} is |
| 298 | equivalent to @samp{?\^I} and to @samp{?\^i}: |
| 299 | |
| 300 | @example |
| 301 | ?\^I @result{} 9 ?\C-I @result{} 9 |
| 302 | @end example |
| 303 | |
| 304 | For use in strings and buffers, you are limited to the control |
| 305 | characters that exist in @sc{ASCII}, but for keyboard input purposes, |
| 306 | you can turn any character into a control character with @samp{C-}. The |
| 307 | character codes for these non-@sc{ASCII} control characters include the |
| 308 | 2**22 bit as well as the code for the corresponding non-control |
| 309 | character. Ordinary terminals have no way of generating non-@sc{ASCII} |
| 310 | control characters, but you can generate them straightforwardly using an |
| 311 | X terminal. |
| 312 | |
| 313 | You can think of the @key{DEL} character as @kbd{Control-?}: |
| 314 | |
| 315 | @example |
| 316 | ?\^? @result{} 127 ?\C-? @result{} 127 |
| 317 | @end example |
| 318 | |
| 319 | For representing control characters to be found in files or strings, |
| 320 | we recommend the @samp{^} syntax; for control characters in keyboard |
| 321 | input, we prefer the @samp{C-} syntax. This does not affect the meaning |
| 322 | of the program, but may guide the understanding of people who read it. |
| 323 | |
| 324 | @cindex meta characters |
| 325 | A @dfn{meta character} is a character typed with the @key{META} |
| 326 | modifier key. The integer that represents such a character has the |
| 327 | 2**23 bit set (which on most machines makes it a negative number). We |
| 328 | use high bits for this and other modifiers to make possible a wide range |
| 329 | of basic character codes. |
| 330 | |
| 331 | In a string, the 2**7 bit indicates a meta character, so the meta |
| 332 | characters that can fit in a string have codes in the range from 128 to |
| 333 | 255, and are the meta versions of the ordinary @sc{ASCII} characters. |
| 334 | (In Emacs versions 18 and older, this convention was used for characters |
| 335 | outside of strings as well.) |
| 336 | |
| 337 | The read syntax for meta characters uses @samp{\M-}. For example, |
| 338 | @samp{?\M-A} stands for @kbd{M-A}. You can use @samp{\M-} together with |
| 339 | octal codes, @samp{\C-}, or any other syntax for a character. Thus, you |
| 340 | can write @kbd{M-A} as @samp{?\M-A}, or as @samp{?\M-\101}. Likewise, |
| 341 | you can write @kbd{C-M-b} as @samp{?\M-\C-b}, @samp{?\C-\M-b}, or |
| 342 | @samp{?\M-\002}. |
| 343 | |
| 344 | The case of an ordinary letter is indicated by its character code as |
| 345 | part of @sc{ASCII}, but @sc{ASCII} has no way to represent whether a |
| 346 | control character is upper case or lower case. Emacs uses the 2**21 bit |
| 347 | to indicate that the shift key was used for typing a control character. |
| 348 | This distinction is possible only when you use X terminals or other |
| 349 | special terminals; ordinary terminals do not indicate the distinction to |
| 350 | the computer in any way. |
| 351 | |
| 352 | @cindex hyper characters |
| 353 | @cindex super characters |
| 354 | @cindex alt characters |
| 355 | The X Window System defines three other modifier bits that can be set |
| 356 | in a character: @dfn{hyper}, @dfn{super} and @dfn{alt}. The syntaxes |
| 357 | for these bits are @samp{\H-}, @samp{\s-} and @samp{\A-}. Thus, |
| 358 | @samp{?\H-\M-\A-x} represents @kbd{Alt-Hyper-Meta-x}. Numerically, the |
| 359 | bit values are 2**18 for alt, 2**19 for super and 2**20 for hyper. |
| 360 | |
| 361 | @cindex @samp{?} in character constant |
| 362 | @cindex question mark in character constant |
| 363 | @cindex @samp{\} in character constant |
| 364 | @cindex backslash in character constant |
| 365 | @cindex octal character code |
| 366 | Finally, the most general read syntax consists of a question mark |
| 367 | followed by a backslash and the character code in octal (up to three |
| 368 | octal digits); thus, @samp{?\101} for the character @kbd{A}, |
| 369 | @samp{?\001} for the character @kbd{C-a}, and @code{?\002} for the |
| 370 | character @kbd{C-b}. Although this syntax can represent any @sc{ASCII} |
| 371 | character, it is preferred only when the precise octal value is more |
| 372 | important than the @sc{ASCII} representation. |
| 373 | |
| 374 | @example |
| 375 | @group |
| 376 | ?\012 @result{} 10 ?\n @result{} 10 ?\C-j @result{} 10 |
| 377 | ?\101 @result{} 65 ?A @result{} 65 |
| 378 | @end group |
| 379 | @end example |
| 380 | |
| 381 | A backslash is allowed, and harmless, preceding any character without |
| 382 | a special escape meaning; thus, @samp{?\+} is equivalent to @samp{?+}. |
| 383 | There is no reason to add a backslash before most characters. However, |
| 384 | you should add a backslash before any of the characters |
| 385 | @samp{()\|;'`"#.,} to avoid confusing the Emacs commands for editing |
| 386 | Lisp code. Also add a backslash before whitespace characters such as |
| 387 | space, tab, newline and formfeed. However, it is cleaner to use one of |
| 388 | the easily readable escape sequences, such as @samp{\t}, instead of an |
| 389 | actual whitespace character such as a tab. |
| 390 | |
| 391 | @node Sequence Type |
| 392 | @subsection Sequence Types |
| 393 | |
| 394 | A @dfn{sequence} is a Lisp object that represents an ordered set of |
| 395 | elements. There are two kinds of sequence in Emacs Lisp, lists and |
| 396 | arrays. Thus, an object of type list or of type array is also |
| 397 | considered a sequence. |
| 398 | |
| 399 | Arrays are further subdivided into strings and vectors. Vectors can |
| 400 | hold elements of any type, but string elements must be characters in the |
| 401 | range from 0 to 255. However, the characters in a string can have text |
| 402 | properties; vectors do not support text properties even when their |
| 403 | elements happen to be characters. |
| 404 | |
| 405 | Lists, strings and vectors are different, but they have important |
| 406 | similarities. For example, all have a length @var{l}, and all have |
| 407 | elements which can be indexed from zero to @var{l} minus one. Also, |
| 408 | several functions, called sequence functions, accept any kind of |
| 409 | sequence. For example, the function @code{elt} can be used to extract |
| 410 | an element of a sequence, given its index. @xref{Sequences Arrays |
| 411 | Vectors}. |
| 412 | |
| 413 | It is impossible to read the same sequence twice, since sequences are |
| 414 | always created anew upon reading. If you read the read syntax for a |
| 415 | sequence twice, you get two sequences with equal contents. There is one |
| 416 | exception: the empty list @code{()} always stands for the same object, |
| 417 | @code{nil}. |
| 418 | |
| 419 | @node List Type |
| 420 | @subsection List Type |
| 421 | @cindex address field of register |
| 422 | @cindex decrement field of register |
| 423 | |
| 424 | A @dfn{list} is a series of cons cells, linked together. A @dfn{cons |
| 425 | cell} is an object comprising two pointers named the @sc{car} and the |
| 426 | @sc{cdr}. Each of them can point to any Lisp object, but when the cons |
| 427 | cell is part of a list, the @sc{cdr} points either to another cons cell |
| 428 | or to the empty list. @xref{Lists}, for functions that work on lists. |
| 429 | |
| 430 | The names @sc{car} and @sc{cdr} have only historical meaning now. The |
| 431 | original Lisp implementation ran on an @w{IBM 704} computer which |
| 432 | divided words into two parts, called the ``address'' part and the |
| 433 | ``decrement''; @sc{car} was an instruction to extract the contents of |
| 434 | the address part of a register, and @sc{cdr} an instruction to extract |
| 435 | the contents of the decrement. By contrast, ``cons cells'' are named |
| 436 | for the function @code{cons} that creates them, which in turn is named |
| 437 | for its purpose, the construction of cells. |
| 438 | |
| 439 | @cindex atom |
| 440 | Because cons cells are so central to Lisp, we also have a word for |
| 441 | ``an object which is not a cons cell''. These objects are called |
| 442 | @dfn{atoms}. |
| 443 | |
| 444 | @cindex parenthesis |
| 445 | The read syntax and printed representation for lists are identical, and |
| 446 | consist of a left parenthesis, an arbitrary number of elements, and a |
| 447 | right parenthesis. |
| 448 | |
| 449 | Upon reading, each object inside the parentheses becomes an element |
| 450 | of the list. That is, a cons cell is made for each element. The |
| 451 | @sc{car} of the cons cell points to the element, and its @sc{cdr} points |
| 452 | to the next cons cell which holds the next element in the list. The |
| 453 | @sc{cdr} of the last cons cell is set to point to @code{nil}. |
| 454 | |
| 455 | @cindex box diagrams, for lists |
| 456 | @cindex diagrams, boxed, for lists |
| 457 | A list can be illustrated by a diagram in which the cons cells are |
| 458 | shown as pairs of boxes. (The Lisp reader cannot read such an |
| 459 | illustration; unlike the textual notation, which can be understood both |
| 460 | humans and computers, the box illustrations can only be understood by |
| 461 | humans.) The following represents the three-element list @code{(rose |
| 462 | violet buttercup)}: |
| 463 | |
| 464 | @example |
| 465 | @group |
| 466 | ___ ___ ___ ___ ___ ___ |
| 467 | |___|___|--> |___|___|--> |___|___|--> nil |
| 468 | | | | |
| 469 | | | | |
| 470 | --> rose --> violet --> buttercup |
| 471 | @end group |
| 472 | @end example |
| 473 | |
| 474 | In this diagram, each box represents a slot that can refer to any Lisp |
| 475 | object. Each pair of boxes represents a cons cell. Each arrow is a |
| 476 | reference to a Lisp object, either an atom or another cons cell. |
| 477 | |
| 478 | In this example, the first box, the @sc{car} of the first cons cell, |
| 479 | refers to or ``contains'' @code{rose} (a symbol). The second box, the |
| 480 | @sc{cdr} of the first cons cell, refers to the next pair of boxes, the |
| 481 | second cons cell. The @sc{car} of the second cons cell refers to |
| 482 | @code{violet} and the @sc{cdr} refers to the third cons cell. The |
| 483 | @sc{cdr} of the third (and last) cons cell refers to @code{nil}. |
| 484 | |
| 485 | Here is another diagram of the same list, @code{(rose violet |
| 486 | buttercup)}, sketched in a different manner: |
| 487 | |
| 488 | @smallexample |
| 489 | @group |
| 490 | --------------- ---------------- ------------------- |
| 491 | | car | cdr | | car | cdr | | car | cdr | |
| 492 | | rose | o-------->| violet | o-------->| buttercup | nil | |
| 493 | | | | | | | | | | |
| 494 | --------------- ---------------- ------------------- |
| 495 | @end group |
| 496 | @end smallexample |
| 497 | |
| 498 | @cindex @samp{(@dots{})} in lists |
| 499 | @cindex @code{nil} in lists |
| 500 | @cindex empty list |
| 501 | A list with no elements in it is the @dfn{empty list}; it is identical |
| 502 | to the symbol @code{nil}. In other words, @code{nil} is both a symbol |
| 503 | and a list. |
| 504 | |
| 505 | Here are examples of lists written in Lisp syntax: |
| 506 | |
| 507 | @example |
| 508 | (A 2 "A") ; @r{A list of three elements.} |
| 509 | () ; @r{A list of no elements (the empty list).} |
| 510 | nil ; @r{A list of no elements (the empty list).} |
| 511 | ("A ()") ; @r{A list of one element: the string @code{"A ()"}.} |
| 512 | (A ()) ; @r{A list of two elements: @code{A} and the empty list.} |
| 513 | (A nil) ; @r{Equivalent to the previous.} |
| 514 | ((A B C)) ; @r{A list of one element} |
| 515 | ; @r{(which is a list of three elements).} |
| 516 | @end example |
| 517 | |
| 518 | Here is the list @code{(A ())}, or equivalently @code{(A nil)}, |
| 519 | depicted with boxes and arrows: |
| 520 | |
| 521 | @example |
| 522 | @group |
| 523 | ___ ___ ___ ___ |
| 524 | |___|___|--> |___|___|--> nil |
| 525 | | | |
| 526 | | | |
| 527 | --> A --> nil |
| 528 | @end group |
| 529 | @end example |
| 530 | |
| 531 | @menu |
| 532 | * Dotted Pair Notation:: An alternative syntax for lists. |
| 533 | * Association List Type:: A specially constructed list. |
| 534 | @end menu |
| 535 | |
| 536 | @node Dotted Pair Notation |
| 537 | @comment node-name, next, previous, up |
| 538 | @subsubsection Dotted Pair Notation |
| 539 | @cindex dotted pair notation |
| 540 | @cindex @samp{.} in lists |
| 541 | |
| 542 | @dfn{Dotted pair notation} is an alternative syntax for cons cells |
| 543 | that represents the @sc{car} and @sc{cdr} explicitly. In this syntax, |
| 544 | @code{(@var{a} .@: @var{b})} stands for a cons cell whose @sc{car} is |
| 545 | the object @var{a}, and whose @sc{cdr} is the object @var{b}. Dotted |
| 546 | pair notation is therefore more general than list syntax. In the dotted |
| 547 | pair notation, the list @samp{(1 2 3)} is written as @samp{(1 . (2 . (3 |
| 548 | . nil)))}. For @code{nil}-terminated lists, the two notations produce |
| 549 | the same result, but list notation is usually clearer and more |
| 550 | convenient when it is applicable. When printing a list, the dotted pair |
| 551 | notation is only used if the @sc{cdr} of a cell is not a list. |
| 552 | |
| 553 | Here's how box notation can illustrate dotted pairs. This example |
| 554 | shows the pair @code{(rose . violet)}: |
| 555 | |
| 556 | @example |
| 557 | @group |
| 558 | ___ ___ |
| 559 | |___|___|--> violet |
| 560 | | |
| 561 | | |
| 562 | --> rose |
| 563 | @end group |
| 564 | @end example |
| 565 | |
| 566 | Dotted pair notation can be combined with list notation to represent a |
| 567 | chain of cons cells with a non-@code{nil} final @sc{cdr}. For example, |
| 568 | @code{(rose violet . buttercup)} is equivalent to @code{(rose . (violet |
| 569 | . buttercup))}. The object looks like this: |
| 570 | |
| 571 | @example |
| 572 | @group |
| 573 | ___ ___ ___ ___ |
| 574 | |___|___|--> |___|___|--> buttercup |
| 575 | | | |
| 576 | | | |
| 577 | --> rose --> violet |
| 578 | @end group |
| 579 | @end example |
| 580 | |
| 581 | These diagrams make it evident why @w{@code{(rose .@: violet .@: |
| 582 | buttercup)}} is invalid syntax; it would require a cons cell that has |
| 583 | three parts rather than two. |
| 584 | |
| 585 | The list @code{(rose violet)} is equivalent to @code{(rose . (violet))} |
| 586 | and looks like this: |
| 587 | |
| 588 | @example |
| 589 | @group |
| 590 | ___ ___ ___ ___ |
| 591 | |___|___|--> |___|___|--> nil |
| 592 | | | |
| 593 | | | |
| 594 | --> rose --> violet |
| 595 | @end group |
| 596 | @end example |
| 597 | |
| 598 | Similarly, the three-element list @code{(rose violet buttercup)} |
| 599 | is equivalent to @code{(rose . (violet . (buttercup)))}. |
| 600 | @ifinfo |
| 601 | It looks like this: |
| 602 | |
| 603 | @example |
| 604 | @group |
| 605 | ___ ___ ___ ___ ___ ___ |
| 606 | |___|___|--> |___|___|--> |___|___|--> nil |
| 607 | | | | |
| 608 | | | | |
| 609 | --> rose --> violet --> buttercup |
| 610 | @end group |
| 611 | @end example |
| 612 | @end ifinfo |
| 613 | |
| 614 | @node Association List Type |
| 615 | @comment node-name, next, previous, up |
| 616 | @subsubsection Association List Type |
| 617 | |
| 618 | An @dfn{association list} or @dfn{alist} is a specially-constructed |
| 619 | list whose elements are cons cells. In each element, the @sc{car} is |
| 620 | considered a @dfn{key}, and the @sc{cdr} is considered an |
| 621 | @dfn{associated value}. (In some cases, the associated value is stored |
| 622 | in the @sc{car} of the @sc{cdr}.) Association lists are often used as |
| 623 | stacks, since it is easy to add or remove associations at the front of |
| 624 | the list. |
| 625 | |
| 626 | For example, |
| 627 | |
| 628 | @example |
| 629 | (setq alist-of-colors |
| 630 | '((rose . red) (lily . white) (buttercup . yellow))) |
| 631 | @end example |
| 632 | |
| 633 | @noindent |
| 634 | sets the variable @code{alist-of-colors} to an alist of three elements. In the |
| 635 | first element, @code{rose} is the key and @code{red} is the value. |
| 636 | |
| 637 | @xref{Association Lists}, for a further explanation of alists and for |
| 638 | functions that work on alists. |
| 639 | |
| 640 | @node Array Type |
| 641 | @subsection Array Type |
| 642 | |
| 643 | An @dfn{array} is composed of an arbitrary number of slots for |
| 644 | referring to other Lisp objects, arranged in a contiguous block of |
| 645 | memory. Accessing any element of an array takes a the same amount of |
| 646 | time. In contrast, accessing an element of a list requires time |
| 647 | proportional to the position of the element in the list. (Elements at |
| 648 | the end of a list take longer to access than elements at the beginning |
| 649 | of a list.) |
| 650 | |
| 651 | Emacs defines two types of array, strings and vectors. A string is an |
| 652 | array of characters and a vector is an array of arbitrary objects. Both |
| 653 | are one-dimensional. (Most other programming languages support |
| 654 | multidimensional arrays, but they are not essential; you can get the |
| 655 | same effect with an array of arrays.) Each type of array has its own |
| 656 | read syntax; see @ref{String Type}, and @ref{Vector Type}. |
| 657 | |
| 658 | An array may have any length up to the largest integer; but once |
| 659 | created, it has a fixed size. The first element of an array has index |
| 660 | zero, the second element has index 1, and so on. This is called |
| 661 | @dfn{zero-origin} indexing. For example, an array of four elements has |
| 662 | indices 0, 1, 2, @w{and 3}. |
| 663 | |
| 664 | The array type is contained in the sequence type and contains both the |
| 665 | string type and the vector type. |
| 666 | |
| 667 | @node String Type |
| 668 | @subsection String Type |
| 669 | |
| 670 | A @dfn{string} is an array of characters. Strings are used for many |
| 671 | purposes in Emacs, as can be expected in a text editor; for example, as |
| 672 | the names of Lisp symbols, as messages for the user, and to represent |
| 673 | text extracted from buffers. Strings in Lisp are constants: evaluation |
| 674 | of a string returns the same string. |
| 675 | |
| 676 | @cindex @samp{"} in strings |
| 677 | @cindex double-quote in strings |
| 678 | @cindex @samp{\} in strings |
| 679 | @cindex backslash in strings |
| 680 | The read syntax for strings is a double-quote, an arbitrary number of |
| 681 | characters, and another double-quote, @code{"like this"}. The Lisp |
| 682 | reader accepts the same formats for reading the characters of a string |
| 683 | as it does for reading single characters (without the question mark that |
| 684 | begins a character literal). You can enter a nonprinting character such |
| 685 | as tab, @kbd{C-a} or @kbd{M-C-A} using the convenient escape sequences, |
| 686 | like this: @code{"\t, \C-a, \M-\C-a"}. You can include a double-quote |
| 687 | in a string by preceding it with a backslash; thus, @code{"\""} is a |
| 688 | string containing just a single double-quote character. |
| 689 | (@xref{Character Type}, for a description of the read syntax for |
| 690 | characters.) |
| 691 | |
| 692 | If you use the @samp{\M-} syntax to indicate a meta character in a |
| 693 | string constant, this sets the 2**7 bit of the character in the string. |
| 694 | This is not the same representation that the meta modifier has in a |
| 695 | character on its own (not inside a string). @xref{Character Type}. |
| 696 | |
| 697 | Strings cannot hold characters that have the hyper, super or alt |
| 698 | modifiers; they can hold @sc{ASCII} control characters, but no others. |
| 699 | They do not distinguish case in @sc{ASCII} control characters. |
| 700 | |
| 701 | In contrast with the C programming language, Emacs Lisp allows |
| 702 | newlines in string literals. But an escaped newline---one that is |
| 703 | preceded by @samp{\}---does not become part of the string; i.e., the |
| 704 | Lisp reader ignores an escaped newline in a string literal. |
| 705 | @cindex newline in strings |
| 706 | |
| 707 | @example |
| 708 | "It is useful to include newlines |
| 709 | in documentation strings, |
| 710 | but the newline is \ |
| 711 | ignored if escaped." |
| 712 | @result{} "It is useful to include newlines |
| 713 | in documentation strings, |
| 714 | but the newline is ignored if escaped." |
| 715 | @end example |
| 716 | |
| 717 | The printed representation of a string consists of a double-quote, the |
| 718 | characters it contains, and another double-quote. However, any |
| 719 | backslash or double-quote characters in the string are preceded with a |
| 720 | backslash like this: @code{"this \" is an embedded quote"}. |
| 721 | |
| 722 | A string can hold properties of the text it contains, in addition to |
| 723 | the characters themselves. This enables programs that copy text between |
| 724 | strings and buffers to preserve the properties with no special effort. |
| 725 | @xref{Text Properties}. Strings with text properties have a special |
| 726 | read and print syntax: |
| 727 | |
| 728 | @example |
| 729 | #("@var{characters}" @var{property-data}...) |
| 730 | @end example |
| 731 | |
| 732 | @noindent |
| 733 | where @var{property-data} consists of zero or more elements, in groups |
| 734 | of three as follows: |
| 735 | |
| 736 | @example |
| 737 | @var{beg} @var{end} @var{plist} |
| 738 | @end example |
| 739 | |
| 740 | @noindent |
| 741 | The elements @var{beg} and @var{end} are integers, and together specify |
| 742 | a range of indices in the string; @var{plist} is the property list for |
| 743 | that range. |
| 744 | |
| 745 | @xref{Strings and Characters}, for functions that work on strings. |
| 746 | |
| 747 | @node Vector Type |
| 748 | @subsection Vector Type |
| 749 | |
| 750 | A @dfn{vector} is a one-dimensional array of elements of any type. It |
| 751 | takes a constant amount of time to access any element of a vector. (In |
| 752 | a list, the access time of an element is proportional to the distance of |
| 753 | the element from the beginning of the list.) |
| 754 | |
| 755 | The printed representation of a vector consists of a left square |
| 756 | bracket, the elements, and a right square bracket. This is also the |
| 757 | read syntax. Like numbers and strings, vectors are considered constants |
| 758 | for evaluation. |
| 759 | |
| 760 | @example |
| 761 | [1 "two" (three)] ; @r{A vector of three elements.} |
| 762 | @result{} [1 "two" (three)] |
| 763 | @end example |
| 764 | |
| 765 | @xref{Vectors}, for functions that work with vectors. |
| 766 | |
| 767 | @node Symbol Type |
| 768 | @subsection Symbol Type |
| 769 | |
| 770 | A @dfn{symbol} in GNU Emacs Lisp is an object with a name. The symbol |
| 771 | name serves as the printed representation of the symbol. In ordinary |
| 772 | use, the name is unique---no two symbols have the same name. |
| 773 | |
| 774 | A symbol can serve as a variable, as a function name, or to hold a |
| 775 | property list. Or it may serve only to be distinct from all other Lisp |
| 776 | objects, so that its presence in a data structure may be recognized |
| 777 | reliably. In a given context, usually only one of these uses is |
| 778 | intended. But you can use one symbol in all of these ways, |
| 779 | independently. |
| 780 | |
| 781 | @cindex @samp{\} in symbols |
| 782 | @cindex backslash in symbols |
| 783 | A symbol name can contain any characters whatever. Most symbol names |
| 784 | are written with letters, digits, and the punctuation characters |
| 785 | @samp{-+=*/}. Such names require no special punctuation; the characters |
| 786 | of the name suffice as long as the name does not look like a number. |
| 787 | (If it does, write a @samp{\} at the beginning of the name to force |
| 788 | interpretation as a symbol.) The characters @samp{_~!@@$%^&:<>@{@}} are |
| 789 | less often used but also require no special punctuation. Any other |
| 790 | characters may be included in a symbol's name by escaping them with a |
| 791 | backslash. In contrast to its use in strings, however, a backslash in |
| 792 | the name of a symbol quotes the single character that follows the |
| 793 | backslash, without conversion. For example, in a string, @samp{\t} |
| 794 | represents a tab character; in the name of a symbol, however, @samp{\t} |
| 795 | merely quotes the letter @kbd{t}. To have a symbol with a tab character |
| 796 | in its name, you must actually use a tab (preceded with a backslash). |
| 797 | But it's rare to do such a thing. |
| 798 | |
| 799 | @cindex CL note---case of letters |
| 800 | @quotation |
| 801 | @b{Common Lisp note:} in Common Lisp, lower case letters are always |
| 802 | ``folded'' to upper case, unless they are explicitly escaped. This is |
| 803 | in contrast to Emacs Lisp, in which upper case and lower case letters |
| 804 | are distinct. |
| 805 | @end quotation |
| 806 | |
| 807 | Here are several examples of symbol names. Note that the @samp{+} in |
| 808 | the fifth example is escaped to prevent it from being read as a number. |
| 809 | This is not necessary in the last example because the rest of the name |
| 810 | makes it invalid as a number. |
| 811 | |
| 812 | @example |
| 813 | @group |
| 814 | foo ; @r{A symbol named @samp{foo}.} |
| 815 | FOO ; @r{A symbol named @samp{FOO}, different from @samp{foo}.} |
| 816 | char-to-string ; @r{A symbol named @samp{char-to-string}.} |
| 817 | @end group |
| 818 | @group |
| 819 | 1+ ; @r{A symbol named @samp{1+}} |
| 820 | ; @r{(not @samp{+1}, which is an integer).} |
| 821 | @end group |
| 822 | @group |
| 823 | \+1 ; @r{A symbol named @samp{+1}} |
| 824 | ; @r{(not a very readable name).} |
| 825 | @end group |
| 826 | @group |
| 827 | \(*\ 1\ 2\) ; @r{A symbol named @samp{(* 1 2)} (a worse name).} |
| 828 | @c the @'s in this next line use up three characters, hence the |
| 829 | @c apparent misalignment of the comment. |
| 830 | +-*/_~!@@$%^&=:<>@{@} ; @r{A symbol named @samp{+-*/_~!@@$%^&=:<>@{@}}.} |
| 831 | ; @r{These characters need not be escaped.} |
| 832 | @end group |
| 833 | @end example |
| 834 | |
| 835 | @node Lisp Function Type |
| 836 | @subsection Lisp Function Type |
| 837 | |
| 838 | Just as functions in other programming languages are executable, |
| 839 | @dfn{Lisp function} objects are pieces of executable code. However, |
| 840 | functions in Lisp are primarily Lisp objects, and only secondarily the |
| 841 | text which represents them. These Lisp objects are lambda expressions: |
| 842 | lists whose first element is the symbol @code{lambda} (@pxref{Lambda |
| 843 | Expressions}). |
| 844 | |
| 845 | In most programming languages, it is impossible to have a function |
| 846 | without a name. In Lisp, a function has no intrinsic name. A lambda |
| 847 | expression is also called an @dfn{anonymous function} (@pxref{Anonymous |
| 848 | Functions}). A named function in Lisp is actually a symbol with a valid |
| 849 | function in its function cell (@pxref{Defining Functions}). |
| 850 | |
| 851 | Most of the time, functions are called when their names are written in |
| 852 | Lisp expressions in Lisp programs. However, you can construct or obtain |
| 853 | a function object at run time and then call it with the primitive |
| 854 | functions @code{funcall} and @code{apply}. @xref{Calling Functions}. |
| 855 | |
| 856 | @node Lisp Macro Type |
| 857 | @subsection Lisp Macro Type |
| 858 | |
| 859 | A @dfn{Lisp macro} is a user-defined construct that extends the Lisp |
| 860 | language. It is represented as an object much like a function, but with |
| 861 | different parameter-passing semantics. A Lisp macro has the form of a |
| 862 | list whose first element is the symbol @code{macro} and whose @sc{cdr} |
| 863 | is a Lisp function object, including the @code{lambda} symbol. |
| 864 | |
| 865 | Lisp macro objects are usually defined with the built-in |
| 866 | @code{defmacro} function, but any list that begins with @code{macro} is |
| 867 | a macro as far as Emacs is concerned. @xref{Macros}, for an explanation |
| 868 | of how to write a macro. |
| 869 | |
| 870 | @node Primitive Function Type |
| 871 | @subsection Primitive Function Type |
| 872 | @cindex special forms |
| 873 | |
| 874 | A @dfn{primitive function} is a function callable from Lisp but |
| 875 | written in the C programming language. Primitive functions are also |
| 876 | called @dfn{subrs} or @dfn{built-in functions}. (The word ``subr'' is |
| 877 | derived from ``subroutine''.) Most primitive functions evaluate all |
| 878 | their arguments when they are called. A primitive function that does |
| 879 | not evaluate all its arguments is called a @dfn{special form} |
| 880 | (@pxref{Special Forms}).@refill |
| 881 | |
| 882 | It does not matter to the caller of a function whether the function is |
| 883 | primitive. However, this does matter if you try to substitute a |
| 884 | function written in Lisp for a primitive of the same name. The reason |
| 885 | is that the primitive function may be called directly from C code. |
| 886 | Calls to the redefined function from Lisp will use the new definition, |
| 887 | but calls from C code may still use the built-in definition. |
| 888 | |
| 889 | The term @dfn{function} refers to all Emacs functions, whether written |
| 890 | in Lisp or C. @xref{Lisp Function Type}, for information about the |
| 891 | functions written in Lisp.@refill |
| 892 | |
| 893 | Primitive functions have no read syntax and print in hash notation |
| 894 | with the name of the subroutine. |
| 895 | |
| 896 | @example |
| 897 | @group |
| 898 | (symbol-function 'car) ; @r{Access the function cell} |
| 899 | ; @r{of the symbol.} |
| 900 | @result{} #<subr car> |
| 901 | (subrp (symbol-function 'car)) ; @r{Is this a primitive function?} |
| 902 | @result{} t ; @r{Yes.} |
| 903 | @end group |
| 904 | @end example |
| 905 | |
| 906 | @node Byte-Code Type |
| 907 | @subsection Byte-Code Function Type |
| 908 | |
| 909 | The byte compiler produces @dfn{byte-code function objects}. |
| 910 | Internally, a byte-code function object is much like a vector; however, |
| 911 | the evaluator handles this data type specially when it appears as a |
| 912 | function to be called. @xref{Byte Compilation}, for information about |
| 913 | the byte compiler. |
| 914 | |
| 915 | The printed representation for a byte-code function object is like that |
| 916 | for a vector, with an additional @samp{#} before the opening @samp{[}. |
| 917 | |
| 918 | @node Autoload Type |
| 919 | @subsection Autoload Type |
| 920 | |
| 921 | An @dfn{autoload object} is a list whose first element is the symbol |
| 922 | @code{autoload}. It is stored as the function definition of a symbol as |
| 923 | a placeholder for the real definition; it says that the real definition |
| 924 | is found in a file of Lisp code that should be loaded when necessary. |
| 925 | The autoload object contains the name of the file, plus some other |
| 926 | information about the real definition. |
| 927 | |
| 928 | After the file has been loaded, the symbol should have a new function |
| 929 | definition that is not an autoload object. The new definition is then |
| 930 | called as if it had been there to begin with. From the user's point of |
| 931 | view, the function call works as expected, using the function definition |
| 932 | in the loaded file. |
| 933 | |
| 934 | An autoload object is usually created with the function |
| 935 | @code{autoload}, which stores the object in the function cell of a |
| 936 | symbol. @xref{Autoload}, for more details. |
| 937 | |
| 938 | @node Editing Types |
| 939 | @section Editing Types |
| 940 | @cindex editing types |
| 941 | |
| 942 | The types in the previous section are common to many Lisp dialects. |
| 943 | Emacs Lisp provides several additional data types for purposes connected |
| 944 | with editing. |
| 945 | |
| 946 | @menu |
| 947 | * Buffer Type:: The basic object of editing. |
| 948 | * Marker Type:: A position in a buffer. |
| 949 | * Window Type:: Buffers are displayed in windows. |
| 950 | * Frame Type:: Windows subdivide frames. |
| 951 | * Window Configuration Type:: Recording the way a frame is subdivided. |
| 952 | * Process Type:: A process running on the underlying OS. |
| 953 | * Stream Type:: Receive or send characters. |
| 954 | * Keymap Type:: What function a keystroke invokes. |
| 955 | * Syntax Table Type:: What a character means. |
| 956 | * Display Table Type:: How display tables are represented. |
| 957 | * Overlay Type:: How an overlay is represented. |
| 958 | @end menu |
| 959 | |
| 960 | @node Buffer Type |
| 961 | @subsection Buffer Type |
| 962 | |
| 963 | A @dfn{buffer} is an object that holds text that can be edited |
| 964 | (@pxref{Buffers}). Most buffers hold the contents of a disk file |
| 965 | (@pxref{Files}) so they can be edited, but some are used for other |
| 966 | purposes. Most buffers are also meant to be seen by the user, and |
| 967 | therefore displayed, at some time, in a window (@pxref{Windows}). But a |
| 968 | buffer need not be displayed in any window. |
| 969 | |
| 970 | The contents of a buffer are much like a string, but buffers are not |
| 971 | used like strings in Emacs Lisp, and the available operations are |
| 972 | different. For example, insertion of text into a buffer is very |
| 973 | efficient, whereas ``inserting'' text into a string requires |
| 974 | concatenating substrings, and the result is an entirely new string |
| 975 | object. |
| 976 | |
| 977 | Each buffer has a designated position called @dfn{point} |
| 978 | (@pxref{Positions}). At any time, one buffer is the @dfn{current |
| 979 | buffer}. Most editing commands act on the contents of the current |
| 980 | buffer in the neighborhood of point. Many other functions manipulate or |
| 981 | test the characters in the current buffer; a whole chapter in this |
| 982 | manual is devoted to describing these functions (@pxref{Text}). |
| 983 | |
| 984 | Several other data structures are associated with each buffer: |
| 985 | |
| 986 | @itemize @bullet |
| 987 | @item |
| 988 | a local syntax table (@pxref{Syntax Tables}); |
| 989 | |
| 990 | @item |
| 991 | a local keymap (@pxref{Keymaps}); and, |
| 992 | |
| 993 | @item |
| 994 | a local variable binding list (@pxref{Buffer-Local Variables}). |
| 995 | @end itemize |
| 996 | |
| 997 | @noindent |
| 998 | The local keymap and variable list contain entries which individually |
| 999 | override global bindings or values. These are used to customize the |
| 1000 | behavior of programs in different buffers, without actually changing the |
| 1001 | programs. |
| 1002 | |
| 1003 | Buffers have no read syntax. They print in hash notation with the |
| 1004 | buffer name. |
| 1005 | |
| 1006 | @example |
| 1007 | @group |
| 1008 | (current-buffer) |
| 1009 | @result{} #<buffer objects.texi> |
| 1010 | @end group |
| 1011 | @end example |
| 1012 | |
| 1013 | @node Marker Type |
| 1014 | @subsection Marker Type |
| 1015 | |
| 1016 | A @dfn{marker} denotes a position in a specific buffer. Markers |
| 1017 | therefore have two components: one for the buffer, and one for the |
| 1018 | position. Changes in the buffer's text automatically relocate the |
| 1019 | position value as necessary to ensure that the marker always points |
| 1020 | between the same two characters in the buffer. |
| 1021 | |
| 1022 | Markers have no read syntax. They print in hash notation, giving the |
| 1023 | current character position and the name of the buffer. |
| 1024 | |
| 1025 | @example |
| 1026 | @group |
| 1027 | (point-marker) |
| 1028 | @result{} #<marker at 10779 in objects.texi> |
| 1029 | @end group |
| 1030 | @end example |
| 1031 | |
| 1032 | @xref{Markers}, for information on how to test, create, copy, and move |
| 1033 | markers. |
| 1034 | |
| 1035 | @node Window Type |
| 1036 | @subsection Window Type |
| 1037 | |
| 1038 | A @dfn{window} describes the portion of the terminal screen that Emacs |
| 1039 | uses to display a buffer. Every window has one associated buffer, whose |
| 1040 | contents appear in the window. By contrast, a given buffer may appear |
| 1041 | in one window, no window, or several windows. |
| 1042 | |
| 1043 | Though many windows may exist simultaneously, at any time one window |
| 1044 | is designated the @dfn{selected window}. This is the window where the |
| 1045 | cursor is (usually) displayed when Emacs is ready for a command. The |
| 1046 | selected window usually displays the current buffer, but this is not |
| 1047 | necessarily the case. |
| 1048 | |
| 1049 | Windows are grouped on the screen into frames; each window belongs to |
| 1050 | one and only one frame. @xref{Frame Type}. |
| 1051 | |
| 1052 | Windows have no read syntax. They print in hash notation, giving the |
| 1053 | window number and the name of the buffer being displayed. The window |
| 1054 | numbers exist to identify windows uniquely, since the buffer displayed |
| 1055 | in any given window can change frequently. |
| 1056 | |
| 1057 | @example |
| 1058 | @group |
| 1059 | (selected-window) |
| 1060 | @result{} #<window 1 on objects.texi> |
| 1061 | @end group |
| 1062 | @end example |
| 1063 | |
| 1064 | @xref{Windows}, for a description of the functions that work on windows. |
| 1065 | |
| 1066 | @node Frame Type |
| 1067 | @subsection Frame Type |
| 1068 | |
| 1069 | A @var{frame} is a rectangle on the screen that contains one or more |
| 1070 | Emacs windows. A frame initially contains a single main window (plus |
| 1071 | perhaps a minibuffer window) which you can subdivide vertically or |
| 1072 | horizontally into smaller windows. |
| 1073 | |
| 1074 | Frames have no read syntax. They print in hash notation, giving the |
| 1075 | frame's title, plus its address in core (useful to identify the frame |
| 1076 | uniquely). |
| 1077 | |
| 1078 | @example |
| 1079 | @group |
| 1080 | (selected-frame) |
| 1081 | @result{} #<frame xemacs@@mole.gnu.ai.mit.edu 0xdac80> |
| 1082 | @end group |
| 1083 | @end example |
| 1084 | |
| 1085 | @xref{Frames}, for a description of the functions that work on frames. |
| 1086 | |
| 1087 | @node Window Configuration Type |
| 1088 | @subsection Window Configuration Type |
| 1089 | @cindex screen layout |
| 1090 | |
| 1091 | A @dfn{window configuration} stores information about the positions, |
| 1092 | sizes, and contents of the windows in a frame, so you can recreate the |
| 1093 | same arrangement of windows later. |
| 1094 | |
| 1095 | Window configurations do not have a read syntax. They print as |
| 1096 | @samp{#<window-configuration>}. @xref{Window Configurations}, for a |
| 1097 | description of several functions related to window configurations. |
| 1098 | |
| 1099 | @node Process Type |
| 1100 | @subsection Process Type |
| 1101 | |
| 1102 | The word @dfn{process} usually means a running program. Emacs itself |
| 1103 | runs in a process of this sort. However, in Emacs Lisp, a process is a |
| 1104 | Lisp object that designates a subprocess created by the Emacs process. |
| 1105 | Programs such as shells, GDB, ftp, and compilers, running in |
| 1106 | subprocesses of Emacs, extend the capabilities of Emacs. |
| 1107 | |
| 1108 | An Emacs subprocess takes textual input from Emacs and returns textual |
| 1109 | output to Emacs for further manipulation. Emacs can also send signals |
| 1110 | to the subprocess. |
| 1111 | |
| 1112 | Process objects have no read syntax. They print in hash notation, |
| 1113 | giving the name of the process: |
| 1114 | |
| 1115 | @example |
| 1116 | @group |
| 1117 | (process-list) |
| 1118 | @result{} (#<process shell>) |
| 1119 | @end group |
| 1120 | @end example |
| 1121 | |
| 1122 | @xref{Processes}, for information about functions that create, delete, |
| 1123 | return information about, send input or signals to, and receive output |
| 1124 | from processes. |
| 1125 | |
| 1126 | @node Stream Type |
| 1127 | @subsection Stream Type |
| 1128 | |
| 1129 | A @dfn{stream} is an object that can be used as a source or sink for |
| 1130 | characters---either to supply characters for input or to accept them as |
| 1131 | output. Many different types can be used this way: markers, buffers, |
| 1132 | strings, and functions. Most often, input streams (character sources) |
| 1133 | obtain characters from the keyboard, a buffer, or a file, and output |
| 1134 | streams (character sinks) send characters to a buffer, such as a |
| 1135 | @file{*Help*} buffer, or to the echo area. |
| 1136 | |
| 1137 | The object @code{nil}, in addition to its other meanings, may be used |
| 1138 | as a stream. It stands for the value of the variable |
| 1139 | @code{standard-input} or @code{standard-output}. Also, the object |
| 1140 | @code{t} as a stream specifies input using the minibuffer |
| 1141 | (@pxref{Minibuffers}) or output in the echo area (@pxref{The Echo |
| 1142 | Area}). |
| 1143 | |
| 1144 | Streams have no special printed representation or read syntax, and |
| 1145 | print as whatever primitive type they are. |
| 1146 | |
| 1147 | @xref{Streams}, for a description of various functions related to |
| 1148 | streams, including various parsing and printing functions. |
| 1149 | |
| 1150 | @node Keymap Type |
| 1151 | @subsection Keymap Type |
| 1152 | |
| 1153 | A @dfn{keymap} maps keys typed by the user to commands. This mapping |
| 1154 | controls how the user's command input is executed. A keymap is actually |
| 1155 | a list whose @sc{car} is the symbol @code{keymap}. |
| 1156 | |
| 1157 | @xref{Keymaps}, for information about creating keymaps, handling prefix |
| 1158 | keys, local as well as global keymaps, and changing key bindings. |
| 1159 | |
| 1160 | @node Syntax Table Type |
| 1161 | @subsection Syntax Table Type |
| 1162 | |
| 1163 | A @dfn{syntax table} is a vector of 256 integers. Each element of the |
| 1164 | vector defines how one character is interpreted when it appears in a |
| 1165 | buffer. For example, in C mode (@pxref{Major Modes}), the @samp{+} |
| 1166 | character is punctuation, but in Lisp mode it is a valid character in a |
| 1167 | symbol. These modes specify different interpretations by changing the |
| 1168 | syntax table entry for @samp{+}, at index 43 in the syntax table. |
| 1169 | |
| 1170 | Syntax tables are only used for scanning text in buffers, not for |
| 1171 | reading Lisp expressions. The table the Lisp interpreter uses to read |
| 1172 | expressions is built into the Emacs source code and cannot be changed; |
| 1173 | thus, to change the list delimiters to be @samp{@{} and @samp{@}} |
| 1174 | instead of @samp{(} and @samp{)} would be impossible. |
| 1175 | |
| 1176 | @xref{Syntax Tables}, for details about syntax classes and how to make |
| 1177 | and modify syntax tables. |
| 1178 | |
| 1179 | @node Display Table Type |
| 1180 | @subsection Display Table Type |
| 1181 | |
| 1182 | A @dfn{display table} specifies how to display each character code. |
| 1183 | Each buffer and each window can have its own display table. A display |
| 1184 | table is actually a vector of length 261. @xref{Display Tables}. |
| 1185 | |
| 1186 | @node Overlay Type |
| 1187 | @subsection Overlay Type |
| 1188 | |
| 1189 | An @dfn{overlay} specifies temporary alteration of the display |
| 1190 | appearance of a part of a buffer. It contains markers delimiting a |
| 1191 | range of the buffer, plus a property list (a list whose elements are |
| 1192 | alternating property names and values). Overlays are used to present |
| 1193 | parts of the buffer temporarily in a different display style. |
| 1194 | |
| 1195 | @xref{Overlays}, for how to create and use overlays. They have no |
| 1196 | read syntax, and print in hash notation, giving the buffer name and |
| 1197 | range of positions. |
| 1198 | |
| 1199 | @node Type Predicates |
| 1200 | @section Type Predicates |
| 1201 | @cindex predicates |
| 1202 | @cindex type checking |
| 1203 | @kindex wrong-type-argument |
| 1204 | |
| 1205 | The Emacs Lisp interpreter itself does not perform type checking on |
| 1206 | the actual arguments passed to functions when they are called. It could |
| 1207 | not do so, since function arguments in Lisp do not have declared data |
| 1208 | types, as they do in other programming languages. It is therefore up to |
| 1209 | the individual function to test whether each actual argument belongs to |
| 1210 | a type that the function can use. |
| 1211 | |
| 1212 | All built-in functions do check the types of their actual arguments |
| 1213 | when appropriate, and signal a @code{wrong-type-argument} error if an |
| 1214 | argument is of the wrong type. For example, here is what happens if you |
| 1215 | pass an argument to @code{+} which it cannot handle: |
| 1216 | |
| 1217 | @example |
| 1218 | @group |
| 1219 | (+ 2 'a) |
| 1220 | @error{} Wrong type argument: integer-or-marker-p, a |
| 1221 | @end group |
| 1222 | @end example |
| 1223 | |
| 1224 | @cindex type predicates |
| 1225 | @cindex testing types |
| 1226 | Lisp provides functions, called @dfn{type predicates}, to test whether |
| 1227 | an object is a member of a given type. (Following a convention of long |
| 1228 | standing, the names of most Emacs Lisp predicates end in @samp{p}.) |
| 1229 | |
| 1230 | Here is a table of predefined type predicates, in alphabetical order, |
| 1231 | with references to further information. |
| 1232 | |
| 1233 | @table @code |
| 1234 | @item atom |
| 1235 | @xref{List-related Predicates, atom}. |
| 1236 | |
| 1237 | @item arrayp |
| 1238 | @xref{Array Functions, arrayp}. |
| 1239 | |
| 1240 | @item bufferp |
| 1241 | @xref{Buffer Basics, bufferp}. |
| 1242 | |
| 1243 | @item byte-code-function-p |
| 1244 | @xref{Byte-Code Type, byte-code-function-p}. |
| 1245 | |
| 1246 | @item case-table-p |
| 1247 | @xref{Case Table, case-table-p}. |
| 1248 | |
| 1249 | @item char-or-string-p |
| 1250 | @xref{Predicates for Strings, char-or-string-p}. |
| 1251 | |
| 1252 | @item commandp |
| 1253 | @xref{Interactive Call, commandp}. |
| 1254 | |
| 1255 | @item consp |
| 1256 | @xref{List-related Predicates, consp}. |
| 1257 | |
| 1258 | @item floatp |
| 1259 | @xref{Predicates on Numbers, floatp}. |
| 1260 | |
| 1261 | @item frame-live-p |
| 1262 | @xref{Deleting Frames, frame-live-p}. |
| 1263 | |
| 1264 | @item framep |
| 1265 | @xref{Frames, framep}. |
| 1266 | |
| 1267 | @item integer-or-marker-p |
| 1268 | @xref{Predicates on Markers, integer-or-marker-p}. |
| 1269 | |
| 1270 | @item integerp |
| 1271 | @xref{Predicates on Numbers, integerp}. |
| 1272 | |
| 1273 | @item keymapp |
| 1274 | @xref{Creating Keymaps, keymapp}. |
| 1275 | |
| 1276 | @item listp |
| 1277 | @xref{List-related Predicates, listp}. |
| 1278 | |
| 1279 | @item markerp |
| 1280 | @xref{Predicates on Markers, markerp}. |
| 1281 | |
| 1282 | @item natnump |
| 1283 | @xref{Predicates on Numbers, natnump}. |
| 1284 | |
| 1285 | @item nlistp |
| 1286 | @xref{List-related Predicates, nlistp}. |
| 1287 | |
| 1288 | @item numberp |
| 1289 | @xref{Predicates on Numbers, numberp}. |
| 1290 | |
| 1291 | @item number-or-marker-p |
| 1292 | @xref{Predicates on Markers, number-or-marker-p}. |
| 1293 | |
| 1294 | @item overlayp |
| 1295 | @xref{Overlays, overlayp}. |
| 1296 | |
| 1297 | @item processp |
| 1298 | @xref{Processes, processp}. |
| 1299 | |
| 1300 | @item sequencep |
| 1301 | @xref{Sequence Functions, sequencep}. |
| 1302 | |
| 1303 | @item stringp |
| 1304 | @xref{Predicates for Strings, stringp}. |
| 1305 | |
| 1306 | @item subrp |
| 1307 | @xref{Function Cells, subrp}. |
| 1308 | |
| 1309 | @item symbolp |
| 1310 | @xref{Symbols, symbolp}. |
| 1311 | |
| 1312 | @item syntax-table-p |
| 1313 | @xref{Syntax Tables, syntax-table-p}. |
| 1314 | |
| 1315 | @item user-variable-p |
| 1316 | @xref{Defining Variables, user-variable-p}. |
| 1317 | |
| 1318 | @item vectorp |
| 1319 | @xref{Vectors, vectorp}. |
| 1320 | |
| 1321 | @item window-configuration-p |
| 1322 | @xref{Window Configurations, window-configuration-p}. |
| 1323 | |
| 1324 | @item window-live-p |
| 1325 | @xref{Deleting Windows, window-live-p}. |
| 1326 | |
| 1327 | @item windowp |
| 1328 | @xref{Basic Windows, windowp}. |
| 1329 | @end table |
| 1330 | |
| 1331 | @node Equality Predicates |
| 1332 | @section Equality Predicates |
| 1333 | @cindex equality |
| 1334 | |
| 1335 | Here we describe two functions that test for equality between any two |
| 1336 | objects. Other functions test equality between objects of specific |
| 1337 | types, e.g., strings. See the appropriate chapter describing the data |
| 1338 | type for these predicates. |
| 1339 | |
| 1340 | @defun eq object1 object2 |
| 1341 | This function returns @code{t} if @var{object1} and @var{object2} are |
| 1342 | the same object, @code{nil} otherwise. The ``same object'' means that a |
| 1343 | change in one will be reflected by the same change in the other. |
| 1344 | |
| 1345 | @code{eq} returns @code{t} if @var{object1} and @var{object2} are |
| 1346 | integers with the same value. Also, since symbol names are normally |
| 1347 | unique, if the arguments are symbols with the same name, they are |
| 1348 | @code{eq}. For other types (e.g., lists, vectors, strings), two |
| 1349 | arguments with the same contents or elements are not necessarily |
| 1350 | @code{eq} to each other: they are @code{eq} only if they are the same |
| 1351 | object. |
| 1352 | |
| 1353 | (The @code{make-symbol} function returns an uninterned symbol that is |
| 1354 | not interned in the standard @code{obarray}. When uninterned symbols |
| 1355 | are in use, symbol names are no longer unique. Distinct symbols with |
| 1356 | the same name are not @code{eq}. @xref{Creating Symbols}.) |
| 1357 | |
| 1358 | @example |
| 1359 | @group |
| 1360 | (eq 'foo 'foo) |
| 1361 | @result{} t |
| 1362 | @end group |
| 1363 | |
| 1364 | @group |
| 1365 | (eq 456 456) |
| 1366 | @result{} t |
| 1367 | @end group |
| 1368 | |
| 1369 | @group |
| 1370 | (eq "asdf" "asdf") |
| 1371 | @result{} nil |
| 1372 | @end group |
| 1373 | |
| 1374 | @group |
| 1375 | (eq '(1 (2 (3))) '(1 (2 (3)))) |
| 1376 | @result{} nil |
| 1377 | @end group |
| 1378 | |
| 1379 | @group |
| 1380 | (setq foo '(1 (2 (3)))) |
| 1381 | @result{} (1 (2 (3))) |
| 1382 | (eq foo foo) |
| 1383 | @result{} t |
| 1384 | (eq foo '(1 (2 (3)))) |
| 1385 | @result{} nil |
| 1386 | @end group |
| 1387 | |
| 1388 | @group |
| 1389 | (eq [(1 2) 3] [(1 2) 3]) |
| 1390 | @result{} nil |
| 1391 | @end group |
| 1392 | |
| 1393 | @group |
| 1394 | (eq (point-marker) (point-marker)) |
| 1395 | @result{} nil |
| 1396 | @end group |
| 1397 | @end example |
| 1398 | |
| 1399 | @end defun |
| 1400 | |
| 1401 | @defun equal object1 object2 |
| 1402 | This function returns @code{t} if @var{object1} and @var{object2} have |
| 1403 | equal components, @code{nil} otherwise. Whereas @code{eq} tests if its |
| 1404 | arguments are the same object, @code{equal} looks inside nonidentical |
| 1405 | arguments to see if their elements are the same. So, if two objects are |
| 1406 | @code{eq}, they are @code{equal}, but the converse is not always true. |
| 1407 | |
| 1408 | @example |
| 1409 | @group |
| 1410 | (equal 'foo 'foo) |
| 1411 | @result{} t |
| 1412 | @end group |
| 1413 | |
| 1414 | @group |
| 1415 | (equal 456 456) |
| 1416 | @result{} t |
| 1417 | @end group |
| 1418 | |
| 1419 | @group |
| 1420 | (equal "asdf" "asdf") |
| 1421 | @result{} t |
| 1422 | @end group |
| 1423 | @group |
| 1424 | (eq "asdf" "asdf") |
| 1425 | @result{} nil |
| 1426 | @end group |
| 1427 | |
| 1428 | @group |
| 1429 | (equal '(1 (2 (3))) '(1 (2 (3)))) |
| 1430 | @result{} t |
| 1431 | @end group |
| 1432 | @group |
| 1433 | (eq '(1 (2 (3))) '(1 (2 (3)))) |
| 1434 | @result{} nil |
| 1435 | @end group |
| 1436 | |
| 1437 | @group |
| 1438 | (equal [(1 2) 3] [(1 2) 3]) |
| 1439 | @result{} t |
| 1440 | @end group |
| 1441 | @group |
| 1442 | (eq [(1 2) 3] [(1 2) 3]) |
| 1443 | @result{} nil |
| 1444 | @end group |
| 1445 | |
| 1446 | @group |
| 1447 | (equal (point-marker) (point-marker)) |
| 1448 | @result{} t |
| 1449 | @end group |
| 1450 | |
| 1451 | @group |
| 1452 | (eq (point-marker) (point-marker)) |
| 1453 | @result{} nil |
| 1454 | @end group |
| 1455 | @end example |
| 1456 | |
| 1457 | Comparison of strings uses @code{string=}, and is case-sensitive. |
| 1458 | |
| 1459 | @example |
| 1460 | @group |
| 1461 | (equal "asdf" "ASDF") |
| 1462 | @result{} nil |
| 1463 | @end group |
| 1464 | @end example |
| 1465 | @end defun |
| 1466 | |
| 1467 | The test for equality is implemented recursively, and circular lists may |
| 1468 | therefore cause infinite recursion (leading to an error). |