emacs-lisp-intro.texi 778 KB
 Glenn Morris committed Sep 06, 2007 1 2 \input texinfo @c -*-texinfo-*- @comment %**start of header  Glenn Morris committed Sep 06, 2007 3 @setfilename ../../info/eintr  Glenn Morris committed Sep 06, 2007 4 5 6 7 8 9 10 11 12 13 14 @c setfilename emacs-lisp-intro.info @c sethtmlfilename emacs-lisp-intro.html @settitle Programming in Emacs Lisp @syncodeindex vr cp @syncodeindex fn cp @finalout @c --------- @c <<<< For hard copy printing, this file is now @c set for smallbook, which works for all sizes @c of paper, and with Postscript figures >>>>  Karl Berry committed Oct 05, 2008 15 16 @set smallbook @ifset smallbook  Glenn Morris committed Sep 06, 2007 17 18 @smallbook @clear largebook  Karl Berry committed Oct 05, 2008 19 @end ifset  Glenn Morris committed Sep 06, 2007 20 21 22 23 24 25 26 @set print-postscript-figures @c set largebook @c clear print-postscript-figures @c --------- @comment %**end of header  Karl Berry committed Oct 05, 2008 27 @c per rms and peterb, use 10pt fonts for the main text, mostly to  Juanma Barranquero committed Feb 20, 2009 28 @c save on paper cost.  Karl Berry committed Oct 05, 2008 29 30 31 32 @c Do this inside @tex for now, so current makeinfo does not complain. @tex @ifset smallbook @fonttextsize 10  Robert J. Chassell committed Oct 28, 2009 33   Karl Berry committed Oct 05, 2008 34 35 36 37 @end ifset \global\hbadness=6666 % don't worry about not-too-underfull boxes @end tex  Robert J. Chassell committed Oct 28, 2009 38 39 @set edition-number 3.10 @set update-date 28 October 2009  Robert J. Chassell committed Oct 27, 2009 40   Glenn Morris committed Sep 06, 2007 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 @ignore ## Summary of shell commands to create various output formats: pushd /usr/local/src/emacs/lispintro/ ## pushd /u/intro/ ## Info output makeinfo --paragraph-indent=0 --verbose emacs-lisp-intro.texi ## ;; (progn (when (bufferp (get-buffer "*info*")) (kill-buffer "*info*")) (info "/usr/local/src/emacs/info/eintr")) ## DVI output texi2dvi emacs-lisp-intro.texi ## xdvi -margins 24pt -topmargin 4pt -offsets 24pt -geometry 760x1140 -s 5 -useTeXpages -mousemode 1 emacs-lisp-intro.dvi & ## HTML output makeinfo --html --no-split --verbose emacs-lisp-intro.texi ## galeon emacs-lisp-intro.html ## Plain text output makeinfo --fill-column=70 --no-split --paragraph-indent=0 \ --verbose --no-headers --output=emacs-lisp-intro.txt emacs-lisp-intro.texi popd # as user root' # insert thumbdrive mtusb # mount -v -t ext3 /dev/sda /mnt cp -v /u/intro/emacs-lisp-intro.texi /mnt/backup/intro/emacs-lisp-intro.texi umtusb # umount -v /mnt # remove thumbdrive ## Other shell commands pushd /usr/local/src/emacs/lispintro/ ## pushd /u/intro/ ## PDF texi2dvi --pdf emacs-lisp-intro.texi # xpdf emacs-lisp-intro.pdf & ## DocBook -- note file extension makeinfo --docbook --no-split --paragraph-indent=0 \ --verbose --output=emacs-lisp-intro.docbook emacs-lisp-intro.texi ## XML with a Texinfo DTD -- note file extension makeinfo --xml --no-split --paragraph-indent=0 \ --verbose --output=emacs-lisp-intro.texinfoxml emacs-lisp-intro.texi ## PostScript (needs DVI) # gv emacs-lisp-intro.ps & # Create DVI if we lack it # texi2dvi emacs-lisp-intro.texi dvips emacs-lisp-intro.dvi -o emacs-lisp-intro.ps ## RTF (needs HTML) # Use OpenOffice to view RTF # Create HTML if we lack it # makeinfo --no-split --html emacs-lisp-intro.texi /usr/local/src/html2rtf.pl emacs-lisp-intro.html ## LaTeX (needs RTF) /usr/bin/rtf2latex emacs-lisp-intro.rtf popd @end ignore @c ================ Included Figures ================ @c Set print-postscript-figures if you print PostScript figures. @c If you clear this, the ten figures will be printed as ASCII diagrams. @c (This is not relevant to Info, since Info only handles ASCII.) @c Your site may require editing changes to print PostScript; in this @c case, search for print-postscript-figures' and make appropriate changes. @c ================ How to Create an Info file ================ @c If you have makeinfo' installed, run the following command @c makeinfo emacs-lisp-intro.texi @c or, if you want a single, large Info file, and no paragraph indents: @c makeinfo --no-split --paragraph-indent=0 --verbose emacs-lisp-intro.texi @c After creating the Info file, edit your Info dir' file, if the @c dircategory' section below does not enable your system to @c install the manual automatically. @c (The dir' file is often in the /usr/local/share/info/' directory.) @c ================ How to Create an HTML file ================ @c To convert to HTML format @c makeinfo --html --no-split --verbose emacs-lisp-intro.texi @c ================ How to Print a Book in Various Sizes ================ @c This book can be printed in any of three different sizes. @c In the above header, set @-commands appropriately. @c 7 by 9.25 inches: @c @smallbook @c @clear largebook @c 8.5 by 11 inches: @c @c smallbook @c @set largebook @c European A4 size paper: @c @c smallbook @c @afourpaper @c @set largebook @c ================ How to Typeset and Print ================ @c If you do not include PostScript figures, run either of the @c following command sequences, or similar commands suited to your @c system: @c texi2dvi emacs-lisp-intro.texi @c lpr -d emacs-lisp-intro.dvi @c or else: @c tex emacs-lisp-intro.texi @c texindex emacs-lisp-intro.?? @c tex emacs-lisp-intro.texi @c lpr -d emacs-lisp-intro.dvi @c If you include the PostScript figures, and you have old software, @c you may need to convert the .dvi file to a .ps file before @c printing. Run either of the following command sequences, or one @c similar: @c @c dvips -f < emacs-lisp-intro.dvi > emacs-lisp-intro.ps @c @c or else: @c @c postscript -p < emacs-lisp-intro.dvi > emacs-lisp-intro.ps @c @c (Note: if you edit the book so as to change the length of the @c table of contents, you may have to change the value of pageno' below.) @c ================ End of Formatting Sections ================ @c For next or subsequent edition: @c create function using with-output-to-temp-buffer @c create a major mode, with keymaps @c run an asynchronous process, like grep or diff @c For 8.5 by 11 inch format: do not use such a small amount of @c whitespace between paragraphs as smallbook format @ifset largebook @tex \global\parskip 6pt plus 1pt @end tex @end ifset @c For all sized formats: print within-book cross @c reference with ...'' rather than [...] @c This works with the texinfo.tex file, version 2003-05-04.08, @c in the Texinfo version 4.6 of the 2003 Jun 13 distribution. @tex \if \xrefprintnodename \global\def\xrefprintnodename#1{\unskip, #1''} \else \global\def\xrefprintnodename#1{ #1''} \fi % \global\def\xrefprintnodename#1{, #1''} @end tex @c ---------------------------------------------------- @dircategory Emacs @direntry * Emacs Lisp Intro: (eintr). A simple introduction to Emacs Lisp programming. @end direntry @copying This is an @cite{Introduction to Programming in Emacs Lisp}, for people who are not programmers. @sp 1 Edition @value{edition-number}, @value{update-date} @sp 1 Copyright @copyright{} 1990, 1991, 1992, 1993, 1994, 1995, 1997, 2001,  Glenn Morris committed Feb 15, 2010 232 233  2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software Foundation, Inc.  Glenn Morris committed Sep 06, 2007 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 @sp 1 @iftex Published by the:@* GNU Press, @hfill @uref{http://www.gnupress.org}@* a division of the @hfill General: @email{press@@gnu.org}@* Free Software Foundation, Inc. @hfill Orders:@w{ } @email{sales@@gnu.org}@* 51 Franklin Street, Fifth Floor @hfill Tel: +1 (617) 542-5942@* Boston, MA 02110-1301 USA @hfill Fax: +1 (617) 542-2652@* @end iftex @ifnottex Published by the: @example GNU Press, Website: http://www.gnupress.org a division of the General: press@@gnu.org Free Software Foundation, Inc. Orders: sales@@gnu.org 51 Franklin Street, Fifth Floor Tel: +1 (617) 542-5942 Boston, MA 02110-1301 USA Fax: +1 (617) 542-2652 @end example @end ifnottex @sp 1 @c Printed copies are available for \$30 each.@* ISBN 1-882114-43-4 Permission is granted to copy, distribute and/or modify this document  Glenn Morris committed Nov 19, 2008 263 under the terms of the GNU Free Documentation License, Version 1.3 or  Glenn Morris committed Sep 06, 2007 264 265 266 267 268 269 any later version published by the Free Software Foundation; there being no Invariant Section, with the Front-Cover Texts being A GNU Manual'', and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled GNU Free Documentation License''.  Robert J. Chassell committed Jan 31, 2008 270 271 272 (a) The FSF's Back-Cover Text is: You have the freedom to copy and modify this GNU manual. Buying copies from the FSF supports it in developing GNU and promoting software freedom.''  Glenn Morris committed Sep 06, 2007 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 @end copying @c half title; two lines here, so do not use shorttitlepage' @tex {\begingroup% \hbox{}\vskip 1.5in \chaprm \centerline{An Introduction to}% \endgroup}% {\begingroup\hbox{}\vskip 0.25in \chaprm% \centerline{Programming in Emacs Lisp}% \endgroup\page\hbox{}\page} @end tex @titlepage @sp 6 @center @titlefont{An Introduction to} @sp 2 @center @titlefont{Programming in Emacs Lisp} @sp 2 @center Revised Third Edition @sp 4 @center by Robert J. Chassell @page @vskip 0pt plus 1filll @insertcopying @end titlepage @iftex @headings off @evenheading @thispage @| @| @thischapter @oddheading @thissection @| @| @thispage @end iftex @ifnothtml @c Keep T.O.C. short by tightening up for largebook @ifset largebook @tex \global\parskip 2pt plus 1pt \global\advance\baselineskip by -1pt @end tex @end ifset @end ifnothtml @shortcontents @contents @ifnottex @node Top, Preface, (dir), (dir) @top An Introduction to Programming in Emacs Lisp @insertcopying This master menu first lists each chapter and index; then it lists every node in every chapter. @end ifnottex @c >>>> Set pageno appropriately <<<< @c The first page of the Preface is a roman numeral; it is the first @c right handed page after the Table of Contents; hence the following @c setting must be for an odd negative number.  Karl Berry committed Oct 05, 2008 335 336 337 @c iftex @c global@pageno = -11 @c end iftex  Glenn Morris committed Sep 06, 2007 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786  @menu * Preface:: What to look for. * List Processing:: What is Lisp? * Practicing Evaluation:: Running several programs. * Writing Defuns:: How to write function definitions. * Buffer Walk Through:: Exploring a few buffer-related functions. * More Complex:: A few, even more complex functions. * Narrowing & Widening:: Restricting your and Emacs attention to a region. * car cdr & cons:: Fundamental functions in Lisp. * Cutting & Storing Text:: Removing text and saving it. * List Implementation:: How lists are implemented in the computer. * Yanking:: Pasting stored text. * Loops & Recursion:: How to repeat a process. * Regexp Search:: Regular expression searches. * Counting Words:: A review of repetition and regexps. * Words in a defun:: Counting words in a @code{defun}. * Readying a Graph:: A prototype graph printing function. * Emacs Initialization:: How to write a @file{.emacs} file. * Debugging:: How to run the Emacs Lisp debuggers. * Conclusion:: Now you have the basics. * the-the:: An appendix: how to find reduplicated words. * Kill Ring:: An appendix: how the kill ring works. * Full Graph:: How to create a graph with labelled axes. * Free Software and Free Manuals:: * GNU Free Documentation License:: * Index:: * About the Author:: @detailmenu --- The Detailed Node Listing --- Preface * Why:: Why learn Emacs Lisp? * On Reading this Text:: Read, gain familiarity, pick up habits.... * Who You Are:: For whom this is written. * Lisp History:: * Note for Novices:: You can read this as a novice. * Thank You:: List Processing * Lisp Lists:: What are lists? * Run a Program:: Any list in Lisp is a program ready to run. * Making Errors:: Generating an error message. * Names & Definitions:: Names of symbols and function definitions. * Lisp Interpreter:: What the Lisp interpreter does. * Evaluation:: Running a program. * Variables:: Returning a value from a variable. * Arguments:: Passing information to a function. * set & setq:: Setting the value of a variable. * Summary:: The major points. * Error Message Exercises:: Lisp Lists * Numbers Lists:: List have numbers, other lists, in them. * Lisp Atoms:: Elemental entities. * Whitespace in Lists:: Formatting lists to be readable. * Typing Lists:: How GNU Emacs helps you type lists. The Lisp Interpreter * Complications:: Variables, Special forms, Lists within. * Byte Compiling:: Specially processing code for speed. Evaluation * How the Interpreter Acts:: Returns and Side Effects... * Evaluating Inner Lists:: Lists within lists... Variables * fill-column Example:: * Void Function:: The error message for a symbol without a function. * Void Variable:: The error message for a symbol without a value. Arguments * Data types:: Types of data passed to a function. * Args as Variable or List:: An argument can be the value of a variable or list. * Variable Number of Arguments:: Some functions may take a variable number of arguments. * Wrong Type of Argument:: Passing an argument of the wrong type to a function. * message:: A useful function for sending messages. Setting the Value of a Variable * Using set:: Setting values. * Using setq:: Setting a quoted value. * Counting:: Using @code{setq} to count. Practicing Evaluation * How to Evaluate:: Typing editing commands or @kbd{C-x C-e} causes evaluation. * Buffer Names:: Buffers and files are different. * Getting Buffers:: Getting a buffer itself, not merely its name. * Switching Buffers:: How to change to another buffer. * Buffer Size & Locations:: Where point is located and the size of the buffer. * Evaluation Exercise:: How To Write Function Definitions * Primitive Functions:: * defun:: The @code{defun} special form. * Install:: Install a function definition. * Interactive:: Making a function interactive. * Interactive Options:: Different options for @code{interactive}. * Permanent Installation:: Installing code permanently. * let:: Creating and initializing local variables. * if:: What if? * else:: If--then--else expressions. * Truth & Falsehood:: What Lisp considers false and true. * save-excursion:: Keeping track of point, mark, and buffer. * Review:: * defun Exercises:: Install a Function Definition * Effect of installation:: * Change a defun:: How to change a function definition. Make a Function Interactive * Interactive multiply-by-seven:: An overview. * multiply-by-seven in detail:: The interactive version. @code{let} * Prevent confusion:: * Parts of let Expression:: * Sample let Expression:: * Uninitialized let Variables:: The @code{if} Special Form * if in more detail:: * type-of-animal in detail:: An example of an @code{if} expression. Truth and Falsehood in Emacs Lisp * nil explained:: @code{nil} has two meanings. @code{save-excursion} * Point and mark:: A review of various locations. * Template for save-excursion:: A Few Buffer--Related Functions * Finding More:: How to find more information. * simplified-beginning-of-buffer:: Shows @code{goto-char}, @code{point-min}, and @code{push-mark}. * mark-whole-buffer:: Almost the same as @code{beginning-of-buffer}. * append-to-buffer:: Uses @code{save-excursion} and @code{insert-buffer-substring}. * Buffer Related Review:: Review. * Buffer Exercises:: The Definition of @code{mark-whole-buffer} * mark-whole-buffer overview:: * Body of mark-whole-buffer:: Only three lines of code. The Definition of @code{append-to-buffer} * append-to-buffer overview:: * append interactive:: A two part interactive expression. * append-to-buffer body:: Incorporates a @code{let} expression. * append save-excursion:: How the @code{save-excursion} works. A Few More Complex Functions * copy-to-buffer:: With @code{set-buffer}, @code{get-buffer-create}. * insert-buffer:: Read-only, and with @code{or}. * beginning-of-buffer:: Shows @code{goto-char}, @code{point-min}, and @code{push-mark}. * Second Buffer Related Review:: * optional Exercise:: The Definition of @code{insert-buffer} * insert-buffer code:: * insert-buffer interactive:: When you can read, but not write. * insert-buffer body:: The body has an @code{or} and a @code{let}. * if & or:: Using an @code{if} instead of an @code{or}. * Insert or:: How the @code{or} expression works. * Insert let:: Two @code{save-excursion} expressions. * New insert-buffer:: The Interactive Expression in @code{insert-buffer} * Read-only buffer:: When a buffer cannot be modified. * b for interactive:: An existing buffer or else its name. Complete Definition of @code{beginning-of-buffer} * Optional Arguments:: * beginning-of-buffer opt arg:: Example with optional argument. * beginning-of-buffer complete:: @code{beginning-of-buffer} with an Argument * Disentangle beginning-of-buffer:: * Large buffer case:: * Small buffer case:: Narrowing and Widening * Narrowing advantages:: The advantages of narrowing * save-restriction:: The @code{save-restriction} special form. * what-line:: The number of the line that point is on. * narrow Exercise:: @code{car}, @code{cdr}, @code{cons}: Fundamental Functions * Strange Names:: An historical aside: why the strange names? * car & cdr:: Functions for extracting part of a list. * cons:: Constructing a list. * nthcdr:: Calling @code{cdr} repeatedly. * nth:: * setcar:: Changing the first element of a list. * setcdr:: Changing the rest of a list. * cons Exercise:: @code{cons} * Build a list:: * length:: How to find the length of a list. Cutting and Storing Text * Storing Text:: Text is stored in a list. * zap-to-char:: Cutting out text up to a character. * kill-region:: Cutting text out of a region. * copy-region-as-kill:: A definition for copying text. * Digression into C:: Minor note on C programming language macros. * defvar:: How to give a variable an initial value. * cons & search-fwd Review:: * search Exercises:: @code{zap-to-char} * Complete zap-to-char:: The complete implementation. * zap-to-char interactive:: A three part interactive expression. * zap-to-char body:: A short overview. * search-forward:: How to search for a string. * progn:: The @code{progn} special form. * Summing up zap-to-char:: Using @code{point} and @code{search-forward}. @code{kill-region} * Complete kill-region:: The function definition. * condition-case:: Dealing with a problem. * Lisp macro:: @code{copy-region-as-kill} * Complete copy-region-as-kill:: The complete function definition. * copy-region-as-kill body:: The body of @code{copy-region-as-kill}. The Body of @code{copy-region-as-kill} * last-command & this-command:: * kill-append function:: * kill-new function:: Initializing a Variable with @code{defvar} * See variable current value:: * defvar and asterisk:: How Lists are Implemented * Lists diagrammed:: * Symbols as Chest:: Exploring a powerful metaphor. * List Exercise:: Yanking Text Back * Kill Ring Overview:: * kill-ring-yank-pointer:: The kill ring is a list. * yank nthcdr Exercises:: The @code{kill-ring-yank-pointer} variable. Loops and Recursion * while:: Causing a stretch of code to repeat. * dolist dotimes:: * Recursion:: Causing a function to call itself. * Looping exercise:: @code{while} * Looping with while:: Repeat so long as test returns true. * Loop Example:: A @code{while} loop that uses a list. * print-elements-of-list:: Uses @code{while}, @code{car}, @code{cdr}. * Incrementing Loop:: A loop with an incrementing counter. * Incrementing Loop Details:: * Decrementing Loop:: A loop with a decrementing counter. Details of an Incrementing Loop * Incrementing Example:: Counting pebbles in a triangle. * Inc Example parts:: The parts of the function definition. * Inc Example altogether:: Putting the function definition together. Loop with a Decrementing Counter * Decrementing Example:: More pebbles on the beach. * Dec Example parts:: The parts of the function definition. * Dec Example altogether:: Putting the function definition together. Save your time: @code{dolist} and @code{dotimes} * dolist:: * dotimes:: Recursion * Building Robots:: Same model, different serial number ... * Recursive Definition Parts:: Walk until you stop ... * Recursion with list:: Using a list as the test whether to recurse. * Recursive triangle function:: * Recursion with cond:: * Recursive Patterns:: Often used templates. * No Deferment:: Don't store up work ... * No deferment solution:: Recursion in Place of a Counter * Recursive Example arg of 1 or 2:: * Recursive Example arg of 3 or 4:: Recursive Patterns * Every:: * Accumulate:: * Keep:: Regular Expression Searches * sentence-end:: The regular expression for @code{sentence-end}. * re-search-forward:: Very similar to @code{search-forward}. * forward-sentence:: A straightforward example of regexp search. * forward-paragraph:: A somewhat complex example. * etags:: How to create your own @file{TAGS} table. * Regexp Review:: * re-search Exercises:: @code{forward-sentence} * Complete forward-sentence:: * fwd-sentence while loops:: Two @code{while} loops. * fwd-sentence re-search:: A regular expression search. @code{forward-paragraph}: a Goldmine of Functions * forward-paragraph in brief:: Key parts of the function definition. * fwd-para let:: The @code{let*} expression. * fwd-para while:: The forward motion @code{while} loop. Counting: Repetition and Regexps * Why Count Words:: * count-words-region:: Use a regexp, but find a problem. * recursive-count-words:: Start with case of no words in region. * Counting Exercise:: The @code{count-words-region} Function * Design count-words-region:: The definition using a @code{while} loop. * Whitespace Bug:: The Whitespace Bug in @code{count-words-region}. Counting Words in a @code{defun} * Divide and Conquer:: * Words and Symbols:: What to count? * Syntax:: What constitutes a word or symbol? * count-words-in-defun:: Very like @code{count-words}. * Several defuns:: Counting several defuns in a file. * Find a File:: Do you want to look at a file? * lengths-list-file:: A list of the lengths of many definitions. * Several files:: Counting in definitions in different files. * Several files recursively:: Recursively counting in different files. * Prepare the data:: Prepare the data for display in a graph. Count Words in @code{defuns} in Different Files * lengths-list-many-files:: Return a list of the lengths of defuns. * append:: Attach one list to another. Prepare the Data for Display in a Graph * Data for Display in Detail:: * Sorting:: Sorting lists. * Files List:: Making a list of files. * Counting function definitions:: Readying a Graph * Columns of a graph:: * graph-body-print:: How to print the body of a graph. * recursive-graph-body-print:: * Printed Axes:: * Line Graph Exercise:: Your @file{.emacs} File * Default Configuration:: * Site-wide Init:: You can write site-wide init files. * defcustom:: Emacs will write code for you. * Beginning a .emacs File:: How to write a @code{.emacs file}. * Text and Auto-fill:: Automatically wrap lines. * Mail Aliases:: Use abbreviations for email addresses. * Indent Tabs Mode:: Don't use tabs with @TeX{} * Keybindings:: Create some personal keybindings. * Keymaps:: More about key binding. * Loading Files:: Load (i.e., evaluate) files automatically. * Autoload:: Make functions available. * Simple Extension:: Define a function; bind it to a key. * X11 Colors:: Colors in X. * Miscellaneous:: * Mode Line:: How to customize your mode line. Debugging * debug:: How to use the built-in debugger. * debug-on-entry:: Start debugging when you call a function. * debug-on-quit:: Start debugging when you quit with @kbd{C-g}. * edebug:: How to use Edebug, a source level debugger. * Debugging Exercises:: Handling the Kill Ring * What the Kill Ring Does:: * current-kill:: * yank:: Paste a copy of a clipped element. * yank-pop:: Insert element pointed to. * ring file:: The @code{current-kill} Function  Glenn Morris committed Jul 10, 2009 787 * Code for current-kill::  Glenn Morris committed Sep 06, 2007 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 * Understanding current-kill:: @code{current-kill} in Outline * Body of current-kill:: * Digression concerning error:: How to mislead humans, but not computers. * Determining the Element:: A Graph with Labelled Axes * Labelled Example:: * print-graph Varlist:: @code{let} expression in @code{print-graph}. * print-Y-axis:: Print a label for the vertical axis. * print-X-axis:: Print a horizontal label. * Print Whole Graph:: The function to print a complete graph. The @code{print-Y-axis} Function * print-Y-axis in Detail:: * Height of label:: What height for the Y axis? * Compute a Remainder:: How to compute the remainder of a division. * Y Axis Element:: Construct a line for the Y axis. * Y-axis-column:: Generate a list of Y axis labels. * print-Y-axis Penultimate:: A not quite final version. The @code{print-X-axis} Function * Similarities differences:: Much like @code{print-Y-axis}, but not exactly. * X Axis Tic Marks:: Create tic marks for the horizontal axis. Printing the Whole Graph * The final version:: A few changes. * Test print-graph:: Run a short test. * Graphing words in defuns:: Executing the final code. * lambda:: How to write an anonymous function. * mapcar:: Apply a function to elements of a list. * Another Bug:: Yet another bug @dots{} most insidious. * Final printed graph:: The graph itself! @end detailmenu @end menu @node Preface, List Processing, Top, Top @comment node-name, next, previous, up @unnumbered Preface 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.) @menu * Why:: Why learn Emacs Lisp? * On Reading this Text:: Read, gain familiarity, pick up habits.... * Who You Are:: For whom this is written. * Lisp History:: * Note for Novices:: You can read this as a novice. * Thank You:: @end menu @node Why, On Reading this Text, Preface, Preface @ifnottex @unnumberedsec Why Study Emacs Lisp? @end ifnottex 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. @node On Reading this Text, Who You Are, Why, Preface @comment node-name, next, previous, up @unnumberedsec On Reading this Text 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 @kbd{M-.} (the key which invokes the @code{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. @ignore In addition, I have written several programs as extended examples. Although these are examples, the programs are real. I use them. Other people use them. You may use them. Beyond the fragments of programs used for illustrations, there is very little in here that is just for teaching purposes'; what you see is used. This is a great advantage of Emacs Lisp: it is easy to learn to use it for work. @end ignore 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. @node Who You Are, Lisp History, On Reading this Text, Preface @comment node-name, next, previous, up @unnumberedsec For Whom This is Written 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: @quotation @i{I prefer to learn from reference manuals. I dive into'' each paragraph, and come up for air'' between paragraphs.} @i{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.} @end quotation 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 @cite{Programming in Emacs Lisp} has a companion document, @iftex @cite{The GNU Emacs Lisp Reference Manual}. @end iftex @ifnottex @ref{Top, , The GNU Emacs Lisp Reference Manual, elisp, The GNU Emacs Lisp Reference Manual}. @end ifnottex 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 @cite{Introduction}, you will find the @cite{Reference Manual} useful when you are writing your own programs. @node Lisp History, Note for Novices, Who You Are, Preface @unnumberedsec Lisp History @cindex Lisp history 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. @cindex Maclisp @cindex Common Lisp 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, @file{cl.el}, that adds many Common Lisp features to Emacs Lisp.) @node Note for Novices, Thank You, Lisp History, Preface @comment node-name, next, previous, up @unnumberedsec A Note for Novices 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 @kbd{C-h t}. (This means you press and release the @key{CTRL} key and the @kbd{h} at the same time, and then press and release @kbd{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: @kbd{M-C-\} (@code{indent-region}). What this means is that the @code{indent-region} command is customarily invoked by typing @kbd{M-C-\}. (You can, if you wish, change the keys that are typed to invoke the command; this is called @dfn{rebinding}. @xref{Keymaps, , Keymaps}.) The abbreviation @kbd{M-C-\} means that you type your @key{META} key, @key{CTRL} key and @key{\} key all at the same time. (On many modern keyboards the @key{META} key is labelled @key{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 @key{META} key, the @key{ESC} key prefix is used in place of it. In this case, @kbd{M-C-\} means that you press and release your @key{ESC} key and then type the @key{CTRL} key and the @key{\} key at the same time. But usually @kbd{M-C-\} means press the @key{CTRL} key along with the key that is labelled @key{ALT} and, at the same time, press the @key{\} key. In addition to typing a lone keychord, you can prefix what you type with @kbd{C-u}, which is called the universal argument'. The @kbd{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 @w{@kbd{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.) @xref{Arguments, , Numeric Arguments, emacs, The GNU Emacs Manual}. If you are reading this in Info using GNU Emacs, you can read through this whole document just by pressing the space bar, @key{SPC}. (To learn about Info, type @kbd{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. @node Thank You, , Note for Novices, Preface @comment node-name, next, previous, up @unnumberedsec Thank You My thanks to all who helped me with this book. My especial thanks to @r{Jim Blandy}, @r{Noah Friedman}, @w{Jim Kingdon}, @r{Roland McGrath}, @w{Frank Ritter}, @w{Randy Smith}, @w{Richard M.@: Stallman}, and @w{Melissa Weisshaus}. My thanks also go to both @w{Philip Johnson} and @w{David Stampe} for their patient encouragement. My mistakes are my own. @flushright Robert J. Chassell  Robert J. Chassell committed Sep 12, 2007 1067 @email{bob@@gnu.org}  Glenn Morris committed Sep 06, 2007 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 @end flushright @c ================ Beginning of main text ================ @c Start main text on right-hand (verso) page @tex \par\vfill\supereject \headings off \ifodd\pageno \par\vfill\supereject \else \par\vfill\supereject \page\hbox{}\page \par\vfill\supereject \fi @end tex @iftex @headings off @evenheading @thispage @| @| @thischapter @oddheading @thissection @| @| @thispage @global@pageno = 1 @end iftex @node List Processing, Practicing Evaluation, Preface, Top @comment node-name, next, previous, up @chapter List Processing To the untutored eye, Lisp is a strange programming language. In Lisp code there are parentheses everywhere. Some people even claim that the name stands for Lots of Isolated Silly Parentheses'. But the claim is unwarranted. Lisp stands for LISt Processing, and the programming language handles @emph{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, @samp{'}@footnote{The single apostrophe or quotation mark is an abbreviation for the function @code{quote}; you need not think about functions now; functions are defined in @ref{Making Errors, , Generate an Error Message}.} Lists are the basis of Lisp. @menu * Lisp Lists:: What are lists? * Run a Program:: Any list in Lisp is a program ready to run. * Making Errors:: Generating an error message. * Names & Definitions:: Names of symbols and function definitions. * Lisp Interpreter:: What the Lisp interpreter does. * Evaluation:: Running a program. * Variables:: Returning a value from a variable. * Arguments:: Passing information to a function. * set & setq:: Setting the value of a variable. * Summary:: The major points. * Error Message Exercises:: @end menu @node Lisp Lists, Run a Program, List Processing, List Processing @comment node-name, next, previous, up @section Lisp Lists @cindex Lisp Lists In Lisp, a list looks like this: @code{'(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: @smallexample @group '(rose violet daisy buttercup) @end group @end smallexample @noindent 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. @cindex Flowers in a field @menu * Numbers Lists:: List have numbers, other lists, in them. * Lisp Atoms:: Elemental entities. * Whitespace in Lists:: Formatting lists to be readable. * Typing Lists:: How GNU Emacs helps you type lists. @end menu @node Numbers Lists, Lisp Atoms, Lisp Lists, Lisp Lists @ifnottex @unnumberedsubsec Numbers, Lists inside of Lists @end ifnottex Lists can also have numbers in them, as in this list: @code{(+ 2 2)}. This list has a plus-sign, @samp{+}, followed by two @samp{2}s, 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 @emph{not} Lisp lists, because they contain @samp{;} and @samp{.} as punctuation marks.) @need 1200 Here is another list, this time with a list inside of it: @smallexample '(this list has (a list inside of it)) @end smallexample The components of this list are the words @samp{this}, @samp{list}, @samp{has}, and the list @samp{(a list inside of it)}. The interior list is made up of the words @samp{a}, @samp{list}, @samp{inside}, @samp{of}, @samp{it}. @node Lisp Atoms, Whitespace in Lists, Numbers Lists, Lisp Lists @comment node-name, next, previous, up @subsection Lisp Atoms @cindex Lisp Atoms In Lisp, what we have been calling words are called @dfn{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 @samp{+}. On the other hand, unlike an ancient atom, a list can be split into parts. (@xref{car cdr & cons, , @code{car} @code{cdr} & @code{cons} Fundamental Functions}.) In a list, atoms are separated from each other by whitespace. They can be right next to a parenthesis. @cindex @samp{empty list} defined 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: @code{()}, and is called the @dfn{empty list}. Unlike anything else, an empty list is considered both an atom and a list at the same time. @cindex Symbolic expressions, introduced @cindex @samp{expression} defined @cindex @samp{form} defined The printed representation of both atoms and lists are called @dfn{symbolic expressions} or, more concisely, @dfn{s-expressions}. The word @dfn{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 @dfn{expression} indiscriminately. (Also, in many texts, the word @dfn{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 @samp{ai}, is completely different from the two words, @samp{a}, and @samp{i}. There are many kinds of atom in nature but only a few in Lisp: for example, @dfn{numbers}, such as 37, 511, or 1729, and @dfn{symbols}, such as @samp{+}, @samp{foo}, or @samp{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.) @need 1250  Chong Yidong committed May 13, 2008 1248 1249 Text between double quotation marks---even sentences or paragraphs---is also an atom. 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2638 2639 2640 2641 2642 2643 2644 2645 2646 2647 2648 2649 2650 2651 2652 2653 2654 2655 2656 2657 2658 2659 2660 2661 2662 2663 2664 2665 2666 2667 2668 2669 2670 2671 2672 2673 2674 2675 2676 2677 2678 2679 2680 2681 2682 2683 2684 2685 2686 2687 2688 2689 2690 2691 2692 2693 2694 2695 2696 2697 2698 2699 2700 2701 2702 2703 2704 2705 2706 2707 2708 2709 2710 2711 2712 2713 2714 2715 2716 2717 2718 2719 2720 2721 2722 2723 2724 2725 2726 2727 2728 2729 2730 2731 2732 2733 @cindex Text between double quotation marks @smallexample '(this list includes "text between quotation marks.") @end smallexample @cindex @samp{string} defined @noindent 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 @dfn{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. @node Whitespace in Lists, Typing Lists, Lisp Atoms, Lisp Lists @comment node-name, next, previous, up @subsection Whitespace in Lists @cindex Whitespace in lists @need 1200 The amount of whitespace in a list does not matter. From the point of view of the Lisp language, @smallexample @group '(this list looks like this) @end group @end smallexample @need 800 @noindent is exactly the same as this: @smallexample '(this list looks like this) @end smallexample Both examples show what to Lisp is the same list, the list made up of the symbols @samp{this}, @samp{list}, @samp{looks}, @samp{like}, and @samp{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.) @node Typing Lists, , Whitespace in Lists, Lisp Lists @comment node-name, next, previous, up @subsection GNU Emacs Helps You Type Lists @cindex Help typing lists @cindex Formatting help 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 @key{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 @kbd{M-C-\}. Indentation is designed so that you can see which elements of a list belong 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. (@xref{Major Modes, , Major Modes, emacs, The GNU Emacs Manual}, for more information about Emacs' modes.) @node Run a Program, Making Errors, Lisp Lists, List Processing @comment node-name, next, previous, up @section Run a Program @cindex Run a program @cindex Program, running one @cindex @samp{evaluate} defined A list in Lisp---any list---is a program ready to run. If you run it (for which the Lisp jargon is @dfn{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!) @c use code for the single apostrophe, not samp. The single apostrophe, @code{'}, that I put in front of some of the example lists in preceding sections is called a @dfn{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 @emph{functions}.) The list @code{(+ 2 2)} shown above did not have a quote in front of it, so Lisp understands that the @code{+} is an instruction to do something with the rest of the list: add the numbers that follow. @need 1250 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 @kbd{C-x C-e}: @smallexample (+ 2 2) @end smallexample @c use code for the number four, not samp. @noindent You will see the number @code{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 @kbd{C-x C-e}: @smallexample '(this is a quoted list) @end smallexample @noindent You will see @code{(this is a quoted list)} appear in the echo area. @cindex Lisp interpreter, explained @cindex Interpreter, Lisp, explained In both cases, what you are doing is giving a command to the program inside of GNU Emacs called the @dfn{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 (@pxref{Variables}), we will discuss what the Lisp interpreter does when you make an error. @node Making Errors, Names & Definitions, Run a Program, List Processing @comment node-name, next, previous, up @section Generate an Error Message @cindex Generate an error message @cindex Error message generation 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 @code{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 @kbd{C-x C-e}: @smallexample (this is an unquoted list) @end smallexample @noindent What you see depends on which version of Emacs you are running. GNU Emacs version 22 provides more information than version 20 and before. First, the more recent result of generating an error; then the earlier, version 20 result. @need 1250 @noindent In GNU Emacs version 22, a @file{*Backtrace*} window will open up and you will see the following in it: @smallexample @group ---------- 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* ---------- @end group @end smallexample @need 1200 @noindent 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: @smallexample q @end smallexample @noindent Please type @kbd{q} right now, so you become confident that you can get out of the debugger. Then, type @kbd{C-x C-e} again to re-enter it. @cindex @samp{function} defined Based on what we already know, we can almost read this error message. You read the @file{*Backtrace*} buffer from the bottom up; it tells you what Emacs did. When you typed @kbd{C-x C-e}, you made an interactive call to the command @code{eval-last-sexp}. @code{eval} is an abbreviation for evaluate' and @code{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 @file{*Backtrace*} buffer because it enables you to track Emacs backwards. @need 800 At the top of the @file{*Backtrace*} buffer, you see the line: @smallexample Debugger entered--Lisp error: (void-function this) @end smallexample @noindent The Lisp interpreter tried to evaluate the first atom of the list, the word @samp{this}. It is this action that generated the error message @samp{void-function this}. The message contains the words @samp{void-function} and @samp{this}. @cindex @samp{function} defined The word @samp{function} was mentioned once before. It is a very important word. For our purposes, we can define it by saying that a @dfn{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: @samp{void-function this}. The function (that is, the word @samp{this}) does not have a definition of any set of instructions for the computer to carry out. The slightly odd word, @samp{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 @code{(+ 2 2)}, we can infer that the symbol @code{+} must have a set of instructions for the computer to obey and those instructions must be to add the numbers that follow the @code{+}. @need 1250 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: @smallexample Symbol's function definition is void:@: this @end smallexample @noindent (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 @samp{Symbol}. It refers to the first atom of the list, the word @samp{this}. The word @samp{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: @samp{Symbol's function definition is void:@: this}. The symbol (that is, the word @samp{this}) lacks instructions for the computer to carry out. @node Names & Definitions, Lisp Interpreter, Making Errors, List Processing @comment node-name, next, previous, up @section Symbol Names and Function Definitions @cindex Symbol names We can articulate another characteristic of Lisp based on what we have discussed so far---an important characteristic: a symbol, like @code{+}, 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 @samp{Bob}; however, I am not the letters @samp{B}, @samp{o}, @samp{b} but am, or was, 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 @code{plus} as well as to the symbol @code{+} (and are in some dialects of Lisp). Among humans, I can be referred to as @samp{Robert} as well as @samp{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 @samp{Bob}. However, the function definition to which the name refers can be changed readily. (@xref{Install, , 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 @samp{texinfo-} and those for functions that deal with reading mail start with @samp{rmail-}. @node Lisp Interpreter, Evaluation, Names & Definitions, List Processing @comment node-name, next, previous, up @section The Lisp Interpreter @cindex Lisp interpreter, what it does @cindex Interpreter, what it does 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. @menu * Complications:: Variables, Special forms, Lists within. * Byte Compiling:: Specially processing code for speed. @end menu @node Complications, Byte Compiling, Lisp Interpreter, Lisp Interpreter @ifnottex @unnumberedsubsec Complications @end ifnottex 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 @dfn{variable}. This situation is described in the section on variables. (@xref{Variables}.) @cindex Special form The second complication occurs because some functions are unusual and do not work in the usual manner. Those that don't are called @dfn{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. @node Byte Compiling, , Complications, Lisp Interpreter @subsection Byte Compiling @cindex Byte compiling 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 @dfn{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 @code{byte-compile-file}. Byte compiled code is usually stored in a file that ends with a @file{.elc} extension rather than a @file{.el} extension. You will see both kinds of file in the @file{emacs/lisp} directory; the files to read are those with @file{.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. @xref{Byte Compilation, , Byte Compilation, elisp, The GNU Emacs Lisp Reference Manual}, for a full description of byte compilation. @node Evaluation, Variables, Lisp Interpreter, List Processing @comment node-name, next, previous, up @section Evaluation @cindex Evaluation When the Lisp interpreter works on an expression, the term for the activity is called @dfn{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 @cite{Webster's New Collegiate Dictionary}. @menu * How the Interpreter Acts:: Returns and Side Effects... * Evaluating Inner Lists:: Lists within lists... @end menu @node How the Interpreter Acts, Evaluating Inner Lists, Evaluation, Evaluation @ifnottex @unnumberedsubsec How the Lisp Interpreter Acts @end ifnottex @cindex @samp{returned value} explained After evaluating an expression, the Lisp interpreter will most likely @dfn{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. @cindex @samp{side effect} defined 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 @dfn{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. @node Evaluating Inner Lists, , How the Interpreter Acts, Evaluation @comment node-name, next, previous, up @subsection Evaluating Inner Lists @cindex Inner list evaluation @cindex Evaluating inner lists 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. @need 1250 We can investigate this process by evaluating another addition example. Place your cursor after the following expression and type @kbd{C-x C-e}: @smallexample (+ 2 (+ 3 3)) @end smallexample @noindent The number 8 will appear in the echo area. What happens is that the Lisp interpreter first evaluates the inner expression, @code{(+ 3 3)}, for which the value 6 is returned; then it evaluates the outer expression as if it were written @code{(+ 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 @kbd{C-x C-e}: the name is @code{eval-last-sexp}. The letters @code{sexp} are an abbreviation for symbolic expression', and @code{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. @need 800 Here is another copy of the expression: @smallexample (+ 2 (+ 3 3)) @end smallexample @noindent If you place the cursor at the beginning of the blank line that immediately follows the expression and type @kbd{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 @code{(+ 3 3)}. Now put the cursor immediately after a number. Type @kbd{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 @code{+}, 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. @node Variables, Arguments, Evaluation, List Processing @comment node-name, next, previous, up @section Variables @cindex Variables 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 @dfn{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''. @ignore (Incidentally, in Emacs Lisp, a symbol can have two other things attached to it, too: a property list and a documentation string; these are discussed later.) @end ignore 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-verse. @menu * fill-column Example:: * Void Function:: The error message for a symbol without a function. * Void Variable:: The error message for a symbol without a value. @end menu @node fill-column Example, Void Function, Variables, Variables @ifnottex @unnumberedsubsec @code{fill-column}, an Example Variable @end ifnottex @findex fill-column, @r{an example variable} @cindex Example variable, @code{fill-column} @cindex Variable, example of, @code{fill-column} The variable @code{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 @kbd{C-x C-e}: @smallexample fill-column @end smallexample @noindent After I typed @kbd{C-x C-e}, Emacs printed the number 72 in my echo area. This is the value for which @code{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 @dfn{bind} the variable to a value: to a number, such as 72; to a string, @code{"such as this"}; to a list, such as @code{(spruce pine oak)}; we can even bind a variable to a function definition. A symbol can be bound to a value in several ways. @xref{set & setq, , Setting the Value of a Variable}, for information about one way to do this. @node Void Function, Void Variable, fill-column Example, Variables @comment node-name, next, previous, up @subsection Error Message for a Symbol Without a Function @cindex Symbol without function error @cindex Error for symbol without function When we evaluated @code{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 @code{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 @code{fill-column} has no function definition. Try evaluating this: @smallexample (fill-column) @end smallexample @need 1250 @noindent In GNU Emacs version 22, you will create a @file{*Backtrace*} buffer that says: @smallexample @group ---------- Buffer: *Backtrace* ----------