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This is emacs-lisp-intro.info, produced by makeinfo version 4.0b from emacs-lisp-intro.texi. INFO-DIR-SECTION Emacs START-INFO-DIR-ENTRY * Emacs Lisp Intro: (eintr). A simple introduction to Emacs Lisp programming. END-INFO-DIR-ENTRY This is an introduction to `Programming in Emacs Lisp', for people who are not programmers. Edition 2.04, 2001 Dec 17 Copyright (C) 1990, '91, '92, '93, '94, '95, '97, 2001 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Section being the Preface, with the Front-Cover Texts being no Front-Cover Texts, and with the Back-Cover Texts being no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License". File: emacs-lisp-intro.info, Node: Mode Line, Prev: Miscellaneous, Up: Emacs Initialization A Modified Mode Line ==================== Finally, a feature I really like: a modified mode line. When I work over a network, I forget which machine I am using. Also, I tend to I lose track of where I am, and which line point is on. So I reset my mode line to look like this: -:-- foo.texi rattlesnake:/home/bob/ Line 1 (Texinfo Fill) Top I am visiting a file called `foo.texi', on my machine `rattlesnake' in my `/home/bob' buffer. I am on line 1, in Texinfo mode, and am at the top of the buffer. My `.emacs' file has a section that looks like this: ;; Set a Mode Line that tells me which machine, which directory, ;; and which line I am on, plus the other customary information. (setq default-mode-line-format (quote (#("-" 0 1 (help-echo "mouse-1: select window, mouse-2: delete others ...")) mode-line-mule-info mode-line-modified mode-line-frame-identification " " mode-line-buffer-identification " " (:eval (substring (system-name) 0 (string-match "\\..+" (system-name)))) ":" default-directory #(" " 0 1 (help-echo "mouse-1: select window, mouse-2: delete others ...")) (line-number-mode " Line %l ") global-mode-string #(" %[(" 0 6 (help-echo "mouse-1: select window, mouse-2: delete others ...")) (:eval (mode-line-mode-name)) mode-line-process minor-mode-alist #("%n" 0 2 (help-echo "mouse-2: widen" local-map (keymap ...))) ")%] " (-3 . "%P") ;; "-%-" ))) Here, I redefine the default mode line. Most of the parts are from the original; but I make a few changes. I set the _default_ mode line format so as to permit various modes, such as Info, to override it. Many elements in the list are self-explanatory: `mode-line-modified' is a variable that tells whether the buffer has been modified, `mode-name' tells the name of the mode, and so on. However, the format looks complicated because of two features we have not discussed. The first string in the mode line is a dash or hyphen, `-'. In the old days, it would have been specified simply as `"-"'. But nowadays, Emacs can add properties to a string, such as highlighting or, as in this case, a help feature. If you place your mouse cursor over the hyphen, some help information appears (By default, you must wait one second before the information appears. You can change that timing by changing the value of `tooltip-delay'.) The new string format has a special syntax: #("-" 0 1 (help-echo "mouse-1: select window, ...")) The `#(' begins a list. The first element of the list is the string itself, just one `-'. The second and third elements specify the range over which the fourth element applies. A range starts _after_ a character, so a zero means the range starts just before the first character; a 1 means that the range ends just after the first character. The third element is the property for the range. It consists of a property list, a property name, in this case, `help-echo', followed by a value, in this case, a string. The second, third, and fourth elements of this new string format can be repeated. *Note Text Properties in String: (elisp)Text Props and Strings, and see *Note Mode Line Format: (elisp)Mode Line Format, for more information. `mode-line-buffer-identification' displays the current buffer name. It is a list beginning `(#("%12b" 0 4 ...'. The `#(' begins the list. The `"%12b"' displays the current buffer name, using the `buffer-name' function with which we are familiar; the `12' specifies the maximum number of characters that will be displayed. When a name has fewer characters, whitespace is added to fill out to this number. (Buffer names can and often should be longer than 12 characters; this length works well in a typical 80 column wide window.) `:eval' is a new feature in GNU Emacs version 21. It says to evaluate the following form and use the result as a string to display. In this case, the expression displays the first component of the full system name. The end of the first component is a `.' (`period'), so I use the `string-match' function to tell me the length of the first component. The substring from the zeroth character to that length is the name of the machine. This is the expression: (:eval (substring (system-name) 0 (string-match "\\..+" (system-name)))) `%[' and `%]' cause a pair of square brackets to appear for each recursive editing level. `%n' says `Narrow' when narrowing is in effect. `%P' tells you the percentage of the buffer that is above the bottom of the window, or `Top', `Bottom', or `All'. (A lower case `p' tell you the percentage above the _top_ of the window.) `%-' inserts enough dashes to fill out the line. Remember, "You don't have to like Emacs to like it" -- your own Emacs can have different colors, different commands, and different keys than a default Emacs. On the other hand, if you want to bring up a plain `out of the box' Emacs, with no customization, type: emacs -q This will start an Emacs that does _not_ load your `~/.emacs' initialization file. A plain, default Emacs. Nothing more. File: emacs-lisp-intro.info, Node: Debugging, Next: Conclusion, Prev: Emacs Initialization, Up: Top Debugging ********* GNU Emacs has two debuggers, `debug' and `edebug'. The first is built into the internals of Emacs and is always with you; the second requires that you instrument a function before you can use it. Both debuggers are described extensively in *Note Debugging Lisp Programs: (elisp)Debugging. In this chapter, I will walk through a short example of each. * Menu: * 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 C-g. * edebug:: How to use Edebug, a source level debugger. * Debugging Exercises:: File: emacs-lisp-intro.info, Node: debug, Next: debug-on-entry, Prev: Debugging, Up: Debugging `debug' ======= Suppose you have written a function definition that is intended to return the sum of the numbers 1 through a given number. (This is the `triangle' function discussed earlier. *Note Example with Decrementing Counter: Decrementing Example, for a discussion.) However, your function definition has a bug. You have mistyped `1=' for `1-'. Here is the broken definition: (defun triangle-bugged (number) "Return sum of numbers 1 through NUMBER inclusive." (let ((total 0)) (while (> number 0) (setq total (+ total number)) (setq number (1= number))) ; Error here. total)) If you are reading this in Info, you can evaluate this definition in the normal fashion. You will see `triangle-bugged' appear in the echo area. Now evaluate the `triangle-bugged' function with an argument of 4: (triangle-bugged 4) In GNU Emacs version 21, you will create and enter a `*Backtrace*' buffer that says: ---------- Buffer: *Backtrace* ---------- Debugger entered--Lisp error: (void-function 1=) (1= number) (setq number (1= number)) (while (> number 0) (setq total (+ total number)) (setq number (1= number))) (let ((total 0)) (while (> number 0) (setq total ...) (setq number ...)) total) triangle-bugged(4) eval((triangle-bugged 4)) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ---------- Buffer: *Backtrace* ---------- (I have reformatted this example slightly; the debugger does not fold long lines. As usual, you can quit the debugger by typing `q' in the `*Backtrace*' buffer.) In practice, for a bug as simple as this, the `Lisp error' line will tell you what you need to know to correct the definition. The function `1=' is `void'. In GNU Emacs 20 and before, you will see: Symbol's function definition is void: 1= which has the same meaning as the `*Backtrace*' buffer line in version 21. However, suppose you are not quite certain what is going on? You can read the complete backtrace. In this case, you need to run GNU Emacs 21, which automatically starts the debugger that puts you in the `*Backtrace*' buffer; or else, you need to start the debugger manually as described below. Read the `*Backtrace*' buffer from the bottom up; it tells you what Emacs did that led to the error. Emacs made an interactive call to `C-x C-e' (`eval-last-sexp'), which led to the evaluation of the `triangle-bugged' expression. Each line above tells you what the Lisp interpreter evaluated next. The third line from the top of the buffer is (setq number (1= number)) Emacs tried to evaluate this expression; in order to do so, it tried to evaluate the inner expression shown on the second line from the top: (1= number) This is where the error occurred; as the top line says: Debugger entered--Lisp error: (void-function 1=) You can correct the mistake, re-evaluate the function definition, and then run your test again. File: emacs-lisp-intro.info, Node: debug-on-entry, Next: debug-on-quit, Prev: debug, Up: Debugging `debug-on-entry' ================ GNU Emacs 21 starts the debugger automatically when your function has an error. GNU Emacs version 20 and before did not; it simply presented you with an error message. You had to start the debugger manually. You can start the debugger manually for all versions of Emacs; the advantage is that the debugger runs even if you do not have a bug in your code. Sometimes your code will be free of bugs! You can enter the debugger when you call the function by calling `debug-on-entry'. Type: M-x debug-on-entry RET triangle-bugged RET Now, evaluate the following: (triangle-bugged 5) All versions of Emacs will create a `*Backtrace*' buffer and tell you that it is beginning to evaluate the `triangle-bugged' function: ---------- Buffer: *Backtrace* ---------- Debugger entered--entering a function: * triangle-bugged(5) eval((triangle-bugged 5)) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ---------- Buffer: *Backtrace* ---------- In the `*Backtrace*' buffer, type `d'. Emacs will evaluate the first expression in `triangle-bugged'; the buffer will look like this: ---------- Buffer: *Backtrace* ---------- Debugger entered--beginning evaluation of function call form: * (let ((total 0)) (while (> number 0) (setq total ...) (setq number ...)) total) * triangle-bugged(5) eval((triangle-bugged 5)) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ---------- Buffer: *Backtrace* ---------- Now, type `d' again, eight times, slowly. Each time you type `d', Emacs will evaluate another expression in the function definition. Eventually, the buffer will look like this: ---------- Buffer: *Backtrace* ---------- Debugger entered--beginning evaluation of function call form: * (setq number (1= number)) * (while (> number 0) (setq total (+ total number)) (setq number (1= number))) * (let ((total 0)) (while (> number 0) (setq total ...) (setq number ...)) total) * triangle-bugged(5) eval((triangle-bugged 5)) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ---------- Buffer: *Backtrace* ---------- Finally, after you type `d' two more times, Emacs will reach the error, and the top two lines of the `*Backtrace*' buffer will look like this: ---------- Buffer: *Backtrace* ---------- Debugger entered--Lisp error: (void-function 1=) * (1= number) ... ---------- Buffer: *Backtrace* ---------- By typing `d', you were able to step through the function. You can quit a `*Backtrace*' buffer by typing `q' in it; this quits the trace, but does not cancel `debug-on-entry'. To cancel the effect of `debug-on-entry', call `cancel-debug-on-entry' and the name of the function, like this: M-x cancel-debug-on-entry RET triangle-bugged RET (If you are reading this in Info, cancel `debug-on-entry' now.) File: emacs-lisp-intro.info, Node: debug-on-quit, Next: edebug, Prev: debug-on-entry, Up: Debugging `debug-on-quit' and `(debug)' ============================= In addition to setting `debug-on-error' or calling `debug-on-entry', there are two other ways to start `debug'. You can start `debug' whenever you type `C-g' (`keyboard-quit') by setting the variable `debug-on-quit' to `t'. This is useful for debugging infinite loops. Or, you can insert a line that says `(debug)' into your code where you want the debugger to start, like this: (defun triangle-bugged (number) "Return sum of numbers 1 through NUMBER inclusive." (let ((total 0)) (while (> number 0) (setq total (+ total number)) (debug) ; Start debugger. (setq number (1= number))) ; Error here. total)) The `debug' function is described in detail in *Note The Lisp Debugger: (elisp)Debugger. File: emacs-lisp-intro.info, Node: edebug, Next: Debugging Exercises, Prev: debug-on-quit, Up: Debugging The `edebug' Source Level Debugger ================================== Edebug is a source level debugger. Edebug normally displays the source of the code you are debugging, with an arrow at the left that shows which line you are currently executing. You can walk through the execution of a function, line by line, or run quickly until reaching a "breakpoint" where execution stops. Edebug is described in *Note Edebug: (elisp)edebug. Here is a bugged function definition for `triangle-recursively'. *Note Recursion in place of a counter: Recursive triangle function, for a review of it. (defun triangle-recursively-bugged (number) "Return sum of numbers 1 through NUMBER inclusive. Uses recursion." (if (= number 1) 1 (+ number (triangle-recursively-bugged (1= number))))) ; Error here. Normally, you would install this definition by positioning your cursor after the function's closing parenthesis and typing `C-x C-e' (`eval-last-sexp') or else by positioning your cursor within the definition and typing `C-M-x' (`eval-defun'). (By default, the `eval-defun' command works only in Emacs Lisp mode or in Lisp Interactive mode.) However, to prepare this function definition for Edebug, you must first "instrument" the code using a different command. You can do this by positioning your cursor within the definition and typing M-x edebug-defun RET This will cause Emacs to load Edebug automatically if it is not already loaded, and properly instrument the function. After instrumenting the function, place your cursor after the following expression and type `C-x C-e' (`eval-last-sexp'): (triangle-recursively-bugged 3) You will be jumped back to the source for `triangle-recursively-bugged' and the cursor positioned at the beginning of the `if' line of the function. Also, you will see an arrowhead at the left hand side of that line. The arrowhead marks the line where the function is executing. (In the following examples, we show the arrowhead with `=>'; in a windowing system, you may see the arrowhead as a solid triangle in the window `fringe'.) =>-!-(if (= number 1) In the example, the location of point is displayed as `-!-' (in a printed book, it is displayed with a five pointed star). If you now press <SPC>, point will move to the next expression to be executed; the line will look like this: =>(if -!-(= number 1) As you continue to press <SPC>, point will move from expression to expression. At the same time, whenever an expression returns a value, that value will be displayed in the echo area. For example, after you move point past `number', you will see the following: Result: 3 = C-c This means the value of `number' is 3, which is ASCII `control-c' (the third letter of the alphabet). You can continue moving through the code until you reach the line with the error. Before evaluation, that line looks like this: => -!-(1= number))))) ; Error here. When you press <SPC> once again, you will produce an error message that says: Symbol's function definition is void: 1= This is the bug. Press `q' to quit Edebug. To remove instrumentation from a function definition, simply re-evaluate it with a command that does not instrument it. For example, you could place your cursor after the definition's closing parenthesis and type `C-x C-e'. Edebug does a great deal more than walk with you through a function. You can set it so it races through on its own, stopping only at an error or at specified stopping points; you can cause it to display the changing values of various expressions; you can find out how many times a function is called, and more. Edebug is described in *Note Edebug: (elisp)edebug. File: emacs-lisp-intro.info, Node: Debugging Exercises, Prev: edebug, Up: Debugging Debugging Exercises =================== * Install the `count-words-region' function and then cause it to enter the built-in debugger when you call it. Run the command on a region containing two words. You will need to press `d' a remarkable number of times. On your system, is a `hook' called after the command finishes? (For information on hooks, see *Note Command Loop Overview: (elisp)Command Overview.) * Copy `count-words-region' into the `*scratch*' buffer, instrument the function for Edebug, and walk through its execution. The function does not need to have a bug, although you can introduce one if you wish. If the function lacks a bug, the walk-through completes without problems. * While running Edebug, type `?' to see a list of all the Edebug commands. (The `global-edebug-prefix' is usually `C-x X', i.e. `<CTL>-x' followed by an upper case `X'; use this prefix for commands made outside of the Edebug debugging buffer.) * In the Edebug debugging buffer, use the `p' (`edebug-bounce-point') command to see where in the region the `count-words-region' is working. * Move point to some spot further down function and then type the `h' (`edebug-goto-here') command to jump to that location. * Use the `t' (`edebug-trace-mode') command to cause Edebug to walk through the function on its own; use an upper case `T' for `edebug-Trace-fast-mode'. * Set a breakpoint, then run Edebug in Trace mode until it reaches the stopping point. File: emacs-lisp-intro.info, Node: Conclusion, Next: the-the, Prev: Debugging, Up: Top Conclusion ********** We have now reached the end of this Introduction. You have now learned enough about programming in Emacs Lisp to set values, to write simple `.emacs' files for yourself and your friends, and write simple customizations and extensions to Emacs. This is a place to stop. Or, if you wish, you can now go onward, and teach yourself. You have learned some of the basic nuts and bolts of programming. But only some. There are a great many more brackets and hinges that are easy to use that we have not touched. A path you can follow right now lies among the sources to GNU Emacs and in *Note The GNU Emacs Lisp Reference Manual: (elisp)Top. The Emacs Lisp sources are an adventure. When you read the sources and come across a function or expression that is unfamiliar, you need to figure out or find out what it does. Go to the Reference Manual. It is a thorough, complete, and fairly easy-to-read description of Emacs Lisp. It is written not only for experts, but for people who know what you know. (The `Reference Manual' comes with the standard GNU Emacs distribution. Like this introduction, it comes as a Texinfo source file, so you can read it on-line and as a typeset, printed book.) Go to the other on-line help that is part of GNU Emacs: the on-line documentation for all functions, and `find-tags', the program that takes you to sources. Here is an example of how I explore the sources. Because of its name, `simple.el' is the file I looked at first, a long time ago. As it happens some of the functions in `simple.el' are complicated, or at least look complicated at first sight. The `open-line' function, for example, looks complicated. You may want to walk through this function slowly, as we did with the `forward-sentence' function. (*Note forward-sentence::.) Or you may want to skip that function and look at another, such as `split-line'. You don't need to read all the functions. According to `count-words-in-defun', the `split-line' function contains 27 words and symbols. Even though it is short, `split-line' contains four expressions we have not studied: `skip-chars-forward', `indent-to', `current-column' and `?\n'. Consider the `skip-chars-forward' function. (It is part of the function definition for `back-to-indentation', which is shown in *Note Review: Review.) In GNU Emacs, you can find out more about `skip-chars-forward' by typing `C-h f' (`describe-function') and the name of the function. This gives you the function documentation. You may be able to guess what is done by a well named function such as `indent-to'; or you can look it up, too. Incidentally, the `describe-function' function itself is in `help.el'; it is one of those long, but decipherable functions. You can look up `describe-function' using the `C-h f' command! In this instance, since the code is Lisp, the `*Help*' buffer contains the name of the library containing the function's source. You can put point over the name of the library and press the RET key, which in this situation is bound to `help-follow', and be taken directly to the source, in the same way as `M-.' (`find-tag'). The definition for `describe-function' illustrates how to customize the `interactive' expression without using the standard character codes; and it shows how to create a temporary buffer. (The `indent-to' function is written in C rather than Emacs Lisp; it is a `built-in' function. `help-follow' only provides you with the documentation of a built-in function; it does not take you to the source. But `find-tag' will take you to the source, if properly set up.) You can look at a function's source using `find-tag', which is bound to `M-.' Finally, you can find out what the Reference Manual has to say by visiting the manual in Info, and typing `i' (`Info-index') and the name of the function, or by looking up `skip-chars-forward' in the index to a printed copy of the manual. Similarly, you can find out what is meant by `?\n'. You can try using `Info-index' with `?\n'. It turns out that this action won't help; but don't give up. If you search the index for `\n' without the `?', you will be taken directly to the relevant section of the manual. (*Note Character Type: (elisp)Character Type. `?\n' stands for the newline character.) Other interesting source files include `paragraphs.el', `loaddefs.el', and `loadup.el'. The `paragraphs.el' file includes short, easily understood functions as well as longer ones. The `loaddefs.el' file contains the many standard autoloads and many keymaps. I have never looked at it all; only at parts. `loadup.el' is the file that loads the standard parts of Emacs; it tells you a great deal about how Emacs is built. (*Note Building Emacs: (elisp)Building Emacs, for more about building.) As I said, you have learned some nuts and bolts; however, and very importantly, we have hardly touched major aspects of programming; I have said nothing about how to sort information, except to use the predefined `sort' function; I have said nothing about how to store information, except to use variables and lists; I have said nothing about how to write programs that write programs. These are topics for another, and different kind of book, a different kind of learning. What you have done is learn enough for much practical work with GNU Emacs. What you have done is get started. This is the end of a beginning. File: emacs-lisp-intro.info, Node: the-the, Next: Kill Ring, Prev: Conclusion, Up: Top The `the-the' Function ********************** Sometimes when you you write text, you duplicate words--as with "you you" near the beginning of this sentence. I find that most frequently, I duplicate "the'; hence, I call the function for detecting duplicated words, `the-the'. As a first step, you could use the following regular expression to search for duplicates: \\(\\w+[ \t\n]+\\)\\1 This regexp matches one or more word-constituent characters followed by one or more spaces, tabs, or newlines. However, it does not detect duplicated words on different lines, since the ending of the first word, the end of the line, is different from the ending of the second word, a space. (For more information about regular expressions, see *Note Regular Expression Searches: Regexp Search, as well as *Note Syntax of Regular Expressions: (emacs)Regexps, and *Note Regular Expressions: (elisp)Regular Expressions.) You might try searching just for duplicated word-constituent characters but that does not work since the pattern detects doubles such as the two occurrences of `th' in `with the'. Another possible regexp searches for word-constituent characters followed by non-word-constituent characters, reduplicated. Here, `\\w+' matches one or more word-constituent characters and `\\W*' matches zero or more non-word-constituent characters. \\(\\(\\w+\\)\\W*\\)\\1 Again, not useful. Here is the pattern that I use. It is not perfect, but good enough. `\\b' matches the empty string, provided it is at the beginning or end of a word; `[^@ \n\t]+' matches one or more occurrences of any characters that are _not_ an @-sign, space, newline, or tab. \\b\\([^@ \n\t]+\\)[ \n\t]+\\1\\b One can write more complicated expressions, but I found that this expression is good enough, so I use it. Here is the `the-the' function, as I include it in my `.emacs' file, along with a handy global key binding: (defun the-the () "Search forward for for a duplicated word." (interactive) (message "Searching for for duplicated words ...") (push-mark) ;; This regexp is not perfect ;; but is fairly good over all: (if (re-search-forward "\\b\\([^@ \n\t]+\\)[ \n\t]+\\1\\b" nil 'move) (message "Found duplicated word.") (message "End of buffer"))) ;; Bind `the-the' to C-c \ (global-set-key "\C-c\\" 'the-the) Here is test text: one two two three four five five six seven You can substitute the other regular expressions shown above in the function definition and try each of them on this list. File: emacs-lisp-intro.info, Node: Kill Ring, Next: Full Graph, Prev: the-the, Up: Top Handling the Kill Ring ********************** The kill ring is a list that is transformed into a ring by the workings of the `rotate-yank-pointer' function. The `yank' and `yank-pop' commands use the `rotate-yank-pointer' function. This appendix describes the `rotate-yank-pointer' function as well as both the `yank' and the `yank-pop' commands. * Menu: * rotate-yank-pointer:: Move a pointer along a list and around. * yank:: Paste a copy of a clipped element. * yank-pop:: Insert first element pointed to. File: emacs-lisp-intro.info, Node: rotate-yank-pointer, Next: yank, Prev: Kill Ring, Up: Kill Ring The `rotate-yank-pointer' Function ================================== The `rotate-yank-pointer' function changes the element in the kill ring to which `kill-ring-yank-pointer' points. For example, it can change `kill-ring-yank-pointer' from pointing to the second element to point to the third element. Here is the code for `rotate-yank-pointer': (defun rotate-yank-pointer (arg) "Rotate the yanking point in the kill ring." (interactive "p") (let ((length (length kill-ring))) (if (zerop length) ;; then-part (error "Kill ring is empty") ;; else-part (setq kill-ring-yank-pointer (nthcdr (% (+ arg (- length (length kill-ring-yank-pointer))) length) kill-ring))))) * Menu: * Understanding rotate-yk-ptr:: * rotate-yk-ptr body:: The body of `rotate-yank-pointer'. File: emacs-lisp-intro.info, Node: Understanding rotate-yk-ptr, Next: rotate-yk-ptr body, Prev: rotate-yank-pointer, Up: rotate-yank-pointer `rotate-yank-pointer' in Outline -------------------------------- The `rotate-yank-pointer' function looks complex, but as usual, it can be understood by taking it apart piece by piece. First look at it in skeletal form: (defun rotate-yank-pointer (arg) "Rotate the yanking point in the kill ring." (interactive "p") (let VARLIST BODY...) This function takes one argument, called `arg'. It has a brief documentation string; and it is interactive with a small `p', which means that the argument must be a processed prefix passed to the function as a number. The body of the function definition is a `let' expression, which itself has a body as well as a VARLIST. The `let' expression declares a variable that will be only usable within the bounds of this function. This variable is called `length' and is bound to a value that is equal to the number of items in the kill ring. This is done by using the function called `length'. (Note that this function has the same name as the variable called `length'; but one use of the word is to name the function and the other is to name the variable. The two are quite distinct. Similarly, an English speaker will distinguish between the meanings of the word `ship' when he says: "I must ship this package immediately." and "I must get aboard the ship immediately.") The function `length' tells the number of items there are in a list, so `(length kill-ring)' returns the number of items there are in the kill ring. File: emacs-lisp-intro.info, Node: rotate-yk-ptr body, Prev: Understanding rotate-yk-ptr, Up: rotate-yank-pointer The Body of `rotate-yank-pointer' --------------------------------- The body of `rotate-yank-pointer' is a `let' expression and the body of the `let' expression is an `if' expression. The purpose of the `if' expression is to find out whether there is anything in the kill ring. If the kill ring is empty, the `error' function stops evaluation of the function and prints a message in the echo area. On the other hand, if the kill ring has something in it, the work of the function is done. Here is the if-part and then-part of the `if' expression: (if (zerop length) ; if-part (error "Kill ring is empty") ; then-part ... If there is not anything in the kill ring, its length must be zero and an error message sent to the user: `Kill ring is empty'. The `if' expression uses the function `zerop' which returns true if the value it is testing is zero. When `zerop' tests true, the then-part of the `if' is evaluated. The then-part is a list starting with the function `error', which is a function that is similar to the `message' function (*note message::), in that it prints a one-line message in the echo area. However, in addition to printing a message, `error' also stops evaluation of the function within which it is embedded. This means that the rest of the function will not be evaluated if the length of the kill ring is zero. * Menu: * Digression concerning error:: How to mislead humans, but not computers. * rotate-yk-ptr else-part:: The else-part of the `if' expression. * Remainder Function:: The remainder, `%', function. * rotate-yk-ptr remainder:: Using `%' in `rotate-yank-pointer'. * kill-rng-yk-ptr last elt:: Pointing to the last element. File: emacs-lisp-intro.info, Node: Digression concerning error, Next: rotate-yk-ptr else-part, Prev: rotate-yk-ptr body, Up: rotate-yk-ptr body Digression about the word `error' ................................. (In my opinion, it is slightly misleading, at least to humans, to use the term `error' as the name of the `error' function. A better term would be `cancel'. Strictly speaking, of course, you cannot point to, much less rotate a pointer to a list that has no length, so from the point of view of the computer, the word `error' is correct. But a human expects to attempt this sort of thing, if only to find out whether the kill ring is full or empty. This is an act of exploration. (From the human point of view, the act of exploration and discovery is not necessarily an error, and therefore should not be labelled as one, even in the bowels of a computer. As it is, the code in Emacs implies that a human who is acting virtuously, by exploring his or her environment, is making an error. This is bad. Even though the computer takes the same steps as it does when there is an `error', a term such as `cancel' would have a clearer connotation.) File: emacs-lisp-intro.info, Node: rotate-yk-ptr else-part, Next: Remainder Function, Prev: Digression concerning error, Up: rotate-yk-ptr body The else-part of the `if' expression .................................... The else-part of the `if' expression is dedicated to setting the value of `kill-ring-yank-pointer' when the kill ring has something in it. The code looks like this: (setq kill-ring-yank-pointer (nthcdr (% (+ arg (- length (length kill-ring-yank-pointer))) length) kill-ring))))) This needs some examination. Clearly, `kill-ring-yank-pointer' is being set to be equal to some CDR of the kill ring, using the `nthcdr' function that is described in an earlier section. (*Note copy-region-as-kill::.) But exactly how does it do this? Before looking at the details of the code let's first consider the purpose of the `rotate-yank-pointer' function. The `rotate-yank-pointer' function changes what `kill-ring-yank-pointer' points to. If `kill-ring-yank-pointer' starts by pointing to the first element of a list, a call to `rotate-yank-pointer' causes it to point to the second element; and if `kill-ring-yank-pointer' points to the second element, a call to `rotate-yank-pointer' causes it to point to the third element. (And if `rotate-yank-pointer' is given an argument greater than 1, it jumps the pointer that many elements.) The `rotate-yank-pointer' function uses `setq' to reset what the `kill-ring-yank-pointer' points to. If `kill-ring-yank-pointer' points to the first element of the kill ring, then, in the simplest case, the `rotate-yank-pointer' function must cause it to point to the second element. Put another way, `kill-ring-yank-pointer' must be reset to have a value equal to the CDR of the kill ring. That is, under these circumstances, (setq kill-ring-yank-pointer ("some text" "a different piece of text" "yet more text")) (setq kill-ring ("some text" "a different piece of text" "yet more text")) the code should do this: (setq kill-ring-yank-pointer (cdr kill-ring)) As a result, the `kill-ring-yank-pointer' will look like this: kill-ring-yank-pointer => ("a different piece of text" "yet more text")) The actual `setq' expression uses the `nthcdr' function to do the job. As we have seen before (*note nthcdr::), the `nthcdr' function works by repeatedly taking the CDR of a list--it takes the CDR of the CDR of the CDR ... The two following expressions produce the same result: (setq kill-ring-yank-pointer (cdr kill-ring)) (setq kill-ring-yank-pointer (nthcdr 1 kill-ring)) In the `rotate-yank-pointer' function, however, the first argument to `nthcdr' is a rather complex looking expression with lots of arithmetic inside of it: (% (+ arg (- length (length kill-ring-yank-pointer))) length) As usual, we need to look at the most deeply embedded expression first and then work our way towards the light. The most deeply embedded expression is `(length kill-ring-yank-pointer)'. This finds the length of the current value of the `kill-ring-yank-pointer'. (Remember that the `kill-ring-yank-pointer' is the name of a variable whose value is a list.) The measurement of the length is inside the expression: (- length (length kill-ring-yank-pointer)) In this expression, the first `length' is the variable that was assigned the length of the kill ring in the `let' statement at the beginning of the function. (One might think this function would be clearer if the variable `length' were named `length-of-kill-ring' instead; but if you look at the text of the whole function, you will see that it is so short that naming this variable `length' is not a bother, unless you are pulling the function apart into very tiny pieces as we are doing here.) So the line `(- length (length kill-ring-yank-pointer))' tells the difference between the length of the kill ring and the length of the list whose name is `kill-ring-yank-pointer'. To see how all this fits into the `rotate-yank-pointer' function, let's begin by analyzing the case where `kill-ring-yank-pointer' points to the first element of the kill ring, just as `kill-ring' does, and see what happens when `rotate-yank-pointer' is called with an argument of 1. The variable `length' and the value of the expression `(length kill-ring-yank-pointer)' will be the same since the variable `length' is the length of the kill ring and the `kill-ring-yank-pointer' is pointing to the whole kill ring. Consequently, the value of (- length (length kill-ring-yank-pointer)) will be zero. Since the value of `arg' will be 1, this will mean that the value of the whole expression (+ arg (- length (length kill-ring-yank-pointer))) will be 1. Consequently, the argument to `nthcdr' will be found as the result of the expression (% 1 length) File: emacs-lisp-intro.info, Node: Remainder Function, Next: rotate-yk-ptr remainder, Prev: rotate-yk-ptr else-part, Up: rotate-yk-ptr body The `%' remainder function .......................... To understand `(% 1 length)', we need to understand `%'. According to its documentation (which I just found by typing `C-h f % <RET>'), the `%' function returns the remainder of its first argument divided by its second argument. For example, the remainder of 5 divided by 2 is 1. (2 goes into 5 twice with a remainder of 1.) What surprises people who don't often do arithmetic is that a smaller number can be divided by a larger number and have a remainder. In the example we just used, 5 was divided by 2. We can reverse that and ask, what is the result of dividing 2 by 5? If you can use fractions, the answer is obviously 2/5 or .4; but if, as here, you can only use whole numbers, the result has to be something different. Clearly, 5 can go into 2 zero times, but what of the remainder? To see what the answer is, consider a case that has to be familiar from childhood: * 5 divided by 5 is 1 with a remainder of 0; * 6 divided by 5 is 1 with a remainder of 1; * 7 divided by 5 is 1 with a remainder of 2. * Similarly, 10 divided by 5 is 2 with a remainder of 0; * 11 divided by 5 is 2 with a remainder of 1; * 12 divided by 5 is 1 with a remainder of 2. By considering the cases as parallel, we can see that * zero divided by 5 must be zero with a remainder of zero; * 1 divided by 5 must be zero with a remainder of 1; * 2 divided by 5 must be zero with a remainder of 2; and so on. So, in this code, if the value of `length' is 5, then the result of evaluating (% 1 5) is 1. (I just checked this by placing the cursor after the expression and typing `C-x C-e'. Indeed, 1 is printed in the echo area.) File: emacs-lisp-intro.info, Node: rotate-yk-ptr remainder, Next: kill-rng-yk-ptr last elt, Prev: Remainder Function, Up: rotate-yk-ptr body Using `%' in `rotate-yank-pointer' .................................. When the `kill-ring-yank-pointer' points to the beginning of the kill ring, and the argument passed to `rotate-yank-pointer' is 1, the `%' expression returns 1: (- length (length kill-ring-yank-pointer)) => 0 therefore, (+ arg (- length (length kill-ring-yank-pointer))) => 1 and consequently: (% (+ arg (- length (length kill-ring-yank-pointer))) length) => 1 regardless of the value of `length'. As a result of this, the `setq kill-ring-yank-pointer' expression simplifies to: (setq kill-ring-yank-pointer (nthcdr 1 kill-ring)) What it does is now easy to understand. Instead of pointing as it did to the first element of the kill ring, the `kill-ring-yank-pointer' is set to point to the second element. Clearly, if the argument passed to `rotate-yank-pointer' is two, then the `kill-ring-yank-pointer' is set to `(nthcdr 2 kill-ring)'; and so on for different values of the argument. Similarly, if the `kill-ring-yank-pointer' starts out pointing to the second element of the kill ring, its length is shorter than the length of the kill ring by 1, so the computation of the remainder is based on the expression `(% (+ arg 1) length)'. This means that the `kill-ring-yank-pointer' is moved from the second element of the kill ring to the third element if the argument passed to `rotate-yank-pointer' is 1. File: emacs-lisp-intro.info, Node: kill-rng-yk-ptr last elt, Prev: rotate-yk-ptr remainder, Up: rotate-yk-ptr body Pointing to the last element ............................ The final question is, what happens if the `kill-ring-yank-pointer' is set to the _last_ element of the kill ring? Will a call to `rotate-yank-pointer' mean that nothing more can be taken from the kill ring? The answer is no. What happens is different and useful. The `kill-ring-yank-pointer' is set to point to the beginning of the kill ring instead. Let's see how this works by looking at the code, assuming the length of the kill ring is 5 and the argument passed to `rotate-yank-pointer' is 1. When the `kill-ring-yank-pointer' points to the last element of the kill ring, its length is 1. The code looks like this: (% (+ arg (- length (length kill-ring-yank-pointer))) length) When the variables are replaced by their numeric values, the expression looks like this: (% (+ 1 (- 5 1)) 5) This expression can be evaluated by looking at the most embedded inner expression first and working outwards: The value of `(- 5 1)' is 4; the sum of `(+ 1 4)' is 5; and the remainder of dividing 5 by 5 is zero. So what `rotate-yank-pointer' will do is (setq kill-ring-yank-pointer (nthcdr 0 kill-ring)) which will set the `kill-ring-yank-pointer' to point to the beginning of the kill ring. So what happens with successive calls to `rotate-yank-pointer' is that it moves the `kill-ring-yank-pointer' from element to element in the kill ring until it reaches the end; then it jumps back to the beginning. And this is why the kill ring is called a ring, since by jumping back to the beginning, it is as if the list has no end! (And what is a ring, but an entity with no end?) File: emacs-lisp-intro.info, Node: yank, Next: yank-pop, Prev: rotate-yank-pointer, Up: Kill Ring `yank' ====== After learning about `rotate-yank-pointer', the code for the `yank' function is almost easy. It has only one tricky part, which is the computation of the argument to be passed to `rotate-yank-pointer'. The code looks like this: (defun yank (&optional arg) "Reinsert the last stretch of killed text. More precisely, reinsert the stretch of killed text most recently killed OR yanked. With just C-U as argument, same but put point in front (and mark at end). With argument n, reinsert the nth most recently killed stretch of killed text. See also the command \\[yank-pop]." (interactive "*P") (rotate-yank-pointer (if (listp arg) 0 (if (eq arg '-) -1 (1- arg)))) (push-mark (point)) (insert (car kill-ring-yank-pointer)) (if (consp arg) (exchange-point-and-mark))) Glancing over this code, we can understand the last few lines readily enough. The mark is pushed, that is, remembered; then the first element (the CAR) of what the `kill-ring-yank-pointer' points to is inserted; and then, if the argument passed the function is a `cons', point and mark are exchanged so the point is put in the front of the inserted text rather than at the end. This option is explained in the documentation. The function itself is interactive with `"*P"'. This means it will not work on a read-only buffer, and that the unprocessed prefix argument is passed to the function. * Menu: * rotate-yk-ptr arg:: Pass the argument to `rotate-yank-pointer'. * rotate-yk-ptr negative arg:: Pass a negative argument. File: emacs-lisp-intro.info, Node: rotate-yk-ptr arg, Next: rotate-yk-ptr negative arg, Prev: yank, Up: yank Passing the argument .................... The hard part of `yank' is understanding the computation that determines the value of the argument passed to `rotate-yank-pointer'. Fortunately, it is not so difficult as it looks at first sight. What happens is that the result of evaluating one or both of the `if' expressions will be a number and that number will be the argument passed to `rotate-yank-pointer'. Laid out with comments, the code looks like this: (if (listp arg) ; if-part 0 ; then-part (if (eq arg '-) ; else-part, inner if -1 ; inner if's then-part (1- arg)))) ; inner if's else-part This code consists of two `if' expression, one the else-part of the other. The first or outer `if' expression tests whether the argument passed to `yank' is a list. Oddly enough, this will be true if `yank' is called without an argument--because then it will be passed the value of `nil' for the optional argument and an evaluation of `(listp nil)' returns true! So, if no argument is passed to `yank', the argument passed to `rotate-yank-pointer' inside of `yank' is zero. This means the pointer is not moved and the first element to which `kill-ring-yank-pointer' points is inserted, as we expect. Similarly, if the argument for `yank' is `C-u', this will be read as a list, so again, a zero will be passed to `rotate-yank-pointer'. (`C-u' produces an unprocessed prefix argument of `(4)', which is a list of one element.) At the same time, later in the function, this argument will be read as a `cons' so point will be put in the front and mark at the end of the insertion. (The `P' argument to `interactive' is designed to provide these values for the case when an optional argument is not provided or when it is `C-u'.) The then-part of the outer `if' expression handles the case when there is no argument or when it is `C-u'. The else-part handles the other situations. The else-part is itself another `if' expression. The inner `if' expression tests whether the argument is a minus sign. (This is done by pressing the <META> and `-' keys at the same time, or the <ESC> key and then the `-' key). In this case, the `rotate-yank-pointer' function is passed `-1' as an argument. This moves the `kill-ring-yank-pointer' backwards, which is what is desired. If the true-or-false-test of the inner `if' expression is false (that is, if the argument is not a minus sign), the else-part of the expression is evaluated. This is the expression `(1- arg)'. Because of the two `if' expressions, it will only occur when the argument is a positive number or when it is a negative number (not just a minus sign on its own). What `(1- arg)' does is decrement the number and return it. (The `1-' function subtracts one from its argument.) This means that if the argument to `rotate-yank-pointer' is 1, it is reduced to zero, which means the first element to which `kill-ring-yank-pointer' points is yanked back, as you would expect.