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diff --git a/doc/acme/acme.ms b/doc/acme/acme.ms new file mode 100644 index 00000000..7314ff2e --- /dev/null +++ b/doc/acme/acme.ms @@ -0,0 +1,1295 @@ +.de EX +.nr x \\$1v +\\!h0c n \\nx 0 +.. +.de FG \" start figure caption: .FG filename.ps verticalsize +.KF +.BP \\$1 \\$2 +.sp .5v +.EX \\$2v +.ps -1 +.vs -1 +.. +.de fg \" end figure caption (yes, it is clumsy) +.ps +.vs +.br +\l'1i' +.KE +.. +.TL +Acme: A User Interface for Programmers +.AU +.I "Rob Pike +.I rob@plan9.bell-labs.com +.SP .22i exactly +.AB +.FS +\l'1i' +.br +Originally appeared in +.I +Proc. of the Winter 1994 USENIX Conf., +.R +pp. 223-234, +San Francisco, CA +.br +Reprinted in +.I "Plan 9 Programmer's Manual: Volume 2" +(Second Edition), AT&T 1995. +.FE +A hybrid of window system, shell, and editor, Acme gives text-oriented +applications a clean, expressive, and consistent style of interaction. +Traditional window systems support interactive client programs and offer libraries of +pre-defined operations such as pop-up menus +and buttons to promote a consistent +user interface among the clients. +Acme instead provides its clients with a fixed user interface and +simple conventions to encourage its uniform use. +Clients access the facilities of Acme through a file system interface; +Acme is in part a file server that exports device-like files that may be +manipulated to access and control the contents of its windows. +Written in a concurrent programming language, +Acme is structured as a set of communicating processes that neatly subdivide +the various aspects of its tasks: display management, input, file server, and so on. +.PP +Acme attaches distinct functions to the three mouse buttons: +the left selects text; +the middle executes textual commands; +and the right combines context search and file opening +functions to integrate the various applications and files in +the system. +.PP +Acme works well enough to have developed +a community that uses it exclusively. +Although Acme discourages the traditional style of interaction +based on typescript windows\(emteletypes\(emits +users find Acme's other services render +typescripts obsolete. +.AE +.SH +History and motivation +.PP +The usual typescript style of interaction with +Unix and its relatives is an old one. +The typescript\(eman intermingling of textual commands and their +output\(emoriginates with the scrolls of paper on teletypes. +The advent of windowed terminals has given each user what +amounts to an array of teletypes, a limited and unimaginative +use of the powers of bitmap displays and mice. +Systems like the Macintosh +that do involve the mouse as an integral part of the interaction +are geared towards general users, not experts, and certainly +not programmers. +Software developers, at least on time-sharing systems, have been left behind. +.FG ./acme.fig1 5i +Figure 1. A small Acme screen\(emnormally it runs on a larger display\(emdemonstrating +some of the details discussed in the text. +The right column contains some guide files, +a mailbox presented by Acme's mail program, +the columnated display of files in Acme's own source directory, +a couple of windows from the OED browser, +a debugger window, +and an error window showing diagnostics from a compilation. +The left column holds a couple of source files +.CW dat.h "" ( +and +.CW acme.l ), +another debugger window displaying a stack trace, +and a third source file +.CW time.l ). ( +.CW Time.l +was opened from the debugger by clicking the right mouse button +on a line in the stack window; +the mouse cursor landed on the offending line of +.CW acme.l +after a click on the compiler message. +.fg +.PP +Some programs have mouse-based editing of +text files and typescripts; +ones I have built include +the window systems +.CW mux +[Pike88] +and +.CW 8½ +[Pike91] +and the text editor +Sam [Pike87]. +These have put the programmer's mouse to some productive work, +but not wholeheartedly. Even experienced users of these programs +often retype text that could be grabbed with the mouse, +partly because the menu-driven interface is imperfect +and partly because the various pieces are not well enough integrated. +.PP +Other programs\(emEMACS [Stal93] is the prime example\(emoffer a high +degree of integration but with a user interface built around the +ideas of cursor-addressed terminals that date from the 1970's. +They are still keyboard-intensive and +dauntingly complex. +.PP +The most ambitious attempt to face these issues was the Cedar +system, developed at Xerox [Swei86]. +It combined a new programming language, compilers, +window system, even microcode\(ema complete system\(emto +construct a productive, highly +integrated and interactive environment +for experienced users of compiled languages. +Although successful internally, the system was so large +and so tied to specific hardware that it never fledged. +.PP +Cedar was, however, the major inspiration for Oberon [Wirt89], +a system of similar scope but much smaller scale. +Through careful selection of Cedar's ideas, Oberon shows +that its lessons can be applied to a small, coherent system +that can run efficiently on modest hardware. +In fact, Oberon probably +errs too far towards simplicity: a single-process system +with weak networking, it seems an architectural throwback. +.PP +Acme is a new program, +a combined window system, editor, and shell, +that applies +some of the ideas distilled by Oberon. +Where Oberon uses objects and modules within a programming language (also called Oberon), +Acme uses files and commands within an existing operating system (Plan 9). +Unlike Oberon, Acme does not yet have support for graphical output, just text. +At least for now, the work on Acme has concentrated on +producing the smoothest user interface possible for a programmer +at work. +.PP +The rest of this paper describes Acme's interface, +explains how programs can access it, +compares it to existing systems, +and finally presents some unusual aspects of its implementation. +.SH +User interface +.PP +.FG ./acme.fig2 3i +Figure 2. An Acme window showing a section of code. +The upper line of text is the tag containing the file name, +relevant commands, and a scratch area (right of the vertical bar); +the lower portion of the window is the +body, or contents, of the file. +Here the scratch area contains a command for the middle button +.CW mk ) ( +and a word to search for with the right button +.CW cxfidalloc ). ( +The user has just +clicked the right button on +.CW cxfidalloc +and Acme has searched for the word, highlighted it, +and moved the mouse cursor there. The file has been modified: +the center of the layout box is black and the command +.CW Put +appears in the tag. +.fg +Acme windows are arrayed in columns (Figure 1) and are used more +dynamically than in an environment like X Windows or +.CW 8½ +[Sche86, Pike91]. +The system frequently creates them automatically and the user +can order a new one with a single mouse button click. +The initial placement of a new window is determined +automatically, but the user may move an existing window anywhere +by clicking or dragging a +.I "layout box +in the upper left corner of +the window. +.PP +Acme windows have two parts: a +.I tag +holding a single line of text, +above a +.I body +holding zero or more lines (Figure 2). +The body typically contains an image of a file being edited +or the editable output of a +program, analogous to an +EMACS shell +window. The tag contains +the name of the window +(usually the name of the associated +file or directory), some built-in commands, and a scratch area to hold arbitrary text. +If a window represents a directory, the name in the tag ends with +a slash and the body contains a list of the names of the files +in the directory. +Finally, each non-empty body holds a scroll bar at the left of the text. +.PP +Each column of windows also has a layout box and a tag. +The tag has no special meaning, although Acme pre-loads it with a few +built-in commands. +There is also a tag across the whole display, also loaded with +helpful commands and a list of active processes started +by Acme. +.PP +Typing with the keyboard and selecting with the left button are as in +many other systems, including the Macintosh, +.CW 8½ , +and Sam. +The middle and right buttons are used, somewhat like the left button, +to `sweep' text, but the indicated text is treated in a way +that depends on the text's location\(em\f2context\f1\(emas well as its content. +This context, based on the directory of the file containing the text, +is a central component of Acme's style of interaction. +.PP +Acme has no single notion of `current directory'. +Instead, every command, file name, +action, and so on is interpreted or executed in the directory named by the +tag of the window containing the command. For example, the string +.CW mammals +in a window labeled +.CW /lib/ +or +.CW /lib/insects +will be interpreted as the file name +.CW /lib/mammals +if such a file exists. +.PP +Throughout Acme, the middle mouse button is used to execute commands +and the right mouse button is used to locate and select files and text. +Even when there are no true files on which to operate\(emfor example +when editing mail messages\(emAcme and its applications use +consistent extensions of these basic functions. +This idea is as vital to Acme as icons are to the Macintosh. +.PP +The middle button executes commands: text swept with the button +pressed is underlined; when the button is released, the underline is +removed and the indicated text is executed. +A modest number of commands are recognized as built-ins: words like +.CW Cut , +.CW Paste , +and +.CW New +name +functions performed directly by Acme. +These words often appear in tags to make them always available, +but the tags are not menus: any text anywhere in Acme may be a command. +For example, in the tag or body of any window one may type +.CW Cut , +select it with the left button, use the middle button to execute it, +and watch it disappear again. +.PP +If the middle button indicates a command that is not recognized as a built-in, +it is executed in the directory +named by the tag of the window holding the text. +Also, the file to be executed is searched for first in that directory. +Standard input is connected to +.CW /dev/null , +but standard and error outputs are connected to an Acme window, +created if needed, called +\f2dir\f(CW/+Errors\f1 where +.I dir +is the directory of the window. +(Programs that need interactive input use a different interface, described below.) +A typical use of this is to type +.CW mk +(Plan 9's +.CW make ) +in the scratch area in the tag of a C source window, say +.CW /sys/src/cmd/sam/regexp.c , +and execute it. +Output, including compiler errors, appears in the window labeled +.CW /sys/src/cmd/sam/+Errors , +so file names in the output are associated with the windows and directory +holding the source. +The +.CW mk +command remains in the tag, serving as a sort of menu item for the associated +window. +.PP +Like the middle button, the right button is used to indicate text by sweeping it out. +The indicated text is not a command, however, but the argument of a generalized +search operator. +If the text, perhaps after appending it to the directory of the window containing it, +is the name of an existing file, Acme creates a new window to hold the file +and reads it in. It then moves the mouse cursor to that window. If the file is +already loaded into Acme, the mouse motion happens but no new window is made. +For example, indicating the string +.CW sam.h +in +.P1 +#include "sam.h" +.P2 +in a window on the file +.CW /sys/src/cmd/sam/regexp.c +will open the file +.CW /sys/src/cmd/sam/sam.h . +.PP +If the file name is followed immediately by a colon and a legal address in +Sam notation (for example a line number or a regular expression delimited in +slashes or a comma-separated compound of such addresses), Acme highlights +the target of that address in the file and places the mouse there. One may jump to +line 27 of +.CW dat.h +by indicating with the right button the text +.CW dat.h:27 . +If the file is not already open, Acme loads it. +If the file name is null, for example if the indicated string is +.CW :/^main/ , +the file is assumed to be that of the window containing the string. +Such strings, when typed and evaluated in the tag of a window, amount to +context searches. +.PP +If the indicated text is not the name of an existing file, it is taken to be literal +text and is searched for in the body of the window containing the text, highlighting +the result as if it were the result of a context search. +.PP +For the rare occasion when a file name +.I is +just text to search for, it can be selected with the left button and used as the +argument to a built-in +.CW Look +command that always searches for literal text. +.SH +Nuances and heuristics +.PP +A user interface should not only provide the necessary functions, it should also +.I feel +right. +In fact, it should almost not be felt at all; when one notices a +user interface, one is distracted from the job at hand [Pike88]. +To approach this invisibility, some of Acme's properties and features +are there just to make the others easy to use. +Many are based on a fundamental principle of good design: +let the machine do the work. +.PP +Acme tries to avoid needless clicking and typing. +There is no `click-to-type', eliminating a button click. +There are no pop-up or pull-down menus, eliminating the mouse action needed to +make a menu appear. +The overall design is intended to make text on the screen useful without +copying or retyping; the ways in which this happens involve +the combination of many aspects of the interface. +.PP +Acme tiles its windows and places them automatically +to avoid asking the user to place and arrange them. +For this policy to succeed, the automatic placement must behave well enough +that the user is usually content with the location of a new window. +The system will never get it right all the time, but in practice most +windows are used at least for a while where Acme first places them. +There have been several complete rewrites of the +heuristics for placing a new window, +and with each rewrite the system became +noticeably more comfortable. The rules are as follows, although +they are still subject to improvement. +The window appears in the `active' column, that most recently used for typing or +selecting. +Executing and searching do not affect the choice of active column, +so windows of commands and such do not draw new windows towards them, +but rather let them form near the targets of their actions. +Output (error) windows always appear towards the right, away from +edited text, which is typically kept towards the left. +Within the column, several competing desires are balanced to decide where +and how large the window should be: +large blank spaces should be consumed; +existing text should remain visible; +existing large windows should be divided before small ones; +and the window should appear near the one containing the action that caused +its creation. +.PP +Acme binds some actions to chords of mouse buttons. +These include +.CW Cut +and +.CW Paste +so these common operations can be done without +moving the mouse. +Another is a way to apply a command in one window to text (often a file name) +in another, avoiding the actions needed to assemble the command textually. +.PP +Another way Acme avoids the need to move the mouse is instead to move the cursor +to where it is likely to be used next. When a new window is made, Acme +moves the cursor to the new window; in fact, to the selected text in that window. +When the user deletes a newly made window, the cursor is +returned to the point it was before the window was made, +reducing the irritation of windows that pop up to report annoying errors. +.PP +When a window is moved, Acme moves the cursor to the layout box in +its new place, to permit further adjustment without moving the mouse. +For example, when a click of the left mouse button on the layout box grows +the window, the cursor moves to the new location of the box so repeated clicks, +without moving the mouse, continue to grow it. +.PP +Another form of assistance the system can offer is to supply precision in +pointing the mouse. The best-known form of this is `double-clicking' to +select a word rather than carefully sweeping out the entire word. +Acme provides this feature, using context to decide whether to select +a word, line, quoted string, parenthesized expression, and so on. +But Acme takes the idea much further by applying it to execution +and searching. +A +.I single +click, that is, a null selection, with either the middle or right buttons, +is expanded automatically to indicate the appropriate text containing +the click. What is appropriate depends on the context. +.PP +For example, to execute a single-word command +such as +.CW Cut , +it is not necessary to sweep the entire word; just clicking the button once with +the mouse pointing at the word is sufficient. `Word' +means the largest string of likely file name characters surrounding the location +of the click: click on a file name, run that program. +On the right button, the rules are more complicated because +the target of the click might be a file name, file name with address, +or just plain text. Acme examines the text near the click to find +a likely file name; +if it finds one, it checks that it names an existing file (in the directory named in the tag, if the name is relative) +and if so, takes that as the result, after extending it with any address +that may be present. If there is no file with that name, Acme +just takes the largest alphanumeric string under the click. +The effect is a natural overloading of the button to refer to plain text as +well as file names. +.PP +First, though, if the click occurs over the left-button-selected text in the window, +that text is taken to be what is selected. +This makes it easy to skip through the occurrences of a string in a file: just click +the right button +on some occurrence of the text in the window (perhaps after typing it in the tag) +and click once for each subsequent occurrence. It isn't even necessary to move +the mouse between clicks; Acme does that. +To turn a complicated command into a sort of menu item, select it: +thereafter, clicking the middle button on it will execute the full command. +.PP +As an extra feature, Acme recognizes file names in angle brackets +.CW <> +as names of files in standard directories of include files, +making it possible for instance to look at +.CW <stdio.h> +with a single click. +.PP +Here's an example to demonstrate how the actions and defaults work together. +Assume +.CW /sys/src/cmd/sam/regexp.c +is +open and has been edited. We write it (execute +.CW Put +in the tag; once the file is written, Acme removes the word from the tag) +and type +.CW mk +in the tag. We execute +.CW mk +and get some errors, which appear in a new window labeled +.CW /sys/src/cmd/sam/+Errors . +The cursor moves automatically to that window. +Say the error is +.P1 +main.c:112: incompatible types on assignment to `pattern' +.P2 +We move the mouse slightly and click the right button +at the left of the error message; Acme +makes a new window, reads +.CW /sys/src/cmd/main.c +into it, selects line 112 +and places the mouse there, right on the offending line. +.SH +Coupling to existing programs +.PP +Acme's syntax for file names and addresses makes it easy for other programs +to connect automatically to Acme's capabilities. For example, the output of +.P1 +grep -n variable *.[ch] +.P2 +can be used to help Acme step through the occurrences of a variable in a program; +every line of output is potentially a command to open a file. +The file names need not be absolute, either: the output +appears in a window labeled with the directory in which +.CW grep +was run, from which Acme can derive the full path names. +.PP +When necessary, we have changed the output of some programs, +such as compiler error messages, to match +Acme's syntax. +Some might argue that it shouldn't be necessary to change old programs, +but sometimes programs need to be updated when systems change, +and consistent output benefits people as well as programs. +A historical example is the retrofitting of standard error output to the +early Unix programs when pipes were invented. +.PP +Another change was to record full path names in +the symbol table of executables, so line numbers reported by the debugger +are absolute names that may be used directly by Acme; it's not necessary +to run the debugger in the source directory. (This aids debugging +even without Acme.) +.PP +A related change was to add lines of the form +.P1 +#pragma src "/sys/src/libregexp" +.P2 +to header files; coupled with Acme's ability to locate a header file, +this provides a fast, keyboardless way to get the source associated with a library. +.PP +Finally, Acme directs the standard output of programs it runs to +windows labeled by the directory in which the program is run. +Acme's splitting of the +output into directory-labeled windows is a small feature that has a major effect: +local file names printed by programs can be interpreted directly by Acme. +By indirectly coupling the output of programs to the input, +it also simplifies the management of software that occupies multiple +directories. +.SH +Coupling to new programs +.PP +Like many Plan 9 programs, +Acme offers a programmable interface to +other programs by acting as a file server. +The best example of such a file server is the window system +.CW 8½ +[Pike91], +which exports files with names such as +.CW screen , +.CW cons , +and +.CW mouse +through which applications may access the I/O capabilities of the windows. +.CW 8½ +provides a +.I distinct +set of files for each window and builds a private file name space +for the clients running `in' each window; +clients in separate windows see distinct files with the same names +(for example +.CW /dev/mouse ). +Acme, like the process file system [PPTTW93], instead associates each +window with a directory of files; the files of each window are visible +to any application. +This difference reflects a difference in how the systems are used: +.CW 8½ +tells a client what keyboard and mouse activity has happened in its window; +Acme tells a client what changes that activity wrought on any window it asks about. +Putting it another way, +.CW 8½ +enables the construction of interactive applications; +Acme provides the interaction for applications. +.PP +The root of +Acme's file system is mounted using Plan 9 operations on the directory +.CW /mnt/acme . +In +that root directory appears a directory for each window, numbered with the window's identifier, +analogous to a process identifier, for example +.CW /mnt/acme/27 . +The window's directory +contains 6 files: +.CW /mnt/acme/27/addr , +.CW body , +.CW ctl , +.CW data , +.CW event , +and +.CW tag . +The +.CW body +and +.CW tag +files contain the text of the respective parts of the window; they may be +read to recover the contents. Data written to these files is appended to the text; +.CW seeks +are ignored. +The +.CW addr +and +.CW data +files provide random access to the contents of the body. +The +.CW addr +file is written to set a character position within the body; the +.CW data +file may then be read to recover the contents at that position, +or written to change them. +(The tag is assumed +small and special-purpose enough not to need special treatment. +Also, +.CW addr +indexes by character position, which is not the same as byte offset +in Plan 9's multi-byte character set [Pike93]). +The format accepted by the +.CW addr +file is exactly the syntax of addresses within the user interface, +permitting regular expressions, line numbers, and compound addresses +to be specified. For example, to replace the contents of lines 3 through 7, +write the text +.P1 +3,7 +.P2 +to the +.CW addr +file, then write the replacement text to the +.CW data +file. A zero-length write deletes the addressed text; further writes extend the replacement. +.PP +The control file, +.CW ctl , +may be written with commands to effect actions on the window; for example +the command +.P1 +name /adm/users +.P2 +sets the name in the tag of the window to +.CW /adm/users . +Other commands allow deleting the window, writing it to a file, and so on. +Reading the +.CW ctl +file recovers a fixed-format string containing 5 textual numbers\(emthe window +identifier, the number of characters in the tag, the number in the body, +and some status information\(emfollowed by the text of the tag, up to a newline. +.PP +The last file, +.CW event , +is the most unusual. +A program reading a window's +.CW event +file is notified of all changes to the text of the window, and +is asked to interpret all middle- and right-button actions. +The data passed to the program is fixed-format and reports +the source of the action (keyboard, mouse, external program, etc.), +its location (what was pointed at or modified), and its nature (change, +search, execution, etc.). +This message, for example, +.P1 +MI15 19 0 4 time +.P2 +reports that actions of the mouse +.CW M ) ( +inserted in the body (capital +.CW I ) +the 4 characters of +.CW time +at character positions 15 through 19; the zero is a flag word. +Programs may apply their own interpretations of searching and +execution, or may simply reflect the events back to Acme, +by writing them back to the +.CW event +file, to have the default interpretation applied. +Some examples of these ideas in action are presented below. +.PP +Notice that changes to the window are reported +after the fact; the program is told about them but is not required to act +on them. Compare this to a more traditional interface in which a program +is told, for example, that a character has been typed on the keyboard and +must then display and interpret it. +Acme's style stems from the basic model of the system, in which any +number of agents\(emthe keyboard, mouse, external programs +writing to +.CW data +or +.CW body , +and so on\(emmay +change the contents of a window. +The style is efficient: many programs are content +to have Acme do most of the work and act only when the editing is completed. +An example is the Acme mail program, which can ignore the changes +made to a message being composed +and just read its body when asked to send it. +A disadvantage is that some traditional ways of working are impossible. +For example, there is no way `to turn off echo': characters appear on the +screen and are read from there; no agent or buffer stands between +the keyboard and the display. +.PP +There are a couple of other files made available by Acme in its root directory +rather than in the directory of each window. +The text file +.CW /mnt/acme/index +holds a list of all window names and numerical identifiers, +somewhat analogous to the output of the +.CW ps +command for processes. +The most important, though, is +.CW /mnt/acme/new , +a directory that makes new windows, similar to the +.CW clone +directory in the Plan 9 network devices [Pres93]. +The act of opening any file in +.CW new +creates a new Acme window; thus the shell command +.P1 +grep -n var *.c > /mnt/acme/new/body +.P2 +places its output in the body of a fresh window. +More sophisticated applications may open +.CW new/ctl , +read it to discover the new window's identifier, and then +open the window's other files in the numbered directory. +.SH +Acme-specific programs +.PP +Although Acme is in part an attempt to move beyond typescripts, +they will probably always have utility. +The first program written for Acme was therefore one +to run a shell or other traditional interactive application +in a window, the Acme analog of +.CW xterm . +This program, +.CW win , +has a simple structure: +it acts as a two-way intermediary between Acme and the shell, +cross-connecting the standard input and output of the shell to the +text of the window. +The style of interaction is modeled after +.CW mux +[Pike88]: standard output is added to the window at the +.I "output point; +text typed after the output point +is made available on standard input when a newline is typed. +After either of these actions, the output point is advanced. +This is different from the working of a regular terminal, +permitting cut-and-paste editing of an input line until the newline is typed. +Arbitrary editing may be done to any text in the window. +The implementation of +.CW win , +using the +.CW event , +.CW addr , +and +.CW data +files, is straightforward. +.CW Win +needs no code for handling the keyboard and mouse; it just monitors the +contents of the window. Nonetheless, it allows Acme's full editing to be +applied to shell commands. +The division of labor between +.CW win +and +.CW Acme +contrasted with +.CW xterm +and the X server demonstrates how much work Acme handles automatically. +.CW Win +is implemented by a single source file 560 lines long and has no graphics code. +.PP +.CW Win +uses the middle and right buttons to connect itself in a consistent way +with the rest of Acme. +The middle button still executes commands, but in a style more suited +to typescripts. Text selected with the middle button is treated as if +it had been typed after the output point, much as a similar feature in +.CW xterm +or +.CW 8½ , +and therefore causes it to be `executed' by the application running in the window. +Right button actions are reflected back to Acme but refer to the appropriate +files because +.CW win +places the name of the current directory in the tag of the window. +If the shell is running, a simple shell function replacing the +.CW cd +command can maintain the tag as the shell navigates the file system. +This means, for example, that a right button click on a file mentioned in an +.CW ls +listing opens the file within Acme. +.PP +Another Acme-specific program is a mail reader that begins by presenting, +in a window, a listing of the messages in the user's mailbox, one per line. +Here the middle and right button actions are modified to refer to +mail commands +and messages, but the change feels natural. +Clicking the right button on a line creates a new window and displays the +message there, or, if it's already displayed, moves the mouse to that window. +The metaphor is that the mailbox is a directory whose constituent files are messages. +The mail program also places some relevant commands in the tag lines of +the windows; for example, executing the word +.CW Reply +in a message's tag creates a new window +in which to compose a message to the sender of the original; +.CW Post +then dispatches it. +In such windows, the addressee is just a list of names +on the first line of the body, which may be edited to add or change recipients. +The program also monitors the mailbox, updating the `directory' as new messages +arrive. +.PP +The mail program is as simple as it sounds; all the work of interaction, +editing, and management of the display is done by Acme. +The only +difficult sections of the 1200 +lines of code concern honoring the external protocols for managing +the mailbox and connecting to +.CW sendmail . +.PP +One of the things Acme does not provide directly is a facility like +Sam's command language to enable actions such as global substitution; +within Acme, all editing is done manually. +It is easy, though, to write external programs for such tasks. +In this, Acme comes closer to the original intent of Oberon: +a directory, +.CW /acme/edit , +contains a set of tools for repetitive editing and a template +or `guide' file that gives examples +of its use. +Acme's editing guide, +.CW /acme/edit/guide , +looks like this: +.P1 +e file | x '/regexp/' | c 'replacement' +e file:'0,$' | x '/.*word.*\en/' | p -n +e file | pipe command args ... +.P2 +The syntax is reminiscent of Sam's command language, but here the individual +one-letter commands are all stand-alone programs connected by pipes. +Passed along the pipes are addresses, analogous to structural expressions +in Sam terminology. +The +.CW e +command, unlike that of Sam, starts the process by generating the address +(default dot, the highlighted selection) in the named files. +The other commands are as in Sam: +.CW p +prints the addressed text on standard output (the +.CW -n +option is analogous to that of +.CW grep , +useful in combination with the right mouse button); +.CW x +matches a regular expression to the addressed (incoming) text, +subdividing the text; +.CW c +replaces the text; and so on. Thus, global substitution throughout a file, +which would be expressed in Sam as +.P1 +0,$ x/regexp/ c/replacement/ +.P2 +in Acme's editor becomes +.P1 +e 'file:0,$' | x '/regexp/' | c 'replacement' +.P2 +.PP +To use the Acme editing commands, open +.CW /acme/edit/guide , +use the mouse and keyboard to edit one of the commands to the right form, +and execute it with the middle button. +Acme's context rules find the appropriate binaries in +.CW /acme/edit +rather than +.CW /bin ; +the effect is to turn +.CW /acme/edit +into a toolbox containing tools and instructions (the guide file) for their use. +In fact, the source for these tools is also there, in the directory +.CW /acme/edit/src . +This setup allows some control of the file name space for binary programs; +not only does it group related programs, it permits the use of common +names for uncommon jobs. For example, the single-letter names would +be unwise in a directory in everyone's search path; here they are only +visible when running editing commands. +.PP +In Oberon, +such a collection would be called a +.I tool +and would consist +of a set of entry points in a module and a menu-like piece of text containing +representative commands that may be edited to suit and executed. +There is, in fact, a tool called +.CW Edit +in Oberon. +To provide related functionality, +Acme exploits the directory and file structure of the underlying +system, rather than the module structure of the language; +this fits well with Plan 9's +file-oriented philosophy. +Such tools are central to the working of Oberon but they are +less used in Acme, at least so far. +The main reason is probably that Acme's program interface permits +an external program to remain executing in the background, providing +its own commands as needed (for example, the +.CW Reply +command in the mail program); Oberon uses tools to +implement such services because its must invoke +a fresh program for each command. +Also, +Acme's better integration allows more +basic functions to be handled internally; the right mouse button +covers a lot of the basic utility of the editing tools in Oberon. +Nonetheless, as more applications are written for Acme, +many are sure to take this Oberon tool-like form. +.SH +Comparison with other systems +.PP +Acme's immediate ancestor is Help [Pike92], an experimental system written +a few years ago as a first try at exploring some of Oberon's ideas +in an existing operating system. +Besides much better engineering, Acme's advances over Help +include the actions of the right button (Help had nothing comparable), +the ability to connect long-running programs to the user interface +(Help had no analog of the +.CW event +file), +and the small but important change to split command output into +windows labeled with the directory in which the commands run. +.PP +Most of Acme's style, however, derives from the user interface and window +system of Oberon [Wirt89, Reis91]. +Oberon includes a programming language and operating system, +which Acme instead borrows from an existing system, Plan 9. +When I first saw Oberon, in 1988, I was struck by the +simplicity of its user interface, particularly its lack of menus +and its elegant use of multiple mouse buttons. +The system seemed restrictive, though\(emsingle process, +single language, no networking, event-driven programming\(emand +failed to follow through on some of its own ideas. +For example, the middle mouse button had to be pointed accurately and +the right button was essentially unused. +Acme does follow through: +to the basic idea planted by Oberon, it adds +the ability to run on different operating systems and hardware, +connection to existing applications including +interactive ones such as shells and debuggers, +support for multiple processes, +the right mouse button's features, +the default actions and context-dependent properties +of execution and searching, +and a host of little touches such as moving the mouse cursor that make the system +more pleasant. +At the moment, though, Oberon does have one distinct advantage: it incorporates +graphical programs well into its model, an issue Acme has not yet faced. +.PP +Acme shares with the Macintosh a desire to use the mouse well and it is +worth comparing the results. +The mouse on the Macintosh has a single button, so menus are essential +and the mouse must frequently move a long way +to reach the appropriate function. +An indication that this style has trouble is that applications provide +keyboard sequences to invoke menu selections and users often prefer them. +A deeper comparison is that the Macintosh uses pictures where Acme uses text. +In contrast to pictures, text can be edited quickly, created on demand, +and fine-tuned to the job at hand; consider adding an option to a command. +It is also self-referential; Acme doesn't need menus because any text can be +in effect a menu item. +The result is that, although a Macintosh screen is certainly prettier and probably +more attractive, especially to beginners, an Acme screen is more dynamic +and expressive, at least for programmers and experienced users. +.PP +For its role in the overall system, +Acme most resembles EMACS [Stal93]. +It is tricky to compare Acme to EMACS, though, because there are +many versions of EMACS and, since it is fully programmable, EMACS +can in principle do anything Acme does. +Also, Acme is much younger and therefore has not +had the time to acquire as many features. +The issue therefore is less what the systems can be programmed to do than +how they are used. +The EMACS versions that come closest to Acme's style are those that +have been extended to provide a programming environment, usually +for a language such as LISP [Alle92, Lucid92]. +For richness of the existing interface, these EMACS versions are certainly superior to Acme. +On the other hand, Acme's interface works equally well already for a variety +of languages; for example, one of its most enthusiastic users works almost +exclusively in Standard ML, a language nothing like C. +.PP +Where Acme excels is in the smoothness of its interface. +Until recently, EMACS did not support the mouse especially well, +and even with the latest version providing features such as `extents' +that can be programmed to behave much like Acme commands, +many users don't bother to upgrade. +Moreover, in the versions that provide extents, +most EMACS packages don't take advantage of them. +.PP +The most important distinction is just that +EMACS is fundamentally keyboard-based, while +Acme is mouse-based. +.PP +People who try Acme find it hard to go back to their previous environment. +Acme automates so much that to return to a traditional interface +is to draw attention to the extra work it requires. +.SH +Concurrency in the implementation +.PP +Acme is about 8,000 lines of code in Alef, a concurrent object-oriented language syntactically similar to C [Alef]. +Acme's structure is a set of communicating +processes in a single address space. +One subset of the processes drives the display and user interface, +maintaining the windows; other processes forward mouse and keyboard +activity and implement the file server interface for external programs. +The language and design worked out well; +as explained elsewhere [Pike89, Gans93, Reppy93], +user interfaces built with concurrent systems +can avoid the clumsy +top-level event loop typical of traditional interactive systems. +.PP +An example of the benefits of the multi-process style +is the management of the state of open +files held by clients of the file system interface. +The problem is that some I/O requests, +such as reading the +.CW event +file, may block if no data is available, and the server must +maintain the state of (possibly many) requests until data appears. +For example, +in +.CW 8½ , +a single-process window system written in C, pending requests were queued in +a data structure associated with each window. +After activity in the window that might complete pending I/O, +the data structure was scanned for requests that could now finish. +This structure did not fit well with the rest of the program and, worse, +required meticulous effort +to guarantee correct behavior under all conditions +(consider raw mode, reads of partial lines, deleting a window, +multibyte characters, etc.). +.PP +Acme instead creates a new dedicated process +for each I/O request. +This process coordinates with the rest of the system +using Alef's synchronous communication; +its state implicitly encodes the state of +the I/O request and obviates the need for queuing. +The passage of the request through Acme proceeds as follows. +.PP +Acme contains a file server process, F, that executes a +.CW read +system call to receive a Plan 9 file protocol (9P) message from the client [AT&T92]. +The client blocks until Acme answers the request. +F communicates with an allocation process, M, +to acquire an object of type +.CW Xfid +(`executing fid'; fid is a 9P term) +to hold the request. +M sits in a loop (reproduced in Figure 2) waiting for either a request for +a new +.CW Xfid +or notification that an existing one has finished its task. +When an +.CW Xfid +is created, an associated process, X, +is also made. +M queues idle +.CW Xfids , +allocating new ones only when the list is empty. +Thus, there is always a pool of +.CW Xfids , +some executing, some idle. +.PP +The +.CW Xfid +object contains a channel, +.CW Xfid.c , +for communication with its process; +the unpacked message; and some associated functions, +mostly corresponding to 9P messages such as +.CW Xfid.write +to handle a 9P write request. +.PP +The file server process F parses the message to see its nature\(emopen, +close, read, write, etc. Many messages, such as directory +lookups, can be handled immediately; these are responded to directly +and efficiently +by F without invoking the +.CW Xfid , +which is therefore maintained until the next message. +When a message, such as a write to the display, requires the attention +of the main display process and interlocked access to its data structures, +F enables X +by sending a function pointer on +.CW Xfid.c . +For example, if the message is a write, F executes +.P1 +x->c <-= Xfid.write; +.P2 +which sends +the address of +.CW Xfid.write +on +.CW Xfid.c , +waking up X. +.PP +The +.CW Xfid +process, X, executes a simple loop: +.P1 +void +Xfid.ctl(Xfid *x) +{ + for(;;){ + (*<-x->c)(x); /* receive and execute message */ + bflush(); /* synchronize bitmap display */ + cxfidfree <-= x; /* return to free list */ + } +} +.P2 +Thus X +will wake up with the address of a function to call (here +.CW Xfid.write ) +and execute it; once that completes, it returns itself to the pool of +free processes by sending its address back to the allocator. +.PP +Although this sequence may seem complicated, it is just a few lines +of code and is in fact far simpler +than the management of the I/O queues in +.CW 8½ . +The hard work of synchronization is done by the Alef run time system. +Moreover, the code worked the first time, which cannot be said for the code in +.CW 8½ . +.SH +Undo +.PP +Acme provides a general undo facility like that of Sam, permitting +textual changes to be unwound arbitrarily. +The implementation is superior to Sam's, though, +with much higher performance and the ability to `redo' changes. +.PP +Sam uses +a multi-pass algorithm that builds +a transcript of changes to be made simultaneously +and then executes them atomically. +This was thought necessary because the elements of a repetitive +command such as a global substitution should all be applied to the same +initial file and implemented simultaneously; forming the complete +transcript before executing any of the changes avoids the +cumbersome management of addresses in a changing file. +Acme, however, doesn't have this problem; global substitution +is controlled externally and may be made incrementally by exploiting +an observation: if the changes are sorted in address order and +executed in reverse, changes will not invalidate the addresses of +pending changes. +.PP +Acme therefore avoids the initial transcript. Instead, changes are applied +directly to the file, with an undo transcript recorded in a separate list. +For example, when text is added to a window, it is added directly and a record +of what to delete to restore the state is appended to the undo list. +Each undo action and the file are marked with a sequence number; +actions with the same sequence number are considered a unit +to be undone together. +The invariant state of the structure +is that the last action in the undo list applies to the current state of the file, +even if that action is one of a related set from, for example, a global substitute. +(In Sam, a related set of actions needed to be undone simultaneously.) +To undo an action, pop the last item on the undo list, apply it to the file, +revert it, and append it to a second, redo list. +To redo an action, do the identical operation with the lists interchanged. +The expensive operations occur +only when actually undoing; in normal editing the overhead is minor. +For example, Acme reads files about seven times faster than Sam, partly +because of this improvement and partly because of a cleaner implementation. +.PP +Acme uses a temporary file to hold the text, keeping in memory only the +visible portion, and therefore can edit large files comfortably +even on small-memory machines such as laptops. +.SH +Future +.PP +Acme is still under development. +Some things are simply missing. +For example, Acme should support non-textual graphics, but this is being +deferred until it can be done using a new graphics model being developed +for Plan 9. Also, it is undecided how Acme's style of interaction should best be +extended to graphical applications. +On a smaller scale, although the system feels smooth and comfortable, +work continues to tune the heuristics and +try new ideas for the user interface. +.PP +There need to be more programs that use Acme. Browsers for +Usenet and AP News articles, the Oxford English Dictionary, and other +such text sources exist, but more imaginative applications will +be necessary to prove that Acme's approach is viable. +One that has recently been started is an interface to the debugger Acid [Wint94], +although it is still +unclear what form it will ultimately take. +.PP +Acme shows that it is possible to make a user interface a stand-alone component +of an interactive environment. By absorbing more of the interactive +functionality than a simple window system, Acme off-loads much of the +computation from its applications, which helps keep them small and +consistent in their interface. Acme can afford to dedicate +considerable effort to making that interface as good as possible; the result +will benefit the entire system. +.PP +Acme is complete and useful enough to attract users. +Its comfortable user interface, +the ease with which it handles multiple tasks and +programs in multiple directories, +and its high level of integration +make it addictive. +Perhaps most telling, +Acme shows that typescripts may not be the most +productive interface to a time-sharing system. +.SH +Acknowledgements +.PP +Howard Trickey, Acme's first user, suffered buggy versions gracefully and made +many helpful suggestions. Chris Fraser provided the necessary insight for the Acme editing +commands. +.SH +References +.LP +[Alef] P. Winterbottom, +``Alef Language Reference Manual'', +.I +Plan 9 Programmer's Manual, +.R +AT&T Bell Laboratories, +Murray Hill, NJ, +1992; +revised in this volume. +.br +[Alle92] +.I +Allegro Common Lisp user Guide, Vol 2, +.R +Chapter 14, "The Emacs-Lisp Interface". +March 1992. +.br +[AT&T92] Plan 9 Programmer's manual, Murray Hill, New Jersey, 1992. +.br +[Far89] Far too many people, XTERM(1), Massachusetts Institute of Technology, 1989. +.br +[Gans93] Emden R. Gansner and John H. Reppy, ``A Multi-threaded Higher-order User Interface Toolkit'', in +.I +Software Trends, Volume 1, +User Interface Software, +.R +Bass and Dewan (Eds.), +John Wiley & Sons 1993, +pp. 61-80. +.br +[Lucid92] Richard Stallman and Lucid, Inc., +.I +Lucid GNU EMACS Manual, +.R +March 1992. +.br +[Pike87] Rob Pike, ``The Text Editor \f(CWsam\fP'', Softw. - Pract. and Exp., Nov 1987, Vol 17 #11, pp. 813-845; reprinted in this volume. +.br +[Pike88] Rob Pike, ``Window Systems Should Be Transparent'', Comp. Sys., Summer 1988, Vol 1 #3, pp. 279-296. +.br +[Pike89] Rob Pike, ``A Concurrent Window System'', Comp. Sys., Spring 1989, Vol 2 #2, pp. 133-153. +.br +[PPTTW93] Rob Pike, Dave Presotto, Ken Thompson, Howard Trickey, and Phil Winterbottom, ``The Use of Name Spaces in Plan 9'', +Op. Sys. Rev., Vol. 27, No. 2, April 1993, pp. 72-76, +reprinted in this volume. +.br +[Pike91] Rob Pike, ``8½, the Plan 9 Window System'', USENIX Summer Conf. Proc., Nashville, June, 1991, pp. 257-265, +reprinted in this volume. +.br +[Pike92] Rob Pike, ``A Minimalist Global User Interface'', Graphics Interface '92 Proc., Vancouver, 1992, pp. 282-293. An earlier version appeared under the same title in USENIX Summer Conf. Proc., Nashville, June, 1991, pp. 267-279. +.br +[Pike93] Rob Pike and Ken Thompson, ``Hello World or Καλημέρα κόσμε or +\f(Jpこんにちは 世界\fP'', USENIX Winter Conf. Proc., San Diego, 1993, pp. 43-50, +reprinted in this volume. +.br +[Pres93] Dave Presotto and Phil Winterbottom, ``The Organization of Networks in Plan 9'', Proc. Usenix Winter 1993, pp. 271-287, San Diego, CA, +reprinted in this volume. +.br +[Reis91] Martin Reiser, \fIThe Oberon System,\fP Addison Wesley, New York, 1991. +.br +[Reppy93] John H. Reppy, +``CML: A higher-order concurrent language'', Proc. SIGPLAN'91 Conf. on Programming, Lang. Design and Impl., June, 1991, pp. 293-305. +.br +[Sche86] Robert W. Scheifler and Jim Gettys, +``The X Window System'', +ACM Trans. on Graph., Vol 5 #2, pp. 79-109. +.br +[Stal93] Richard Stallman, +.I +Gnu Emacs Manual, 9th edition, Emacs version 19.19, +.R +MIT. +.br +[Swei86] Daniel Sweinhart, Polle Zellweger, Richard Beach, and Robert Hagmann, +``A Structural View of the Cedar Programming Environment'', +ACM Trans. Prog. Lang. and Sys., Vol. 8, No. 4, pp. 419-490, Oct. 1986. +.br +[Wint94], Philip Winterbottom, ``Acid: A Debugger based on a Language'', USENIX Winter Conf. Proc., San Francisco, CA, 1993, +reprinted in this volume. +.br +[Wirt89] N. Wirth and J. Gutknecht, ``The Oberon System'', Softw. - Prac. and Exp., Sep 1989, Vol 19 #9, pp 857-894. |