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Edlib - a library for building a document editor

Edlib is an extensible document editor. It is inspired in part by Emacs, both by its strengths and its weaknesses.

Emacs provides a programming language — E-lisp — for configuring and extending the editor. Edlib doesn't. It allows various pre-existing languages to be used to configure and extend the editor. It does this by providing a library of core editing tools and providing bindings to various languages.

At least, that is the plan. At time if writing, edlib only provides bindings for C and Python. Other languages should be fairly easy.

The particular value-add of edlib over Emacs (apart from the obvious “NIH” issues) is that both document storage and document rendering are fully extensible. A document is not always a text buffer, it could also be a mem-mapped files, a directory, or an internal data structure. Any document can have multiple views, and each view can show very different things: scriptable code is run whenever rendering is required. This should make implementing documents with nontrivial structures a lot easier.

Edlib is designed to have well defined abstractions that can be exported to various languages for them to manipulate. They include primarily commands, panes, and marks, and also attributes, documents, displays, events, and keymaps.


Commands are the single mechanism by which control is transferred from one part of the editor implementation to another. They provide the only way for code in one implementation language to invoke code written in another language, and they are the preferred way to interact between different modules even in the same language.

All commands receive the same types of arguments and produce an integer result. The arguments include two panes (“home” and “focus”), two marks (“mark” and “mark2”), three strings (“key”, “str”, “str2”), two numbers (“num” and “num2”), a coordinate pair (“x”, “y”) and two commands (“comm1” and “comm2”). Extra result values can be effected by passing them to a call to the “comm2” argument - i.e the command can be used as a callback.

Each “pane” has a dedicate command which handles messages sent to the pane, as will be described later. Commands can also be passed to other commands, which can either call them directly (like the callback mentioned above) or store them for later use, or both.

Three of the arguments provided to a command have very special meanings. One of the strings, known as “key”, identifies what action should be performed. The pane handler will normally use this key to select some other command to actually handle the message. Other commands may ignore the key, or use it however they please.

When a message is sent to a pane and the handler command is called, the “home” argument is set to the pane that owns the handle, so it acts a bit like the “self” argument in some object-oriented languages. One of the primary uses of “home” is to access “home->data” which is a private data structure owned by the pane. The command associated with a particular pane is typically the only code which can understand the data.

Finally the “comm1” argument passed to a command always identifies exactly the command that is being run. “comm1” is a pointer to a structure containing a pointer to the function being called. This structure can be embedded is some other data structure which contains context for the command. This context might be read-only, to refine the behaviour of the command, or read/write to provide storage for the command to use. For example, the call-back commands described earlier are normally embedded in a data structure that can store the extra values to return.

Apart from the return value of zero which indicates “command not understood”, a command can return a positive result on success or a negative result indicating lack of success. Known error codes include:

  • Enoarg : missing argument
  • Einval : something is wrong with the context of the request
  • Efail : request makes sense, but didn't work
  • Enosup: request makes sense, but isn't allowed for some reason
  • Efalse: Not really an error, just a Boolean status


A pane combines an optional rectangular area of display with some data storage and some functionality. As such it can receive mouse and keyboard events, can draw on a display, and can send commands to other panes.

All panes are arranged as a tree with all but the root having a parent and many having siblings and children. When a pane represents a rectangle of display all children are restricted to just that rectangle or less. Often a child will cover exactly the same area as its parent. In other cases several children will share out the area.

An “event” is a set of arguments to a command which is being sent to a pane. Events are often generated at a leaf of the tree of panes (i.e. a pane with no children). They travel up the tree towards the root until they find a pane which can handle them. That pane (or its handler command) might handle the event by generating other events. They will typically start looking for a handler at the same leaf which is available as the “focus” argument. For this reason branches of the pane tree usually have more generic panes closer to the root and more special-purpose panes near the leaves.

It is quite normal for there to be panes in the tree that are not directly involves in displaying anything - these are just useful containers for data and functionality. Documents, described below, exist as panes that are not directly displayed. Instead there are display pane which link to the document and display its content.

As well as a dedicated command (the “handler”) and private data, each pane has:

  • x,y coordinates together with width and height. The coordinates are relative to the parent, and by recursive addition can be made absolute.
  • a “z” value which indicates display priority with respect to siblings. When siblings overlap, the sibling with the higher “z” value will be drawn “over” siblings with a lower “z” value, including all the children of that sibling (independent of their z value).
  • a selected child referred to as the “focus”. Keyboard input at a display is forwarded down the chain of focus links until it reaches a leaf pane. This is where handling of the keystroke starts.
  • a set of “damaged” flags which record if any changes have been made which might affect the display.
  • an arbitrary set of attributes with assigned values.

Each pane may also request notifications from other panes. These include, but are not limited to, a notification when the pane is destroyed and a notification when a document attached to the pane changes. These notifications are effected by calling the pane's command with a key like “Notify:Close” and with the second pane argument (the “focus”) set to the pane which is sending the notification.


A document provides access to whatever data is being edited, or just being displayed. There can be multiple implementations for documents but they all have a common interface.

A “document” is assumed to be a linear sequence of elements each of which presents as a single character and may have some attributes associated with it. For a “text” document, the characters are typically Unicode characters stored as UTF-8 and the attributes, if any, are hints for parsing or display. For a “directory” document, the elements are entries in the directory and the associated character reflects the type of entry. The attributes contain other information such as file name, size, modify time etc.

A document is represented by a non-display pane. These panes are typically collected together as children of a “document-list” pane which can be asked to add or find documents. To display a document, a document-display pane is normally created. This contains, in its private data, a reference to the document pane and a “point” (see below) indicating where changes will happen. Events that arrive at the document-display pane will typically be forwarded to the document, though they maybe be handled directly by the display pane.


An attribute is a simple name=value pair, both being strings. Attributes can be associated with various other objects, including marks, panes, and elements in a document. Parsing code can annotate a buffer with attributes, and rendering code can use these attributes to guide rendering. e.g. parsing code can attach “spelling=wrong” or “spelling=doubtful” and rendering can underline in red or whatever is appropriate.

Marks and Points

A “mark” identifies a location in a document. The location is between two elements in the document, or at the start or end, and the mark remains at that location despite any edits that do not affect neighbouring elements.

Marks come in three different sorts: ungrouped, grouped, and points. All of these appear in document-order in a linked list, all have a sequence number in the list so ordering-tests are easy, and each can have a set of attributes attached.

An ungrouped mark has no property beyond the above. A grouped marked is included in a second linked list with all the other marks in the same group. This group is owned by a specific pane and keeps information relevant to the task of that pane. A pane responsible for rendering part of a document might have marks identifying the start and end of the visible portion, and maybe even the start of each line in the visible portion. A grouped mark also has a reference to an arbitrary data structure which is understood by the pane which owns the group.

A “point” is a special grouped-mark which is included in all of the other lists of grouped marks. This is achieved by using the external reference to hold an auxiliary data structure which is linked in to all of the lists. Every document-display pane owns a point. This point is usually where changes to the document happen. When the notification mechanism mentioned earlier tells other panes of a chance to the document, the point where the change happened is also reported. From this point it is easy to find and update nearby marks of any mark-group.

An example use is to have a group of marks which are used to track line numbers. “line-count” marks are placed every 500 lines (or so) with an attribute recording exactly how many lines between this and the next “line-count” mark. When a change happens, the recorded line count on the preceding mark is cleared. When a line count or line number is needed, the list of “line-count” marks is walked from the start. If any has its count cleared, the lines in that section are counted and the record is updated. Otherwise all that is required is simply adding up a few numbers.

Marks could be used by a parser to identify key locations which would allow a renderer to find the important content quickly if it was only rendering a partial view - such as the headings in outline mode.


A “display” is just a pane which can create an image somehow, and responds to commands like “pane-clear”, “text-display”, and “image-display”. Displays are typically quite close to the root of the “pane” tree, but this is not a requirement.

A display is also expected to call “Keystroke” and “Mouse-event” commands in response to appropriate events. These will propagate towards the root and normally hit an input-management pane which will find the appropriate target leaf, will convert to a full event, e.g. adding a repeat count or indication of a prefix key, and will submit the new event at the target.


A keymap is a mapping from command names to commands. While a pane handler could use any mapping it likes, the keymap implemented in edlib has one small advantage in that a range of strings can be mapped to a command, then exceptions can be recorded.

The handler for a pain typically looks up the passed “key” in a keymap, locates the target command, and passes control to that command.

Handling Commands

Now that we have plenty of context, it is time to revisit commands to discuss how they are called. It is possible to invoke a specific command directly if you have a reference to it but most often a more general mechanism is used to find the appropriate command. The most common mechanism is to pass and "Event" (i.e. a set of arguments) to a “home” pane. The handler for that pane and each ancestor will be tried in turn until a handler returns a non-zero value (i.e. it accepts the event), or until the root pane has been tried. Very often the starting home pane will also be the focus pane so when the two are the same it is not necessary to specify both.

The other common mechanism is a “notification event” which follows the "notifier" chain from a pane. This chain lists a number of panes which have requested notifications. When calling notifiers, all target panes have their handler called and if any return a non-zero value, the over-all return value will be non-zero. More precisely it will be the value returned which has the largest absolute value.

Each handler can perform further lookup however it likes. It may just compare the “key” against a number of supported keys, or it might perform a lookup in a key-table. One particularly useful approach is to look up all commands with a prefix matching the key and call all of them in order until one returns a non-zero value. This can be used to allow multiple handlers to register for a service where each will handle different instances. For example when choosing a document type to open a given file, all document types will be tried but some would be expected to return zero. e.g. if the file is actually a directory, everything but the directory document type would reject the request.

Another example worth understanding is the document-display pane type. When this receives an event it will handle it directly if it understands the key, otherwise if it starts with “doc:” it will pass it to the document pane. If that doesn't recognise the event it will continue up the tree from the document-display pane.

Like document-display, other pane types are free to direct events elsewhere as appropriate. The “input” handler takes keystroke events and redirects them to the current focus pane, and take mouse events and redirects them to the pane with the greatest 'z' depth which covers the mouse location.

Core Extensions

These are the basic common objects which can (I hope) be used to build a rich document editor. There needs to be lots of extensions of course to make them useful. The current extensions that are available include:

Text Document

A text document stores text in various linked data structures designed to make simple edits easy and to support unlimited undo/redo. There are a number of allocations, and a list of “chunks” which each identify a start and end in one of those allocations. Edits can add text to the last allocation, can change the endpoints of a chuck, and can insert new chunks or delete old chunks.

Each chunk has a list of attributes each with an offset into the allocation, so they each apply to a single character, though are often interpreted to apply to follow characters as well.

Directory Document

A directory document contains a list of directory entries from a directory and provides a variety of attributes for each entry. The directory can be re-read at any time with incremental changes made to the document.

Documents Document

There is typically one pane of this type and it registers an “doc:appeared-” handler with the root pane to get notified when documents are created. It will reparent the document so that it becomes a child of a separate “collection” pane. Then all documents can be found in the list of children.

The “documents” pane presents as a document which can be viewed and appears as a list of document names. Various keystroke events allow documents to be opened, deleted, etc.

Rendering virtual document

Working directory with the "directory" or "documents" document is sometimes a bit awkward as a mark can only point to a while directory entry or document. When these are displayed one-per-line it isn't possible to move the cursor within that line, which can feel strange, particularly if the line is wider than that display pane. Also selecting an copying text cannot select part of a line.

To overcome these problems, a "rendering" document can be layered over the main document. It presents a virtual document which contains all the character that are use to display (to render) the underlying document. This then "feels" more like a regular document and can respond to select and copy just like a text document.

Multipart document, and "crop" filter

Multipart is another virtual document, which appears to contain the content of a sequence of other documents. This allows a sequence of documents to appear to be combined into one. This can co-operate with the "crop" filter which limits access to a given document to the section between two marks. By combining multipart and crop, one document can be divided up and re-assembled in any order, or parts of multiple documents can be merged.

Email document; base64, qprint, utf8, rfc822header filters

The Email document handler uses crop and multipart and other tools to present an email message as a readable document. The different parts of an email message (header, body, attachments) are identified and cropped out with appropriate filters attached.

Notmuch email reader

"notmuch" as a email indexing and management tools. The notmuch email reader provides one document which displays various saved searches with a count of the number of items, and another document type which can show a summary line for each message found by a given search. They work with the email document pane to allow reading and managing an e-mail mailbox.

Ncurses Display

The “ncurses” display can draw text on a terminal window and can set various attributes such as colour, bold, underline etc. It also receives keyboard and mouse input and sends “Mouse-event” or “Keystroke” command up to the input manage.

There can be multiple ncurses displays, each attached to a different terminal.

Pygtk Display

This is a display module written in python and using pygtk for drawing.

When a “text” or “clear” request is made on a pane, the module allocates a pixmap (arranging for it to be destroyed when the pane is closed) and performs the drawings there. When a refresh is required, the various pixmaps are combined and drawn to the target window.

Variable width fonts are supported as are images. An image is typically the only thing drawn in a pane, so sub-panes must be used to draw images within a document.

Lines-Renderer and render-line filter.

The lines renderer is designed to work with any document that presents as a list of lines. Lines that are wider than the pane can either be truncated (with side-scrolling) or wrapped. The lines renderer moves backwards and forwards from the cursor “point” to determine which lines should be drawn and sends a “render-line” command to get the displayed text for those lines.

There is “render-line” pane type which provides the “render-line” function for a simple text document. It looks for particular attributes in the document and on marks in the document and interprets them to allow highlighting different parts of the text. It returns a line with markup which the line renderer understands.

Attribute Format Renderer

The attribute formatter is given a format for each line into which it interpolates attributes from each element in the document. These lines are provided in response to “render-line” requests so that if the line-render and the attribute formatter are both stacked on a document, then the attributes of elements in the document can be easily displayed.

The format specification is found by requesting the attribute “Line-format” from panes starting at the focus and moving towards the root.

Completion Render

The “completion” render is a filter. In response to a “render-line” call it calls “render-line” on its parent and only returns lines that start with a given prefix, or contain a given substring. It can also add highlights to rendered text to distinguish the common prefix from the remainder.

A prefix is set by a “set-prefix” command. The response to this indicates if the selected lines have a longer common prefix, and if there is in fact only a single matching line. This supports the implementation of filename, document name, command name completion etc. To complete from a set of names, you just need to provide a document which responds to “render-line” with the various options to choose from.

Hex Render

The HEX renderer provides an alternate “render-line” for a document which starts each line at a multiple of 16 bytes from the start of the document, and formats the next 16 bytes as hex and ASCII. Each rendered line starts with the byte offset of the start of the line.

Presentation renderer

The presentation rendering accepts a text document and interprets it as describing pages of a presentation in a language similar to MarkDown. It generates a marked-up rendering the various lines of text which are given to the lines renderer to produce a single page of the presentation.

There is a partner "markdown" mode which interacts with a presentation pane and can ask it to move forward or backward one page, or to redraw some other arbitrary page.


The “tile” handler takes a pane (typically the root pane of a display) and divides it up into 1 or more non-overlapping tiles. Tiles are grouped in horizontal and vertical stacks. Tiles can be split, can be discarded, or can be resized. Any of these operations may affect other tiles.

The leaves of the tile tree need to have some other pane attached. The tiler doesn't render anything itself, not even borders. That is left to the children.


A “view” draws borders around a pane and provides a child pane which is slightly smaller to allow for those borders (so it should probably be called “border”).

The borders can contain scroll-bars, a document name, or other information provided by child panes.

Popup manager

The popup manager places a small window with an elevated “z” value somewhere relevant on the display and can provide a simple text document for text entry - or can use a provided document. Various key strokes are captured to allow the popup to be aborted, or to send the content of the mini document to the originating pane.

The popup requests notifications from that pane so that if it is closed, the popup automatically closes too.


The line-counter uses the model described earlier of placing marks every few hundred lines in a document and using them to expedite line counting. This is implemented as a pane which attaches directly to the document pane.


“Keymap” pane allows both global and local keys (or arbitrary commands) to be defined. The global mappings are handled at a pane which must be stacked before the tiler. A pane to handle local mappings is added on demand at the current focus pane.


“search” provides a global command rather than a pane. This command can perform a reg-ex search through a document.


“Messageline” trims the bottom line off a pane (providing a pane which is slightly smaller) and will display messages in this pane until the next keyboard command.


A pane of this module stores some state related to the current input context, including a modifier prefix and a repeat count.

When a “keystroke” command is received the prefix is added to the key and this is sent as a command to the focus. The command includes the repeat count, which gets cleared.

Commands are provided to set a new prefix or repeat count. So for example “Alt-1” might multiply the repeat count in the command by 10, add 1, and then ask “input” to set that as the new repeat count for the next keystroke.


This pane makes it easy to run a command in the background and capture the output in a text document, which can then be displayed like any other document.


This allows make or grep to be running with the output captured and parsed. A simple keystroke then causes the editor to open a file mentioned in the output, and to go to the identified line.


The viewer pane suppresses any commands that would modify the document, and repurposes some of them to make it easier to move around a document being viewed - so 'space' pages forward, and 'backspace' pages backwards.


A history pane stores data a bit like a document, but provides access in a different way. Individual lines can be recalled and new lines can be appended. It makes it easy to provide a history of lines of text entered for some purpose, such as running a shell command or an editor command selected by name.


Similar in principle to "history", a copybuf pane can store a series of arbitrary slabs of text. It is used to provide copy/paste functionality.


The server pane listens on a socket for request to open a file, or to create an ncurses pane on the current terminal. It also reports to the request when the file has been edited, or when the ncurses pane is closed.

Emacs Mode

This provides a set of named commands which can be given to “keymap” as a global key map. In provides a number of Emacs-like bindings.

C/Python mode

This pane capture various editing command and tailors them to suite editing C or Python code. It helps with correct indenting, highlight matching brackets, and will eventually do a lot more.

Python Interface

This module allows python code to be run, provides an interface to panes, marks, and commands, and allows commands to be called on a given pane. It also allows commands to be defined in python that can be called from other modules just like any other command. It is a complete two-way interface between python and other languages to access the core edlib functionality.


edlib needs an event loop to wait for input, capture signals, and run tasks. Any module can register an event loop by registering handlers for various “event:*” events with the root pane. When pygtk is being used, the glib event loop must be used. Otherwise some other event loop is needed. To this end, the libevent module registers a low-priority set of event handler which use libevent.

I'm not entirely happy about this arrangement. In particular I would like to be able to have multiple event loops running in separate threads. So expect things to change here.

Next steps

The TO-DO list is now a separate document.


Because one more editor is never enough




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