viper: Emacs Preliminaries
1.1 Emacs Preliminaries
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Emacs can edit several files at once. A file in Emacs is placed in a
“buffer” that usually has the same name as the file. Buffers are also
used for other purposes, such as shell interfaces, directory editing,
etc. 
Directory Editor (emacs)Dired, for an example.
A buffer has a distinguished position called the “point”. A “point”
is always between 2 characters, and is “looking at” the right hand
character. The cursor is positioned on the right hand character. Thus,
when the “point” is looking at the end-of-line, the cursor is on the
end-of-line character, i.e., beyond the last character on the line.
This is the default Emacs behavior.
The default settings of Viper try to mimic the behavior of Vi,
preventing the cursor from going beyond the last character on the line.
By using Emacs commands directly (such as those bound to arrow keys), it
is possible to get the cursor beyond the end-of-line. However, this
won’t (or shouldn’t) happen if you restrict yourself to standard Vi
keys, unless you modify the default editing style.
Customization.
In addition to the “point”, there is another distinguished buffer
position called the “mark”. Mark (emacs)Mark, for more info on
the mark. The text between the “point” and the “mark” is called the
“region” of the buffer. For the Viper user, this simply means that in
addition to the Vi textmarkers a–z, there is another marker called
“mark”. This is similar to the unnamed Vi marker used by the jump
commands ‘``’ and ‘''’, which move the cursor to the position of the
last absolute jump. Viper provides access to the region in most text
manipulation commands as ‘r’ and ‘R’ suffix to commands that operate on
text regions, e.g., ‘dr’ to delete region, etc.
Furthermore, Viper lets Ex-style commands to work on the current
region. This is done by typing a digit argument before ‘:’. For
instance, typing ‘1:’ will prompt you with something like _:123,135_,
assuming that the current region starts at line 123 and ends at line
135. There is no need to type the line numbers, since Viper inserts
them automatically in front of the Ex command.
Basics, for more info.
Emacs divides the screen into tiled “windows”. You can see the
contents of a buffer through the window associated with the buffer. The
cursor of the screen is positioned on the character after “point”.
Every window has a “mode line” that displays information about the
buffer. You can change the format of the mode line, but normally if you
see ‘**’ at the beginning of a mode line it means that the buffer is
“modified”. If you write out the contents of a buffer to a file, then
the buffer will become not modified. Also if you see ‘%%’ at the
beginning of the mode line, it means that the file associated with the
buffer is write protected. The mode line will also show the buffer name
and current major and minor modes (see below). A special buffer called
“Minibuffer” is displayed as the last line in a minibuffer window. The
minibuffer window is used for command input output. Viper uses
minibuffer window for ‘/’ and ‘:’ commands.
An Emacs buffer can have a “major mode” that customizes Emacs for
editing text of a particular sort by changing the functionality of the
keys. Keys are defined using a “keymap” that records the bindings
between keystrokes and functions. The “global keymap” is common to all
the buffers. Additionally, each buffer has its “local keymap” that
determines the “mode” of the buffer. If a function is bound to some key
in the local keymap then that function will be executed when you type
the key. If no function is bound to a key in the local map, however,
the function bound to the key in the global map will be executed.
Major Modes (emacs)Major Modes, for more information.
A buffer can also have a “minor mode”. Minor modes are options that
you can use or not. A buffer in ‘text-mode’ can have ‘auto-fill-mode’
as minor mode, which can be turned off or on at any time. In Emacs, a
minor mode may have it own keymap, which overrides the local keymap when
the minor mode is turned on. For more information, Minor Modes
(emacs)Minor Modes.
Viper is implemented as a collection of minor modes. Different minor
modes are involved when Viper emulates Vi command mode, Vi insert mode,
etc. You can also turn Viper on and off at any time while in Vi command
mode. States in Viper, for more information.
Emacs uses Control and Meta modifiers. These are denoted as C and M,
e.g., ‘^Z’ as ‘C-z’ and ‘Meta-x’ as ‘M-x’. The Meta key is usually
located on each side of the Space bar; it is used in a manner similar to
the Control key, e.g., ‘M-x’ means typing ‘x’ while holding the Meta key
down. For keyboards that do not have a Meta key, <ESC> is used as Meta.
Thus ‘M-x’ is typed as ‘<ESC> x’. Viper uses <ESC> to switch from
Insert state to Vi state. Therefore Viper defines ‘C-\’ as its Meta key
in Vi state. Vi State, for more info.
Emacs is structured as a Lisp interpreter around a C core. Emacs
keys cause Lisp functions to be called. It is possible to call these
functions directly, by typing ‘M-x function-name’.
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