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              Notes on Defining Commands and Modes
                           Cris Perdue
                             8/9/82
                      File: pe:dispatch.doc

These notes should be of use to anyone wishing to customize EMODE
by defining commands (keystrokes) or new modes.  Most of the
current mode and keystroke definitions are contained in
PE:DISPCH.SL.  Read it for examples and the keystroke-function
associations.

define_prefix_character(char, prompt)

Char must be a single character, possibly with Control and/or
Meta turned on.  This is used for "true prefix characters" such
as CTRL-X and META-X, not prefixes for obtaining control or meta
through multiple keystrokes.  Those are defined using
AddToKeyList and EstablishCurrentMode.

AddToKeyList(listname, char, opr)

Adds a keystroke-operation association to a "key list", whose
name, an atom, is passed in.  The value of the atom must be the
actual list.  See the information on CharSequence, below, for the
format of the chr parameter.  The opr must be a function of no
arguments.  Its value is ignored.  AddToKeyList may also be used
to change an association in a keylist.  Three existing lists are
BasicDispatchList, ReadOnlyTextDispatchList, and
TextDispatchList.

BasicDispatchList includes commands that do not modify the buffer
  and do not have to do with manipulating text in any way.

ReadOnlyTextDispatchList contains the commands that have to do
  with manipulating text, but that do not modify the buffer.
  This list is for support of read-only buffers.

TextDispatchList contains commands that modify the buffer.

CharSequence([char])

This is a macro analogous to "char".  Where char takes a single
"character specification", CharSequence takes a sequence.  Both
char and CharSequence forms may be used in the specification of
KeyLists.  At present two characters is the maximum sequence, due
to the implementation of the actual dispatcher used when the user
types commands to EMODE.

SetKey(char opr)

It is generally a mistake to use this function directly, but it
is used internally be EstablishCurrentMode to activate a keylist.

Takes a character as produced by "char" or a character sequence
as produced by "CharSequence" and installs it in the (global)
command key lookup tables.  The first character of any character
sequence must be defined as a prefix character.  If the specified
character is upper case, the corresponding lower case character
is also defined.

Does not add the definition to any mode, nor permanently to the
buffer, so use things like AddToKeyList at user level.

MODES

AlterBufferEnv(BufferName, 'ModeEstablishExpressions, Exprs)

Every buffer carries around an environment, which includes a list
of PSL expressions that set up its current mode.  To change
modes, alter the ModeEstblishExpressions part of the buffer's
environment as shown.  The expressions will be evaluated in
reverse order (first one last) immediately and then whenever the
mode is "established" with EstablishCurrentMode.  See
PE:DISPCH.SL for examples of modes, including FundamentalTextMode.
Expressions of the form (SetKeys <variable>) set up the
keystroke-operation associations in a keylist.

EstablishCurrentMode()

Activates the current mode with its keylists.  Key definitions
made by AddToKeyList don't take effect until this is performed
even if the keylist changed is part of the current mode.