Initially edit/1 
uses the editor specified in the EDITOR environment 
variable. There are two ways to force it to use the built-in editor. One 
is to set the Prolog flag editor 
to
pce_emacs and the other is by starting the editor 
explicitly using the emacs[0,1] predicates.
PceEmacs closely mimics Richard Stallman's GNU-Emacs commands, adding features from modern window-based editors to make it more acceptable for beginners.38Decent merging with MS-Windows control-key conventions is difficult as many conflict with GNU-Emacs. Especially the cut/copy/paste commands conflict with important GNU-Emacs commands.
At the basis, PceEmacs maps keyboard sequences to methods defined on 
the extended object. Some frequently used commands are, with their 
key-binding, presented in the menu bar above each editor window. A 
complete overview of the bindings for the current mode is 
provided through Help/Show key bindings (Control-h 
Control-b).
Modes are the heart of (Pce)Emacs. Modes define dedicated editing support for a particular kind of (source) text. For our purpose we want Prolog mode. There are various ways to make PceEmacs use Prolog mode for a file.
.pl or the selected alternative (e.g. .pro) 
extension, Prolog mode is selected.
#!/path/to/.../swipl#!/path/to/swipl options, Prolog mode is 
selected regardless of the extension.
-*- Prolog -*-
Below we list a few important commands and how to activate them.
DEL key or by 
typing something else at the location. Paste is achieved using 
the middle-mouse (or wheel) button. If you don't have a middle-mouse 
button, pressing the left- and right-button at the same time is 
interpreted as a middle-button click. If nothing helps, there is the Edit/Paste 
menu entry. Text is pasted at the caret location.
Control-_ as well as the 
MS-Windows
Control-Z sequence.
Control-G.
Control-S (forward) or
Control-R (backward). PceEmacs implements incremental 
search. This is difficult to use for novices, but very powerful 
once you get the clue. After one of the above start keys, the system 
indicates search mode in the status line. As you are typing the search 
string, the system searches for it, extending the search with every 
character you type. It illustrates the current match using a green 
background.
If the target cannot be found, PceEmacs warns you and no longer extends the search string.39GNU-Emacs keeps extending the string, but why? Adding more text will not make it match. During search, some characters have special meaning. Typing anything but these characters commits the search, re-starting normal edit mode. Special commands are:
Control-SControl-RControl-WControl-GESCBackspace
Alt-/, causing 
PceEmacs to search backwards for identifiers that start the same and use 
it to complete the text you typed. A second Alt-/ searches 
further backwards. If there are no hits before the caret, it starts 
searching forwards. With some practice, this system allows for entering 
code very fast with nice and readable identifiers (or other difficult 
long words).
Control-x Control-f). 
By default the file is loaded into the current window. If you want to 
keep this window, press Alt-s or click the little icon at 
the bottom left to make the window sticky.
Control-x 2 to create a new window pointing to the same 
file. Do not worry, you can edit as well as move around in both.
Control-x 1 kills all other windows running on the same 
file.
These are the most commonly used commands. In section 3.4.3 we discuss specific support for dealing with Prolog source code.
In the previous section (section 3.4.2) we explained the basics of PceEmacs. Here we continue with Prolog-specific functionality. Possibly the most interesting is Syntax highlighting. Unlike most editors where this is based on simple patterns, PceEmacs syntax highlighting is achieved by Prolog itself actually reading and interpreting the source as you type it. There are three moments at which PceEmacs checks (part of) the syntax.
..
Control-c Control-s
Control-l Control-lControl-l command re-centers the window (scrolls the 
window to make the caret the center of the window). Typing this command 
twice starts the same process as above.
The colour schema itself is defined in
library(emacs/prolog_colour). The colouring can be extended 
and modified using multifile predicates. Please check this source file 
for details. In general, underlined objects have a popup (right-mouse 
button) associated with common commands such as viewing the 
documentation or source. Bold text is used to indicate the 
definition of objects (typically predicates when using plain Prolog). 
Other colours follow intuitive conventions. See table 
3.
| Clauses | |
| Blue bold | Head of an exported predicate | 
| Red bold | Head of a predicate that is not called | 
| Black bold | Head of remaining predicates | 
| Calls in the clause body | |
| Blue | Call to built-in or imported predicate | 
| Red | Call to undefined predicate | 
| Purple | Call to dynamic predicate | 
| Other entities | |
| Dark green | Comment | 
| Dark blue | Quoted atom or string | 
| Brown | Variable | 
Layout support
Layout is not `just nice', it is essential for writing 
readable code. There is much debate on the proper layout of Prolog. 
PceEmacs, being a rather small project, supports only one particular 
style for layout.41Defined in 
Prolog in the file library(emacs/prolog_mode), you may wish 
to extend this. Please contribute your extensions! Below 
are examples of typical constructs.
head(arg1, arg2).
head(arg1, arg2) :- !.
head(Arg1, arg2) :- !,
        call1(Arg1).
head(Arg1, arg2) :-
        (   if(Arg1)
        ->  then
        ;   else
        ).
head(Arg1) :-
        (   a
        ;   b
        ).
head :-
        a(many,
          long,
          arguments(with,
                    many,
                    more),
          and([ a,
                long,
                list,
                with,
                a,
              | tail
              ])).
PceEmacs uses the same conventions as GNU-Emacs. The TAB 
key indents the current line according to the syntax rules. Alt-q 
indents all lines of the current clause. It provides support for head, 
calls (indented 1 tab), if-then-else, disjunction and argument lists 
broken across multiple lines as illustrated above.
The command Alt-. extracts name and arity from the caret 
location and jumps (after conformation or edit) to the definition of the 
predicate. It does so based on the source-location database of loaded 
predicates also used by edit/1. 
This makes locating predicates reliable if all sources are loaded and 
up-to-date (see make/0).
In addition, references to files in use_module/[1,2], consult/1, etc. are red if the file cannot be found and underlined blue if the file can be loaded. A popup allows for opening the referenced file.