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
explictely 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. (9)
At the basis, PceEmacs maps keyboard sequences to methods defined on
the extended editor 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. Their 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/pl#!/path/to/pl options -s, Prolog mode is selected
regardless of the extension
-*- Prolog -*-
File/Mode/Prolog)ou
can switch to Prolog mode explicitly.
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. (10) During search some characters have special meaning. Typing anything but these characters commits the search, re-starting normal edit mode. Special commands are:
Control-S
Control-R
Control-W
Control-G
ESC
Backspace
Alt-/, causing PceEmacs to search backwards for
identifiers that start the same and using 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 very fast entering code 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, Hit 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 were the most commonly used commands. In section 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 commands re-centers the window (scrolls the
window to make the caret the center of the window). Hitting 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 for 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 not-defined predicate |
| Purple | Call to dynamic predicate |
| Other entities | |
| Dark green | Comment |
| Dark blue | Quoted atom or string |
| Brown | Variable |
| Table 3 : Colour conventions |
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. (12) 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.