|Did you know ...
|Using the PceEmacs built-in editor
uses the editor specified in the
variable. There are two ways to force it to use the built-in editor. One
is to set the Prolog flag editor
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.44Decent 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 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 (
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.
extension, Prolog mode is selected.
#!/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
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
If the target cannot be found, PceEmacs warns you and no longer extends the search string.45GNU-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:
PceEmacs to search backwards for identifiers that start the same and use
it to complete the text you typed. A second
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
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
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.
that is not preceded by a symbol
character, the system assumes you completed a clause, tries to find the
start of this clause and verifies the syntax. If this process succeeds
it colours the elements of the clause according to the rules given
below. Colouring is done using information from the last full check on
this file. If it fails, the syntax error is displayed in the status line
and the clause is not coloured.
Control-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
|Head of an exported predicate
|Head of a predicate that is not called
|Head of remaining predicates
|Calls in the clause body
|Call to built-in or imported predicate
|Call to undefined predicate
|Call to dynamic predicate
|Quoted atom or string
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.47Defined 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
key indents the current line according to the syntax rules.
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.
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.