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<h2 id="sec:pceemacs"><a id="sec:3.4"><span class="sec-nr">3.4</span> <span class="sec-title">Using 
the PceEmacs built-in editor</span></a></h2>

<a id="sec:pceemacs"></a>

<p><h3 id="sec:runpceemacs"><a id="sec:3.4.1"><span class="sec-nr">3.4.1</span> <span class="sec-title">Activating 
PceEmacs</span></a></h3>

<a id="sec:runpceemacs"></a>

<p>Initially <a id="idx:edit1:276"></a><a class="pred" href="edit.html#edit/1">edit/1</a> 
uses the editor specified in the <code>EDITOR</code> environment 
variable. There are two ways to force it to use the built-in editor. One 
is to set the Prolog flag <a class="flag" href="flags.html#flag:editor">editor</a> 
to
<code>pce_emacs</code> and the other is by starting the editor 
explicitly using the <a id="idx:emacs01:277"></a><span class="pred-ext">emacs/[0,1]</span> 
predicates.

<p><h3 id="sec:emacsbluff"><a id="sec:3.4.2"><span class="sec-nr">3.4.2</span> <span class="sec-title">Bluffing 
through PceEmacs</span></a></h3>

<a id="sec:emacsbluff"></a>

<p>PceEmacs closely mimics Richard Stallman's GNU-Emacs commands, adding 
features from modern window-based editors to make it more acceptable for 
beginners.<sup class="fn">29<span class="fn-text">Decent 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.</span></sup>

<p>At the basis, PceEmacs maps keyboard sequences to methods defined on 
the extended <b>editor</b> 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 <em>mode</em> 
is provided through <strong>Help/Show key bindings</strong> (<code>Control-h 
Control-b</code>).

<p><h4 id="sec:pceemacsmodes"><a id="sec:3.4.2.1"><span class="sec-nr">3.4.2.1</span> <span class="sec-title">Edit 
modes</span></a></h4>

<a id="sec:pceemacsmodes"></a>

<p>Modes are the heart of (Pce)Emacs. Modes define dedicated editing 
support for a particular kind of (source) text. For our purpose we want
<em>Prolog mode</em>. There are various ways to make PceEmacs use Prolog 
mode for a file.

<p>
<ul class="latex">
<li><i>Using the proper extension</i><br>
If the file ends in <code>.pl</code> or the selected alternative (e.g. <code>.pro</code>) 
extension, Prolog mode is selected.

<p>
<li><i>Using <code>#!/path/to/pl</code></i><br>
If the file is a <em>Prolog Script</em> file, starting with the line
<code>#!/path/to/pl options -s</code>, Prolog mode is selected 
regardless of the extension.

<p>
<li><i>Using <code>-*- Prolog -*-</code></i><br>
If the above sequence appears in the first line of the file (inside a 
Prolog comment) Prolog mode is selected.

<p>
<li><i>Explicit selection</i><br>
Finally, using <strong>File/Mode/Prolog</strong> (<code>File/Mode/Prolog</code>)ou 
can switch to Prolog mode explicitly.
</ul>

<p><h4 id="sec:pceemacscommands"><a id="sec:3.4.2.2"><span class="sec-nr">3.4.2.2</span> <span class="sec-title">Frequently 
used editor commands</span></a></h4>

<a id="sec:pceemacscommands"></a>

<p>Below we list a few important commands and how to activate them.

<p>
<ul class="latex">
<li><i>Cut/Copy/Paste</i><br>
These commands follow Unix/X11 traditions. You're best suited with a 
three-button mouse. After selecting using the left-mouse (double-click 
uses word-mode and triple line-mode), the selected text is
<em>automatically</em> copied to the clipboard (X11 primary selection on 
Unix). <em>Cut</em> is achieved using the <code>DEL</code> key or by 
typing something else at the location. <em>Paste</em> 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 <strong>Edit/Paste</strong> 
menu entry. Text is pasted at the caret location.

<p>
<li><i>Undo</i><br>
Undo is bound to the GNU-Emacs <code>Control-_</code> as well as the 
MS-Windows
<code>Control-Z</code> sequence.

<p>
<li><i>Abort</i><br>
Multi-key sequences can be aborted at any stage using <code>Control-G</code>.

<p>
<li><i>Find</i><br>
Find (Search) is started using <code>Control-S</code> (forward) or
<code>Control-R</code> (backward). PceEmacs implements <em>incremental 
search</em>. 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.

<p>If the target cannot be found, PceEmacs warns you and no longer 
extends the search string.<sup class="fn">30<span class="fn-text">GNU-Emacs 
keeps extending the string, but why? Adding more text will not make it 
match.</span></sup> During search, some characters have special meaning. 
Typing anything but these characters commits the search, re-starting 
normal edit mode. Special commands are:

<dl class="latex">
<dt><b><code>Control-S</code></b></dt>
<dd class="defbody">
Search forwards for next.
</dd>
<dt><b><code>Control-R</code></b></dt>
<dd class="defbody">
Search backwards for next.
</dd>
<dt><b><code>Control-W</code></b></dt>
<dd class="defbody">
Extend search to next word boundary.
</dd>
<dt><b><code>Control-G</code></b></dt>
<dd class="defbody">
Cancel search, go back to where it started.
</dd>
<dt><b><code>ESC</code></b></dt>
<dd class="defbody">
Commit search, leaving caret at found location.
</dd>
<dt><b><code>Backspace</code></b></dt>
<dd class="defbody">
Remove a character from the search string.
</dd>
</dl>

<p>
<li><i>Dynamic Abbreviation</i><br>
Also called <em>dabbrev</em>, dynamic abbreviation is an important 
feature of Emacs clones to support programming. After typing the first 
few letters of an identifier, you may press <code>Alt-/</code>, causing 
PceEmacs to search backwards for identifiers that start the same and use 
it to complete the text you typed. A second <code>Alt-/</code> 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).

<p>
<li><i>Open (a file)</i><br>
Is called <strong>File/Find file</strong> (<code>Control-x Control-f</code>). 
By default the file is loaded into the current window. If you want to 
keep this window, press <code>Alt-s</code> or click the little icon at 
the bottom left to make the window <em>sticky</em>.

<p>
<li><i>Split view</i><br>
Sometimes you want to look at two places in the same file. To do this, 
use <code>Control-x 2</code> to create a new window pointing to the same 
file. Do not worry, you can edit as well as move around in both.
<code>Control-x 1</code> kills all other windows running on the same 
file.
</ul>

<p>These are the most commonly used commands. In
<a class="sec" href="pceemacs.html">section 3.4.3</a> we discuss 
specific support for dealing with Prolog source code.

<p><h3 id="sec:emacsprologmode"><a id="sec:3.4.3"><span class="sec-nr">3.4.3</span> <span class="sec-title">Prolog 
Mode</span></a></h3>

<a id="sec:emacsprologmode"></a>

<p>In the previous section (<a class="sec" href="pceemacs.html">section 
3.4.2</a>) we explained the basics of PceEmacs. Here we continue with 
Prolog-specific functionality. Possibly the most interesting is <em>Syntax 
highlighting</em>. 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.

<p>
<ul class="latex">
<li><i>After typing a <code><code>.</code></code></i><br>
After typing a <code><code>.</code></code> that is not preceded by a <em>symbol</em> 
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.

<p>
<li><i>After the command <code>Control-c Control-s</code></i><br>
Acronym for <b>C</b>heck <b>S</b>yntax, it performs the same checks as 
above for the clause surrounding the caret. On a syntax error, however, 
the caret is moved to the expected location of the error.<sup class="fn">31<span class="fn-text">In 
most cases the location where the parser cannot proceed is further down 
the file than the actual error location.</span></sup>

<p>
<li><i>After pausing for two seconds</i><br>
After a short pause (2 seconds), PceEmacs opens the edit buffer and 
reads it as a whole, creating an index of defined, called, dynamic, 
imported and exported predicates. After completing this, it re-reads the 
file and colours all clauses and calls with valid syntax.

<p>
<li><i>After typing <code>Control-l Control-l</code></i><br>
The <code>Control-l</code> 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.
</ul>

<p><b>The colour schema</b> itself is defined in
<code>library(emacs/prolog_colour)</code>. 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. <b>Bold</b> text is used to indicate the 
definition of objects (typically predicates when using plain Prolog). 
Other colours follow intuitive conventions. See <a class="tab" href="pceemacs.html#tab:plcolour">table 
3</a>.

<p><table class="latex frame-box center">
<tr><td colspan=2 align=center>Clauses</tr>
<tr class="hline"><td>Blue bold</td><td>Head of an exported predicate </td></tr>
<tr><td>Red bold</td><td>Head of a predicate that is not called </td></tr>
<tr><td>Black bold</td><td>Head of remaining predicates </td></tr>
<tr class="hline"><td colspan=2 align=center>Calls in the clause body</tr>
<tr class="hline"><td>Blue</td><td>Call to built-in or imported 
predicate </td></tr>
<tr><td>Red</td><td>Call to undefined predicate </td></tr>
<tr><td>Purple</td><td>Call to dynamic predicate </td></tr>
<tr class="hline"><td colspan=2 align=center>Other entities</tr>
<tr class="hline"><td>Dark green</td><td>Comment </td></tr>
<tr><td>Dark blue</td><td>Quoted atom or string </td></tr>
<tr><td>Brown</td><td>Variable </td></tr>
</table>
<a id="tab:plcolour"></a>
<div class="caption"><b>Table 3 : </b>Colour conventions</div>

<p><b>Layout support</b> 

<p>Layout is not `just nice', it is <em>essential</em> 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.<sup class="fn">32<span class="fn-text">Defined in 
Prolog in the file <code>library(emacs/prolog_mode)</code>, you may wish 
to extend this. Please contribute your extensions!</span></sup> Below 
are examples of typical constructs.

<pre class="code">
head(arg1, arg2).

head(arg1, arg2) :- !.

head(Arg1, arg2) :- !,
        call1(Arg1).

head(Arg1, arg2) :-
        (   if(Arg1)
        -&gt;  then
        ;   else
        ).

head(Arg1) :-
        (   a
        ;   b
        ).

head :-
        a(many,
          long,
          arguments(with,
                    many,
                    more),
          and([ a,
                long,
                list,
                with,
                a,
              | tail
              ])).
</pre>

<p>PceEmacs uses the same conventions as GNU-Emacs. The <code>TAB</code> 
key indents the current line according to the syntax rules. <code>Alt-q</code> 
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.

<p><h4 id="sec:pceemacshelp"><a id="sec:3.4.3.1"><span class="sec-nr">3.4.3.1</span> <span class="sec-title">Finding 
your way around</span></a></h4>

<a id="sec:pceemacshelp"></a>

<p>The command <code>Alt-.</code> 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 <a id="idx:edit1:278"></a><a class="pred" href="edit.html#edit/1">edit/1</a>. 
This makes locating predicates reliable if all sources are loaded and 
up-to-date (see <a id="idx:make0:279"></a><a class="pred" href="consulting.html#make/0">make/0</a>).

<p>In addition, references to files in <a id="idx:usemodule12:280"></a><span class="pred-ext">use_module/[1,2]</span>, <a id="idx:consult1:281"></a><a class="pred" href="consulting.html#consult/1">consult/1</a>, 
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.

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