3.4.3 Prolog Mode

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.

• After typing a .
  After typing a . 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.

• After the command Control-c Control-s
  Acronym for Check Syntax, 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.^{48In most cases the location where the parser cannot proceed is further down the file than the actual error location.}

• After pausing for two seconds
  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.

• After typing Control-l Control-l
  The 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 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.^{49Defined 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.

3.4.3.1 Finding your way around

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.