Months later, after much contorsion, I've managed to put a viable (even if ugly) solution together... thereby making a liar out of myself -_-'
Automatic recognition of Prolog predicates, atoms and variables
The solution shown below automatically distinguishes (and highlights in a style very similar to Pygments') Prolog predicates, atoms and variables. No need to manually enter long lists of keywords! In a nutshell, the approach consists in the following:
- Make
listings
treat the opening-parenthesis character as a "letter", in order to test whether that character occurs at the end of identifiers; if it does, then the identifier in question is a predicate.
- Otherwise, test the first character of the identifier against
[A-Z_]
; if the test passes, the identifier is a variable.
- Otherwise, the identifier is an atom.
For a comparison, see the screenshot below.

Improvement over Pygments' Prolog lexer
I noticed that Pygments indiscriminately highlights underscores in dark green, and highlights whatever word immediately follows that character as an atom... That doesn't make sense to me, because, according to the Wikipedia page on Prolog syntax, an identifier starting by an underscore is a variable, not an atom.
The approach shown below improves upon the Pygments Prolog lexer insofar as it highlights underscores differently in different contexts:
- it correctly highlights identifiers starting by an underscore, but composed of at least two characters, as (non-anonymous) Prolog variables, and
- it highlights lone underscores, i.e. Prolog anonymous variables, in a distinct style (dark green).
Correct me if I'm wrong, here; I'm no Prolog expert.
Missing feature
I didn't bother replicating the highlighting style for numbers; as other questions on the subject attest, that is notoriously difficult, so I'll pass on that, at least for now.
Code
\documentclass{article}
\usepackage[top=1in]{geometry}
\usepackage{textcomp}
\usepackage{listings}
%\usepackage{minted} % (requires -shell-escape)
\usepackage{xcolor}
\usepackage{filecontents}
% --- ugly internals for language definition ---
%
\makeatletter
% initialisation of user macros
\newcommand\PrologPredicateStyle{}
\newcommand\PrologVarStyle{}
\newcommand\PrologAnonymVarStyle{}
\newcommand\PrologAtomStyle{}
\newcommand\PrologOtherStyle{}
\newcommand\PrologCommentStyle{}
% useful switches (to keep track of context)
\newif\ifpredicate@prolog@
\newif\ifwithinparens@prolog@
% save definition of underscore for test
\lst@SaveOutputDef{`_}\underscore@prolog
% local variables
\newcount\currentchar@prolog
\newcommand\@testChar@prolog%
{%
% if we're in processing mode...
\ifnum\lst@mode=\lst@Pmode%
\detectTypeAndHighlight@prolog%
\else
% ... or within parentheses
\ifwithinparens@prolog@%
\detectTypeAndHighlight@prolog%
\fi
\fi
% Some housekeeping...
\global\predicate@prolog@false%
}
% helper macros
\newcommand\detectTypeAndHighlight@prolog
{%
% First, assume that we have an atom.
\def\lst@thestyle{\PrologAtomStyle}%
% Test whether we have a predicate and modify the style accordingly.
\ifpredicate@prolog@%
\def\lst@thestyle{\PrologPredicateStyle}%
\else
% Test whether we have a predicate and modify the style accordingly.
\expandafter\splitfirstchar@prolog\expandafter{\the\lst@token}%
% Check whether the identifier starts by an underscore.
\expandafter\ifx\@testChar@prolog\underscore@prolog%
% Check whether the identifier is '_' (anonymous variable)
\ifnum\lst@length=1%
\let\lst@thestyle\PrologAnonymVarStyle%
\else
\let\lst@thestyle\PrologVarStyle%
\fi
\else
% Check whether the identifier starts by a capital letter.
\currentchar@prolog=65
\loop
\expandafter\ifnum\expandafter`\@testChar@prolog=\currentchar@prolog%
\let\lst@thestyle\PrologVarStyle%
\let\iterate\relax
\fi
\advance \currentchar@prolog by 1
\unless\ifnum\currentchar@prolog>90
\repeat
\fi
\fi
}
\newcommand\splitfirstchar@prolog{}
\def\splitfirstchar@prolog#1{\@splitfirstchar@prolog#1\relax}
\newcommand\@splitfirstchar@prolog{}
\def\@splitfirstchar@prolog#1#2\relax{\def\@testChar@prolog{#1}}
% helper macro for () delimiters
\def\beginlstdelim#1#2%
{%
\def\endlstdelim{\PrologOtherStyle #2\egroup}%
{\PrologOtherStyle #1}%
\global\predicate@prolog@false%
\withinparens@prolog@true%
\bgroup\aftergroup\endlstdelim%
}
% language name
\newcommand\lang@prolog{Prolog-pretty}
% ``normalised'' language name
\expandafter\lst@NormedDef\expandafter\normlang@prolog%
\expandafter{\lang@prolog}
% language definition
\expandafter\expandafter\expandafter\lstdefinelanguage\expandafter%
{\lang@prolog}
{%
language = Prolog,
keywords = {}, % reset all preset keywords
showstringspaces = false,
alsoletter = (,
alsoother = @$,
moredelim = **[is][\beginlstdelim{(}{)}]{(}{)},
MoreSelectCharTable =
\lst@DefSaveDef{`(}\opparen@prolog{\global\predicate@prolog@true\opparen@prolog},
}
% Hooking into listings to test each ``identifier''
\newcommand\@ddedToOutput@prolog\relax
\lst@AddToHook{Output}{\@ddedToOutput@prolog}
\lst@AddToHook{PreInit}
{%
\ifx\lst@language\normlang@prolog%
\let\@ddedToOutput@prolog\@testChar@prolog%
\fi
}
\lst@AddToHook{DeInit}{\renewcommand\@ddedToOutput@prolog{}}
\makeatother
%
% --- end of ugly internals ---
% --- definition of a custom style similar to that of Pygments ---
% custom colors
\definecolor{PrologPredicate}{RGB}{000,031,255}
\definecolor{PrologVar} {RGB}{024,021,125}
\definecolor{PrologAnonymVar}{RGB}{000,127,000}
\definecolor{PrologAtom} {RGB}{186,032,032}
\definecolor{PrologComment} {RGB}{063,128,127}
\definecolor{PrologOther} {RGB}{000,000,000}
% redefinition of user macros for Prolog style
\renewcommand\PrologPredicateStyle{\color{PrologPredicate}}
\renewcommand\PrologVarStyle{\color{PrologVar}}
\renewcommand\PrologAnonymVarStyle{\color{PrologAnonymVar}}
\renewcommand\PrologAtomStyle{\color{PrologAtom}}
\renewcommand\PrologCommentStyle{\itshape\color{PrologComment}}
\renewcommand\PrologOtherStyle{\color{PrologOther}}
% custom style definition
\lstdefinestyle{Prolog-pygsty}
{
language = Prolog-pretty,
upquote = true,
stringstyle = \PrologAtomStyle,
commentstyle = \PrologCommentStyle,
literate =
{:-}{{\PrologOtherStyle :-}}2
{,}{{\PrologOtherStyle ,}}1
{.}{{\PrologOtherStyle .}}1
}
% global settings
\lstset
{
captionpos = below,
frame = single,
columns = fullflexible,
basicstyle = \ttfamily,
}
% write some sample code to an external file
\begin{filecontents*}{sample.pl}
somePredicate(_, B) :-
arbitraryPredicate(A, _variable, 1, 2),
predicateWithAtom(someAtom),
anotherPredicate(B, someAtom, myPredicate(A, _)),
findall(X, ('testString'(X), myPredicate(A, X)), L1),
member(A, L1),
!.
/*
block comment: blah blah blah
*/
% to-end-of-line comment: blah blah blah
\end{filecontents*}
\begin{document}
\section{With \textsf{minted} / Pygments}
%\begin{listing}
%\inputminted[frame=single]{prolog}{sample.pl}
%\caption{Sample Prolog code}
%\end{listing}
\section{With \textsf{Listings}}
\setcounter{lstlisting}{1}
\lstinputlisting[
style = Prolog-pygsty,
caption = {Using my custom \textsf{listings} style},
]{sample.pl}
\lstinputlisting[
language = Prolog,
caption = {Using \textsf{listings}' default settings for Prolog},
]{sample.pl}
\end{document}
predicate(anotherPredicate(X))
. TheanotherPredicate
is then highlighted just as any other predicate (light blue) and not as an atom.listings
lovers will be able to figure out whether implementing such a style inlistings
is feasible.(
is found immediately after the identifier. If a(
is found, then the identifier is a predicate; otherwise, the identifier is an atom. In theory, you could declare all characters as "letters" in order to allow for such a look-ahead; however, that would seriously upsetlistings
and keyword recognition would be, by and large, completely broken. In summary: no, that type of highlighting is not possible withlistings
.