Emacs syntax highlighting for DCG specs

[dcnorris]

Just sharing something in the early stages of experimenting with it, partly in the hope of hearing somebody has already done it better!

Feeling that an editor should celebrate Prolog best-practices such as defining and using DCGs, I've added the following to my .emacs, choosing font-lock-type-face for DCGs because its purple color seems appropriately 'noble':

;; Let's see what we can do to improve syntax highlighting of specifiers
;; for DCGs and operators.  Currently, prolog-mode highlights neither
;; `(-->)/2` nor `phrase//2`.
;; We also should aim to highlight _operator_ DCG exports like `... //0`,
;; with a separating space before //.
;; Furthermore, we must take care to allow for a leading comma preceding
;; such DCG-operator specs.
(let* ((atom-char "[[:alnum:]_$]")
       (atom (format "[[:lower:]$]%s*" atom-char))
       (op-char "[[:punct:]]") ; TODO: refine?
       )
  (defconst dcg-regexp
    (format "\\<\\(%s:\\|\\)%s//[0-9]+" atom atom)
    "Recognizes [possibly qualified] DCG identifiers [module:]dcg//arity")
  (defconst op-regexp (format "(%s*)/[0-9]+" op-char)
    "Recognizes [unqualified] operator functors (<operator>)/arity")
  )
;; Augment the 'Font Lock Patterns' defined in prolog.el
(defconst opspecs  ; (<operator-atom>)/arity
  (list op-regexp
        0 'font-lock-function-name-face))
(defconst dcgspecs  ; [module:]dcg//arity (cf. existing 'predspecs')
  (list dcg-regexp
        0 'font-lock-type-face))
(defconst dcgopspecs  ; <op-atom> //arity -- eg, `... //0` exported by library(dcgs)
  (list "[.$-][[:punct:]]+ //[0-9]+" ; restrict 1st char to allow for leading comma
        0 'font-lock-type-face 'prepend))

(add-hook 'prolog-mode-hook
          (lambda ()
            (font-lock-add-keywords nil
                                    (list opspecs
                                          dcgspecs
                                          dcgopspecs)
                                    t)))

[jjtolton]

I've actually been waiting for the embedded Scryer shared library to become more stable again to begin working on such a quest, buuuut I've been doing some socket work lately and having figured out the trick for non-blocking get_n_chars/3 I was considering doing the emacs parsing with Scryer itself.

My previous efforts with emacs tools for Scryer proved to me that parsing Prolog with elisp is ... not fun!