;; - Maybe accept two juxtaposed non-terminals in the BNF under the condition
;; that the first always ends with a terminal, or that the second always
;; starts with a terminal.
+;; - Permit EBNF-style notation.
+;; - If the grammar has conflicts, the only way is to make the lexer return
+;; different tokens for the different cases. This extra work performed by
+;; the lexer can be costly and unnecessary: we perform this extra work every
+;; time we find the conflicting token, regardless of whether or not the
+;; difference between the various situations is relevant to the current
+;; situation. E.g. we may try to determine whether a ";" is a ";-operator"
+;; or a ";-separator" in a case where we're skipping over a "begin..end" pair
+;; where the difference doesn't matter. For frequently occurring tokens and
+;; rarely occurring conflicts, this can be a significant performance problem.
+;; We could try and let the lexer return a "set of possible tokens
+;; plus a refinement function" and then let parser call the refinement
+;; function if needed.
+;; - Make it possible to better specify the behavior in the face of
+;; syntax errors. IOW provide some control over the choice of precedence
+;; levels within the limits of the constraints. E.g. make it possible for
+;; the grammar to specify that "begin..end" has lower precedence than
+;; "Module..EndModule", so that if a "begin" is missing, scanning from the
+;; "end" will stop at "Module" rather than going past it (and similarly,
+;; scanning from "Module" should not stop at a spurious "end").
;;; Code:
;; the trouble, and it lets the writer of the BNF
;; be a bit more sloppy by skipping uninteresting base
;; cases which are terminals but not OPs.
- (assert (not (member (cadr rhs) nts)))
+ (when (member (cadr rhs) nts)
+ (error "Adjacent non-terminals: %s %s"
+ (car rhs) (cadr rhs)))
(pushnew (cadr rhs) first-ops)))
(let ((shr (reverse rhs)))
(if (not (member (car shr) nts))
(pushnew (car shr) last-ops)
(pushnew (car shr) last-nts)
(when (consp (cdr shr))
- (assert (not (member (cadr shr) nts)))
+ (when (member (cadr shr) nts)
+ (error "Adjacent non-terminals: %s %s"
+ (cadr shr) (car shr)))
(pushnew (cadr shr) last-ops)))))
(push (cons nt first-ops) first-ops-table)
(push (cons nt last-ops) last-ops-table)