wanted: way to enable/disable specific rules at runtime

[3b]

For example, supporting multiple versions of a grammar, or selecting optional extensions.

3b/esrap@7f3d9d907f08ce1a6060c31278a449545b52a404 adds support for this by allowing (<predicate>) without arguments, but it seems a bit inconvenient for the common case of just checking a global variable. Possibly it would be better to just add a rule to check a variable directly, or evaluate a chunk of code if that can be implemented efficiently.

See parser.lisp in 3b/3bmd@519b35ba85b678d04d990d4b682470ff092da4a7 and 3b/3bmd@4aad4867ca5d6e89ba4e6637f3316ad09d498d2d for example use cases.

[nikodemus]

I added support for guards onto "guards" branch.

(defrule foo (and "foo" (or "bar" "foo"))
  (:when *use-foo*))

Does that do the right thing for you?

I'm a bit hesitant to add support for no-arg predicates, since a predicate that isn't purely functional (and consistent for the whole parse) breaks the basic caching assumption of packrat parsing:

(defrule bad ...
   (:lambda (res)
       (setf *going-bad* t)
       res))

(defun worse () *going-bad*)

(defrule worst (and (worse) ...)
   ...)

is never going to work right.

[3b]

That works (with small fix at 3b/esrap@1d020a3).

[nikodemus]

Thanks, merged onto master.