Tcl Library Source Code

Hi!

As promised, I reworked a series of bug fixes on PEG transformations (realizability, dropping) into a fix branch: [https://core.tcl.tk/tcllib/timeline?r=pt-container-ssoberni].

In short:

1. minimize: the drop operations were in wrong order.

2. realizability: kleene star and optionals should be set realizable by definition (is consistent with other PEG environments, incl. tcllib's page).

3. Drop: 

o wrong variable name in one, previously untested code branch.

o drop was too permissive on expressions other than choice. In line with the realizability ruling, only choice should survive any removed children.

Generally, offering the otherwise CFG-aware grammar transformations should maybe prominently be marked with a disclaimer in the docs? This is because treating a PEG as a CFG is not necessarily a good idea, although limited use of transformations is warranted. A disclaimer should state that realizability of an ordered choice cannot be decided statically and is conceptually itchy (non-disjointness). Also, realizability of predicates is certainly debatable (in the sense of a non-productive or a non-recognising expression). In any case, the message should be that the result of a minimisation will not yield a minimal PEG, rather a conservative approximation of a minimisation, at best.

What do you think?

Final observation, but also debatable: ::pt::peg::op::drop::unrealizable starts from the set of defined non-terminals (RHS of rules), and not all reachable ones. This is not an issue per se, but if one composes PEGs or the corresponding rules (e.g., with deferred non-terminals) then they will be just dropped, along with most of the rest of the rules set. A more permissive, conservative setting would switching from

   set all [$container nonterminals]

to

   set all [::pt::peg::op reachable $container]

Let me know what you think!

Thx,
Stefan