User:Mlforcada/Robust LR for Transfer

We need a way to implement Apertium4 Bison grammars that are robust.

Here, Bison GRAMMAR is a LALR(1) grammar with actions between braces.

There should be a way (or a procedure) to complete handwritten rules, if possible automatically, to generate a robust parser (and translator). The idea is to take the handwritten Bison grammar $G$ and complement it with automatically-generated glue rules in such a way that conflicts are not produced (or are harmless) to produce a new grammar $G'$.

One possible way to do so is to model left-to-right restarts as follows:

• by accepting the longest possible constituent (in the original grammar $G$ that cannot be merged with the remaining output and generating some kind of translation for it
• and treating the remaining output as a complete sentence again

For instance, if the grammar $G$ is:

S : NP VP  { write(agree(nom($1),$2)) };
NP : n   { write($1) } ;
VP : v  { write($1) } 
   | v NP  { write(acc($2),$1) };

the sequence $vnn$ could not be parsed and would lead to an error. However, it could be seen as a NP followed by a VP, and a translation could be generated for both. One could augment the grammar by systematically adding some rules:

S : NP VP  { write(agree(nom($1),$2)) }
  | NP TryS { write($1); write($2) }      # translate an NP and skip
  | VP TryS { write($1); write($2) }      # translate a VP and skip
  ;
TryS : S  { write($1); }
     | ;                                  # the remaining part may be empty
NP : n   { write($1) } ;
VP : v  { write($1) } 
   | v NP  { write(acc($2),$1) };