Marpa is a general parser: it can parse any context-free grammar. Owl can only parse the visibly-pushdown grammars, which are a much smaller class, even smaller

Marpa is a general parser: it can parse any context-free grammar. Owl can only parse the visibly-pushdown grammars, which are a much smaller class, even smaller than the LR(k) grammars that traditional parser-generators can parse. Besides guaranteed O(n) parsing time, it's not clear to me what the advantages of Owl are, and Marpa runs in O(n) time on any grammar that Owl accepts, and many more (and I think it can or could easily be modified to warn if it can't guarantee O(n) parsing time for a particular grammar).