In the blog post Parsing: The Solved Problem That Isn't Laurence Tratt discusses some interesting unsolved practical problems with parsing especially in combining grammars
The general consensus, therefore, is that parsing is a solved problem. If you've got a parsing problem for synthetic languages, one of the existing tools should do the job. [...]
One of the things that's become increasingly obvious to me over the past few years is that the general consensus breaks down for one vital emerging trend: language composition. "Composition" is one of those long, complicated, but often vague terms that crops up a lot in theoretical work. Fortunately, for our purposes it means something simple: grammar composition, which is where we add one grammar to another and have the combined grammar parse text in the new language (exactly the sort of thing we want to do with Domain Specific Languages (DSLs)). To use a classic example, imagine that we wish to extend a Java-like language with SQL [...]
He goes on to mention several example problems:
- Two LL or LR grammars may combine to produce a grammar that is neither.
- Two unambiguous grammars may combine to produce an ambiguous grammar.
- Two PEG grammars may combine to produce something that doesn't do what you want due to left bias.
What's the current state of the art?
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