26.5 Parse diagnosis checklist

A checklist for diagnosing parse problems will depend, clearly, on the facilities offered by the diagnostics layer in the application. What follows is a “bare bones” checklist, based on the diagnostic facilities recommended in this chapter.

• Check the location of the failure, and any error message. This is an obvious step and, if you are reading this checklist, you most likely have already taken it.
• Use the token tracing facility. Even if the problem is not directly with the tokens, the token trace is a good way to see how far the parse got, and what it was doing. If you are lucky, looking at it may make it apparent where the problem is.
• List the rules and the symbols. Perhaps you’ll see an issue. In any case, you will need these listings for the next step on this checklist.
• List the latest Earley set. Form a theory of what should be happening in the parse and check it against what the listing of the latest Earley shows you is actually happening.
• If the fix is still not apparent, list earlier Earley sets, until you see the fix, or you reach the first one with an error. Figure out a way to change the grammar to fix the first erroneous Earley set, then work forward.