Semantic Errors

Semantic errors in the grammar can be trivial typos or the sign of a bad design of the FD structure. In any case check the following points:

• Mispellings: wrong feature-name, wrong leaf-value-name

• Paths: in both forms, i.e., ((f1 ((f2 ((f3 and {f1 f2 f3. Make sure that paths actually point to the location in the total FD that you expect.

• The most common source of error is to include the wrong number of up-arrows ^ in a path expression. Always double-check your relative paths, especially those appearing embedded in ALTs, CONTROL, EXTERNAL when the up-arrow notation can be quite counter-intuitive.    

• Observe the structure of the unified graph before unification. To this end, use the function uni-fd and pretty-print its output as in:

  (pprint (clean-fd (uni-fd input :grammar gr)))
  

  This is often instructive. Use the function top-gdp to explore this output FD in detail.      

• Draw a graph of the total FD with all features instantiated to help you check all your paths and levels of embedding.  

• If you do not understand the current structure of the FD, insert a %break% in your grammar at some critical point, and use the function path-value to inspect the total FD during the time of the unification.    

• It is risky to use absolute paths in general (SURGE does not include a single absolute path for example). Using an absolute path is a sign of desperation.