Suddenly filler-gap dependencies

Theoreticians say: in sentences like I know who you saw the deep structure is in fact I know [you saw who]. And in the surface structure there's an invisible gap after saw which is filled by who. Many psycholinguistic studies also seem to confirm that: upon seeing who people start to wait for a right place for it and only settle down after finding it.

Due to Russian free word order, I've had a luxury to ignore this complex thing for a while and treat wh-words as normal verbal arguments, like pronouns. But then two surprises have come.

One surprise was that implementing filler-gap dependencies was the easiest way to resolve a nasty ambiguity. Russian has a word что which can be either a complementizer (я знаю, что ты видел его; I know that you saw him) or a wh-word (я знаю, что ты видел; I know what you saw). The first one is higher in structure than the verb, the second one is lower. This made my parser suffer, it still doesn't like visibility ambiguities very well. Now что is no longer a verbal argument directly, it's a filler and is also higher in the hierarchy, just like in many syntactic theories.

Another surprise was that all this was actually very easy to add in the current parser architecture (given that there's no pied-piping yet). The filler is just a special construction which listens for what the incoming words contribute. If a contribution looks as a right head for the filler's grammatical functions, the contribution is enriched accordingly.

Example: Russian wh-word что can be in nominative or accusative case. For normal nouns that would mean it should generate nom and acc construction mites with noun attribute defined, pointing to a frame with some special wh semantic type. In the filler-gap approach it generates a filler construction instead which then sits and waits until it sees a contribution with nom or acc mites with head attribute defined. E.g. saw as a verb can be a head to both nominative and accusative arguments. The filler construction then adds a nom/acc mite having both head and noun attributes, where the noun points to a frame with wh type, and the head comes from the verb.

So how my parser works in this aspect now is quite similar to human sentence processing: a wh-word creates an active filler that finds a gap when there comes a verb with suitable argument requirements.