Week5-8Alternative Parsing Formalisms

Mildly Context-sensitive grammars

-Tree substitution grammars(TSG)

- Terminals generate entire tree fragments

- TSG and CFG are formally equivalent

- Tree adjoining grammar(TAG)

- Combinatory categorical grammar(CCG)

TAG

Like TSG but allow adjunction

Adjunction: suppose we have a parse tree with verb phrase embedded in it. Adjunction allows you to create a new node and to put it inside the tree and push the existing sub-tree further down. So we will have a new verb phrase that have the original verb phrase as its descendents.

It can generate languages like anbncn or ww(cross-serial dependencies)(repeated same strings):

Mary gave a book and a magazine to Chen and Mike, respectively. (cannot be generated by CFG but can by TAG)

expressive power

TAG is formally more powerful than CFG

TAG is less powerful than CSG

CCG

-Complex type

- X/Y, X\Y

- Take an argument of type Y, and return an object of type X.

- X/Y means that Y should appear on the right

- X\Y means that Y should appear on the left



Expressive power

CCGs can generate the language anbncndn, n>0

Semantic Parsing

Associate a semantic expression with each node



One of the hottest areas in the next few years.