Text extraction systems ought to determine who did what to whom, but it can be difficult to simply extract names, dates, places, etc. Most information retrieval systems still treat text as merely a bag of words with little or no linguistic structure . There have been numerous attempts to make use of richer linguistic structures such as phrases , predicate argument relations , and even morphology , but, thus far, most of these attempts have not resulted in significant improvements in retrieval performance. Current natural language processing systems lack lexical and grammatical resources with sufficient coverage for unrestricted text . Time flies like an arrow .
Fruit flies like a banana .
It would be useful for many applications to know that fruit flies is a phrase and time flies is not. Most systems currently do not have access to this kind of information. Parsers currently operate at the level of parts of speech , without looking at the words. Ultimately, parsers and other natural language applications will have to make greater use of collocational constraints and other constraints on words. The task may require a monumental effort like Murray's Oxford English Dictionary project .