"There are very many functionalist approaches which have been put forward, and they are often very different from one another. Two prominent ones are Role-and-Reference Grammar (RRG), developed by William Foley and Robert Van Valin, and Systemic Linguistics (SL), developed by Michael Halliday. RRG approaches linguistic description by asking what communicative purposes need to be served and what grammatical devices are available to serve them. SL is chiefly interested in examining the structure of a large linguistic unit--a text or a discourse--and it attempts to integrate a great deal of structural information with other information (social information, for example) in the hope of constructing a coherent account of what speakers are doing. "Functionalist approaches have proved fruitful, but they are usually hard to formalize, and they often work with 'patterns,' 'preferences,' 'tendencies,' and 'choices,' in place of the explicit rules preferred by non-functional linguists."