The Correct And Efficient Implementation Of Appropriateness Specifications For Typed Feature Structures

in this pa,per, we argue tha, t type inferencing incorrect ly implements a.pl)rolwiateness specifica.tions for typed [ea.ture structures, p romote a combina.tion of l;ype resolut ion and unfil l ing a,s a. correct a.nd ef'~ ticient Mternat ive , and consider the expressive l imits of this a.lterna.tive approa.ch. ![ 'hroughout, we use feature cooccurence restr ict ions as i l lus t ra t ion and linguistic mot iva t ion . 1 I N T R O D U C T I O N Unificat ion lbrmMisms ma.y be either untyped (DCC~s, PATR-II, 1,F(;) or typed (npsG). A m~L,ior reason for adding types to ~ forma,lism is to express restr ict ions on fea.ture cooccurences a.s in (;l's(:: [5] in order to rule out nonexista.nt tyl)es of objects . For example, there a.re no verbs which have the [km.ture +R. The simplest way to express such restrictions is by mea.ns of a.n a.ppropria.teness pa.rt im flmction Approp: Type × Feat ~ Type. With such a.n a.pl)rol)riatleness specifica.t ion lrla.tly Sllch restrictioi,s may be expressed, though no restrictions involving reentrancies ma.y be expressed. In this pal)er, we will first in §2 survey the range of type eonstra.ints tha.t ma.y be expressed with jus t a. type hiera.rchy and *']'he resea.rch pl'eS(!lllL('d ill |,his; paper was pay tia.lly sponsored hy '[kfilprojekt B4 "(;onsl.rahH.s on Grammar fl~r Efficient Ck:neration" of the Soi,der forschungsbereich 340 of the Deutsche [ "orschungsgemeinscha , ft. "VVe would also like to thank 'l'hilo GStz for helph,l comments ou thc ideas present.ed here. All mistakes a.rc of collrsc our OWll. IKI. Wilhehnstr. 113, |)-721174Tfilfi,,ge,, (lerma.ny, {rig,King} g'~sfs.n phil.uni-I uebingen.de. a.n N)propria.teness specification. Then in ~3, we discuss how such type cons | f ronts linty be mainta. ined under unif icat ion as exemplilied in the na.tura.1 language D~rsing /genera t ion system '.l'ro]l [7]. 1 Unlike previous systems such as ALl,:, Troll does not employ a.ny type infereneing, inste~M, a, l imited a m o u n t of nam ed dis junct ion ([1 1], [12], [6])is in t roduced to record type resol u tion possibilities. The a.lnount of disjunc t ion is a.lso kept small by the technique of unlilli,g described in [9]. This s t ra tegy a.ctua.lly ma.inta.ins apl)ropri~tteness conditions in some ca.ses in which a. type inferencing stra.tegy would fa.il, l)'inMly, in §4, we discuss the possibilities for genera lizillg this a.pl)roa.ch to ha.ndle a bro~Mer r~tnge of constra.ints, inc luding cons t ra in ts inw)lving reent ran cies. 2 A P P R O P R I A T E N E S S FOR, M A L I S M S As discussed iu Ge rdem ann ,~ King [8], one ca.n view a.pl}rol)ria.teness CO[lditions as (lelining GPSG style fea,1;tl r e cooccurence restrict:ions (FCRs) . In [8], we divided FCRs into co,j,,ctive and di.q,,~ctive ct~sses. A conjunct ive FCI/. is a constra.int of the following fornl : i[' a.n object is of ;~ cert;fin kind then ill deserves certa.in fea.tures wi th wdues of cert~till kinds An FCI~ stat:ing tha,2: a. verb mus t h~we v and N t 'eatures with values Aand respectively is a.ll example of a. conjunct ive FCI{. A dis junct ive I"CI{. is of the form: l rl'he "]'roll ,qysl.em was implemented in Quintus Prolog by Dale (lerdemann and '['hilo (]Stz.