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This presentation explores the implementation of PATRR bibliographic note material, leveraging Natural Language Processing (NLP) techniques in Prolog as described by Gazdar and Mellish (1989). We discuss the differences between PATRR and Prolog, covering explicit variable requirements, syntactic paths, grammar rules, and lexical entries. Key concepts such as path values and left-corner recognizers are analyzed, providing insights into their definitions and applications. This structured approach enhances understanding of NLP's role in linguistic processing and Prolog's capabilities in handling complex data structures.
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Bibliographic Note Material in this presentation is based on Natural Language Processing in Prolog G. Gazdar and C. Mellish, Addison-Wesley 1989.
Some Differences between Patr and Prolog • Prolog requires explicit variables • Syntactic appearance of paths • Grammar Rules • Lexical Entries • Templates
Rule Format R ule S ---> [NP, VP] :-S:cat === s,NP:cat === np,VP:cat === vp,S:head === VP:head,NP === VP:subcat:first.
Operator Definitions op(500,xfy,:).op(500,xfx,--->).op(600,xfx,===). op(400,xfx,ule).op(500,xfx,ord).
Definition of === • X===Y if and only if X and Y denote unifiable items. • We achieve this by saying that there is a third item that they both denote, i.e. X === Y :- denotes(X,Z), denotes(Y,Z).
Definition of denote(X,Y). • We must take account of the fact that X can be one of three things: variable, atom, or path denotes(Var,Var) :- var(Var), !. denotes(Atom,Atom) :- atomic(Atom), !. denotes(V:Path, Value) :- pathval(V,Path,Value).
Modifying pathval • Notice that the second argument to pathval need not be atomic. However, our existing definition for pathval assumes that it is. • We therefore add a clause that applies pathval applies to the first element of the path to obtain a second value, and then applies the rest of the path the new value.
New Definition of pathval pathval([Attr:Val1 | X], Attr, Val2, X):- !,unify(Val1, Val2). pathval([AV | X], Attr, Val, [AV | Rem] ) :- pathval(X, Attr, Val, Rem). pathval(V1, Attr:Path, Value, Rem) :- !, pathval(V1,Attr,V2,Rem), pathval(V2,Path,Value,_).
Lexical Entries W ord uther :- W:cat = n, W:head:trans = uther. • Uppercase can cause problems. • Use quote (e.g. ‘Uther’) if necessary.
Connecting Words to Lexical Entries • Represent strings as difference lists. • Define a predicate that associates words in string with lexical entry. leaf(W, [Word|Rest], Rest) :- W ord Word. leaf(C,X,X) :- R ule C ---> [].
In Other (DCG) Words …. leaf(FS)--> [Word], {FS ord Word}. leaf(FS) –-> [_ ule Dag --- []}
Left Corner Recogniser • To recognise a string as an instance of FS1, we need to consider an initial leaf FS0 and prove that FS0 is a “left corner” of FS1, i.e. that FS0 is a category that would appear at the bottom left of a parse tree for it. rec(FS1) --> leaf(FS0), lc(FS0,FS1}.
Definition of left cornerlc(FS1,FS2) • If the end of the string has been reached, then FS1 is a left corner of FS2 if they unify. • Otherwise there is a rule FS0 ---> [FS1|Rest] such that • The rest of the string is recognised as Rest • FS0 is a left corner of FS1
Code for lc lc(FS1,FS2) --> [], {unify(FS1,FS2)}. lc(FS1,FS2) --> { R ule FS0 ---> [FS1 | Rest] }, recognise_rest(Rest), lc(FS0,FS2).
recognise_rest recognise_rest( [ ] ) --> [ ]. recognise_rest([FS | FSs]) :- recognise(FS), recognise_rest(FSs).
