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Implementing Patr

Implementing Patr. 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

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Implementing Patr

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  1. Implementing Patr

  2. Bibliographic Note Material in this presentation is based on Natural Language Processing in Prolog G. Gazdar and C. Mellish, Addison-Wesley 1989.

  3. Some Differences between Patr and Prolog • Prolog requires explicit variables • Syntactic appearance of paths • Grammar Rules • Lexical Entries • Templates

  4. Rule Format R ule S ---> [NP, VP] :-S:cat === s,NP:cat === np,VP:cat === vp,S:head === VP:head,NP === VP:subcat:first.

  5. Operator Definitions op(500,xfy,:).op(500,xfx,--->).op(600,xfx,===). op(400,xfx,ule).op(500,xfx,ord).

  6. 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).

  7. 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).

  8. 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.

  9. 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,_).

  10. Lexical Entries W ord uther :- W:cat = n, W:head:trans = uther. • Uppercase can cause problems. • Use quote (e.g. ‘Uther’) if necessary.

  11. 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 ---> [].

  12. In Other (DCG) Words …. leaf(FS)--> [Word], {FS ord Word}. leaf(FS) –-> [_ ule Dag --- []}

  13. 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}.

  14. 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

  15. Code for lc lc(FS1,FS2) --> [], {unify(FS1,FS2)}. lc(FS1,FS2) --> { R ule FS0 ---> [FS1 | Rest] }, recognise_rest(Rest), lc(FS0,FS2).

  16. recognise_rest recognise_rest( [ ] ) --> [ ]. recognise_rest([FS | FSs]) :- recognise(FS), recognise_rest(FSs).

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