Notes on computational linguistics.pdf - UCLA Department of ...

Stabler - Lx 185/209 2003
/* earley.pl
* E Stabler, Feb 2000
* Earley parser, adapted from Shieber et al.
* NB: the grammar must specify: startCategory(XXX).
*/
:- op(1200,xfx,: ˜ ). % this is our object language "if"
:- [’closure-sics’]. % Shieber et al’s definition of closure/2, uses inference/4
%verbose. % comment to reduce verbosity of chart construction
computeClosure(Input) :retractall(word(_,_)),
% get rid of words from previous parse
lexAxioms(0,Input,Axioms),
closure(Axioms, Chart),
nl, portray_clauses(Chart).
computeClosure(Input,Chart) :retractall(word(_,_)),
% get rid of words from previous parse
lexAxioms(0,Input,Axioms),
closure(Axioms, Chart).
% for Earley, lexAxioms *asserts* word(i,WORDi) for each input word,
% and then returns the single input axiom: item(start,[],[s],0,0)
lexAxioms(_Pos,[],[item(start,[],[S],0,0)]) :startCategory(S).
lexAxioms(Pos,[W|Ws],Axioms) :-
Pos1 is Pos+1,
assert(word(Pos1,W)),
lexAxioms(Pos1,Ws,Axioms).
inference( scan,
[ item(A, Alpha, [W|Beta], I, J) ],
% -----------------------------------item(A,
[W|Alpha], Beta, I, J1),
% where
[J1 is J + 1,
word(J1, W)] ).
inference( predict,
[ item(_A, _Alpha, [B|_Beta], _I,J) ],
% --------------------------------------item(B,
[], Gamma, J,J),
% where
[(B : ˜Gamma), eligible(Gamma,J)] ).
inference( complete,
[ item(A, Alpha, [B|Beta], I,J),
item(B, _Gamma, [], J,K) ],
% ------------------------------item(A,
[B|Alpha], Beta, I,K),
% where [] ).
eligible([],_).
eligible([A|_],_) :- \+ (\+ (A : ˜_)), !. % the double negation leaves A unbound
eligible([A|_],J) :- J1 is J+1, word(J1,A).
portray_clauses([]).
portray_clauses([C|Cs]) :- portray_clause(C), portray_clauses(Cs).
114