Prolog for Linguists Symbolic Systems 139P/239P

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Prolog for Linguists Symbolic Systems 139P/239P

• John Dowding
• Week 7, November 12, 2001
• jdowding_at_stanford.edu

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Office Hours

• We have reserved 4 workstations in the Unix
  Cluster in Meyer library, fables 1-4
• No office hours this week or next week
• Contact me to make other arrangements

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Course Schedule

• Oct. 8
• Oct. 15
• Oct. 22
• Oct. 29
• Nov. 5 (double up)
• Nov. 12
• Nov. 26 (double up) Iterative Deepening and
  Logic
• Dec. 3
• No class on Nov. 19

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Dynamic predicates and assert

• Add or remove clauses from a dynamic predicate at
  run time.
• To specify that a predicate is dynamic, add
• - dynamic predicate/Arity.
• to your program.
• assert/1, asserta/1, assertz/1 adds a new clause
  for the predicate
• retract/1 removes one or more clauses
• retractall/1 removes all clauses for the
  predicate
• abolish/1 removes all information about a
  predicate
• Cant modify compiled predicates at run time
• Modifying a program while it is running is
  dangerous

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Aggregation findall/3.

• findall/3 is a meta-predicate that collects
  values from multiple solutions to a Goal
• findall(Value, Goal, Values)
• findall(Child, parent(james, Child), Children)
• Prolog has other aggregation predicates setof/3
  and bagof/3, but well ignore them for now.

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Built-in current_op/3

• current_op/3 gives the precedence and
  associativity of all current operators.
• current_op(Precedence, Associativity, Operator)
• where Precedence in an integer 1-1200
• and Associativity is of
• fx or fy for prefix operators
• xf or yf for postfix operators
• xfx, xfy, yfx, yfy for infix operators

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Associativity

• These atoms fx, fy, xf, yf, xfx, xfy, yfx, yfy
  draw a picture of the associativity of the
  operator
• The location of the f tells if the operator is
  prefix, infix, or postfix.
• x means that the argument must be of lower
  precedence
• y means that the argument must be of equal or
  lower precedence.
• A y on the left means the operator is left
  associative
• A y on the right means the operator is right
  associative
• yfy is not in Sicstus Prolog, or Quintus Prolog

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Creating new operators

• Built-in op/3 creates new operators
• op(Precedence, Associativity, Operator)
• - op(700, xfx, equals).
• - op(650, fx, ).
• - op(650, xf, cents).
• Dollars equals Cents cents -
• Cents is 100 Dollars.

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Declarations

• The syntax
• - Goal.
• indicates to execute Goal when consulting the
  file
• Weve seen a few of these so far
• - dynamic known_prime/1.
• - op(700, xfx, equals).

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Consult

• The operation for reading in a file of Prolog
  clauses and treating them as a program is
  traditional known as consulting the file.
• We will write a simple consult/1 predicate, and
  build on it over time.
• We will write similar

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Parsing, grammars, and language theory

• The development of Prolog (by Colmeraur at
  Marseilles) was motivated in part by a desire to
  study logic and language.
• Grammars are formal specifications of languages
• Prolog takes these specifications and treats them
  as logical theories about language, and as
  computations
• Grammar ? Proof ? Computation
• Pereira and Warren, Parsing as Deduction, 1984.
• Ideas from Prolog/Logic Programming, particularly
  unification, are found in modern Linguistics.

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Difference lists as indicies

• Traditional parsing uses indicies to keep track
  of phrase boundaries
• the man likes the dog
• 0 1 2 3 4 5
• the man is an NP spanning 0-2
• likes the dog is a VP spanning 2-5
• Well use difference lists to indicate spans,
• the dog is an NP spanning the,dog-
• the man is an NP spanning the,man,likes,the,dog
  -likes,the,dog

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Difference list grammar rule translation

• s ? np, vp.
• Translates to
• s(S0, SN) - np(S0, S1), vp(S1, SN).
• Instead of one variable, we have two, for the
  start and end points of the phrase,
• And the phrases are linked so that the end of one
  phrase is the same as the start of the adjacent
  phrase.

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Ruling out ungrammatical phrases

• Weve got a little grammar, but it accepts a lot
  of ungrammatical sentences
• First, lets deal with number agreement between
  subject NP and the verb
• Conventional to indicate ungrammatical sentences
  with a
• The man sleeps.
• The man sleep.

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Features

• But, this leads to duplicating a lot of rules
• What if we want to eliminate other ungrammatical
  sentences
• Number agreement between determiner and noun
• Transitive and Intransitive verbs
• A man sleeps.
• A men sleep.
• The men like the cat.
• The men like.
• The men sleep.
• The men sleep the cat.

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Features

• We can add features on rules to express these
  constraints concisely.
• s(Number) ? np(Number), vp(Number).
• np(Number) ? det(Number), n(Number).
• vp(Number) ? v(Number, intranitive).
• vp(Number) ? v(Number, transitive), np(_).
• det(singular) ? a.
• det(_) ? the.
• n(singular) ? man.
• n(plural) ? men.
• v(singular, transitive) ? likes.
• v(singular, intransitive) ? sleeps.

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Improved Consult

• consult_term((NT --gt Rule))-
• !,
• grammar_rule_body(Rule, Body, Start, End),
• make_nonterminal(NT, Start, End, Goal),
• assertz((Goal - Body)).
• make_nonterminal(NT, Start, End, Goal)-
• NT .. List,
• append(List, Start,End, FullList),
• Goal .. FullList.

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Improved Consult (cont)

• grammar_rule_body((Rule1, Rule2),(Body1, Body2),
  Start, End)-
• !,
• grammar_rule_body(Rule1, Body1, Start, Next),
• grammar_rule_body(Rule2, Body2, Next, End).
• grammar_rule_body(List, true, Start, End)-
• is_list(List),
• !,
• append(List, End, Start).
• grammar_rule_body(NT, Goal, Start, End)-
• make_nonterminal(NT, Start, End, Goal).

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Using Features to return results Parse trees

• In addition to just judging grammatical and
  ungrammatical sentences, we can also use the
  grammar to build up results to pass back.
• One example building a parse tree.
• This doesnt require any changes to consult

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Parse tree as a Prolog term
S
VP
NP
N
V
NP
DET
man
the
likes
N
DET
cats
the
s(np(det(the),n(man)),vp(v(likes),np(det(the),n(ca
ts)))
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Grammar1

• s(s(NP, VP)) --gt np(Number, NP), vp(Number, VP).
• np(Number, np(DET, N)) --gt det(Number, DET),
  n(Number, N).
• vp(Number, vp(V)) --gt v(Number, intransitive, V).
• vp(Number, vp(V, NP)) --gt v(Number, transitive,
  V), np(_, NP).
• det(_, det(the)) --gt the.
• det(singular, det(a)) --gt a
• n(singular, n(man)) --gt man.
• n(plural, n(men)) --gt men.

• n(singular, n(woman)) --gt woman.
• n(plural, n(women)) --gt women.
• n(singular, n(cat)) --gt cat.
• n(plural, n(cats)) --gt cats.
• n(singular, n(dog)) --gt dog.
• n(plural, n(dogs)) --gt dogs.
• v(singular, transitive, v(likes)) --gt likes.
• v(plural, transitive, v(like)) --gt like.
• v(singular, intransitive, v(sleeps)) --gt
  sleeps.
• v(plural, intransitive, v(sleep)) --gt sleep.

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Utility to type in and parse sentences

• parse_sentences -
• repeat,
• write('Type in a sentence to parse'),
• nl,
• read_line(Line),
• tokenize(Line, Tokens),
• s(Result, Tokens, ),
• write('Parse tree '), nl,
• write(Result), nl,
• at_end_of_stream(user_input).

• read_line(CharRestLine)-
• get_code(Char),
• Char \ 0'\n,
• !,
• read_line(RestLine).
• read_line().

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One more extension

• Our consult_file/1 is almost all the way to
  handling full Definite Clause Grammars. (DCGs)
• Its common practice to separate out the grammar
  from the lexicon. This can make the grammar
  more concise.
• Read Ch. 9 of Clocksin and Mellish for more about
  DCGs

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Grammar 2

• s(s(NP, VP)) --gt
• np(Number, NP),
• vp(Number, VP).
• np(Number, np(DET, N)) --gt
• det(Number, DET),
• n(Number, N).
• vp(Number, vp(V)) --gt
• v(Number, intransitive, V).
• vp(Number, vp(V, NP)) --gt
• v(Number, transitive, V),
• np(_, NP).

• det(Number, det(Det)) --gt
• Det,
• det(Det, Number).
• n(Number, n(Noun)) --gt
• Noun,
• n(Noun,Number).
• v(Number, Transitivity, v(Verb)) --gt
• Verb,
• v(Verb, Number, Transitivity).

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Lexicon

• det(the, _).
• det(a, singular).
• v(likes, singular, transitive).
• v(like, plural, transitive).
• v(sleeps, singular, intransitive).
• v(sleep, plural, intransitive).

• n(man, singular).
• n(men, plural).
• n(woman, singular).
• n(women, plural).
• n(cat, singular).
• n(cats, plural).
• n(dog, singular).
• n(dogs, plural).

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Consult modified to allow Goal

• Goal allows a regular Prolog Goal to be called,
  without treating it as a nonterminal
• grammar_rule_body(Body, call(Body), Start,
  Start)-
• !.

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Example Syntactic Gaps

• Questions and relative clauses are often missing
  a phrase that would normally be required
• Who likes cats?
• The man who likes cats sleeps.
• The man who cats like sleeps.
• Syntactic elements like wh-words (who, what,
  where, when, how) and relative pronouns (who,
  that) introduce gaps.

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Example Syntactic Gaps (cont)

• The gapped elements are linked to the source
• The man who likes cats sleeps
• The man who cats like sleeps
• The man who like cats sleeps.

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Handling gaps

• We will use difference lists to pass information
  through the parser to handle gaps
• Gap introduction rules
• Gap discharging rules
• Gap threading rules
• Well just worry about the relative clause case

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Gap Introduction Rules

• np(GapsIn, GapsOut, Number, np(DET, N, RC)) --gt
• det(Number, DET),
• n(Number, N),
• relative_clause(GapsIn, GapsOut, Number, RC).
• relative_clause(GapsIn, GapsOut, Number, rc(Rel,
  S)) --gt
• rel_pronoun(Rel),
• s(np_gap(Number)GapsIn, GapsOut, S).

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Gap Discharging

• np(np_gap(Number)RestGaps, RestGaps, Number,
  Gap) --gt
• .

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Gap Threading

• Any category that the gap might pass through will
  have to allow for gaps
• s(GapsIn, GapsOut, s(NP, VP)) --gt
• np(GapsIn, GapsNext, Number, NP),
• vp(GapsNext, GapsOut, Number, VP).

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Example Quasi-Logical Forms (QLFs)

• To really make use of a grammar, we will need
  more semantically useful representations.
• We can augment our DCGs to produce various kinds
  of semantic/logical representations
• Quasi-Logical Forms because it doesnt deal with
  quantifier scoping.
• This is meant as an example only

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Examples

• The man likes the cats
• qterm(the, X, man(X)), qterm(the,Y,cat(Y)),
  likes(X, Y)
• The man that likes the cats sleeps
• qterm(the,X,man(X), qterm(the,Y,cat(X)),
  likes(X,Y), sleeps(X)
• The man that the cats like sleeps
• qterm(the,X, man(X)), qterm(the,Y, cat(Y)),
  likes(Y,X), sleeps(X)

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QLF bits

• qterm(Quantifier, Variable, Predicate)
• 1-place predicates for nouns
• cat(X)
• man(X)
• 1- and 2- place predicates for verbs
• likes(X,Y)
• sleeps(X)

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Noun Phrases

• Two new argument positions for the QLF and the
  semantic variable
• np(Gaps, Gaps, Number, QLF, Var) --gt
• det(Number, QLF, Pred, Var),
• n(Number, Pred, Var).
• det(Number, QLF, Pred, Var) --gt
• Det,
• det(Det, Number, QLF, Pred, Var).
• n(Number, Pred, Var) --gt
• Noun,
• n(Noun,Number, Pred, Var).

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Noun Phrase Lexical Items

• det(the, _, qterm(the, X, Pred), Pred, X).
• det(a, singular, qterm(a, X, Pred), Pred, X).
• n(man, singular, man(X), X).
• n(men, plural, man(X), X).

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Verb Phrase Rules

• The semantic variable from the subject is passed
  into the verb phrase
• vp(Gaps, Gaps, Number, SubjVar, Pred) --gt
• Verb,
• v(Verb, Number, intransitive, SubjVarPred).
• vp(GapsIn, GapsOut, Number, SubjVar,
  (ObjQLF,Pred)) --gt
• Verb,
• v(Verb, Number, transitive, SubjVarObjVarPred)
  ,
• np(GapsIn, GapsOut, _, ObjQLF, ObjVar).

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Verb Phrase Lexical Items

• Using as an infix operator like lambda
• v(sleeps, singular, intransitive, Xsleeps(X)).
• v(sleep, plural, intransitive, Xsleeps(X)).
• v(likes, singular, transitive, XYlikes(X,Y)).
• v(like, plural, transitive, XYlikes(X,Y)).

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Possible Class Projects

• Should demonstrate competence in Prolog
  programming
• Expect problems with solutions in 5-20 pages of
  code range.
• Talk/email with me about your project

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What to cover in remaining weeks

• Weve got 4 more sessions, I have these plans
• Another session on DCGs
• A session on iterative deepening
• Some time on logical foundations/theorem proving
• Any thoughts on other things yould like to
  cover?
• More review?
• Help with class projects?

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Assignment

• Definite Clause Grammars
• I expect it will take 2-4 hours effort
• Due on Nov. 19th (no class that day!)