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LING 581: Advanced Computational Linguistics

LING 581: Advanced Computational Linguistics. Lecture Notes April 24th. Administrivia. Today: Interim report from Kristen TCEs Semantic grammars. Semantics. Last time: To get computers to understand sentences, m odel the world,

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LING 581: Advanced Computational Linguistics

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  1. LING 581: Advanced Computational Linguistics Lecture Notes April 24th

  2. Administrivia • Today: • Interim report from Kristen • TCEs • Semantic grammars

  3. Semantics • Last time: • To getcomputers to understand sentences, • model the world, • compute truth conditions, truth values, entailments, presuppositions, tautologies, etc. • propositional logic in Prolog, truth tables • Turing testand Searle’s Chinese room experiment

  4. Propositional Logic Program: plogic3.pl

  5. Semantic Grammars • Use slides from course • LING 324 – Introduction to Semantics • Simon Frasier University, Prof. F.J. Pelletier • http://www.sfu.ca/~jeffpell/Ling324/fjpSlides4.pdf • Difference is we’re computational linguists… so we’re going to implement the slides • We’ll do the syntax part this lecture, and the semantics next time

  6. Syntax • fjpSlides4.pdf • Slide 3

  7. Syntax • We already know how to build Prolog grammars • See • http://www.swi-prolog.org/pldoc/doc_for?object=section(2,'4.12',swi('/doc/Manual/DCG.html')) for the executive summary

  8. Syntax • Step 1: let’s build the simplest possible Prolog grammar for this • fjpSlides4.pdf • Slide 4

  9. Simplest possible grammar g1.pl Excluding (2b) for the time being

  10. Simplest possible grammar Examples (3), (4) and (5) from two slides back

  11. Syntax • Step 2: let’s add the parse tree component to our grammar … Recall: grammar rules can have extra arguments Parse tree Implement agreement etc.

  12. Syntax Note: on handling left recursion in Prolog grammar rules • techniques: • use a bottom-up parser • rewrite grammar (left recursive -> right recursive) • or use lookahead (today’s lecture) lookahead is a dummy nonterminal that does not contribute to the parse, it is a “guard” that prevents rule from firing unless appropriate lookahead succeeds if it can find a conjunction in the input and marks it (so it can’t find it twice)

  13. Grammar: version 2 g2.pl

  14. Grammar: version 2

  15. Grammar: version 2 Examples (3), (4) and (5) again from slide 9

  16. Grammar: version 2 Examples (6) and (7) from slide 9

  17. Semantics • We want to obtain a semantic parse for our sentences that we can “run” (i.e. evaluate) against the Prolog database (i.e. situation or possible world). • So the semantic parse should be valid Prolog code (that we can call) • We’ll need (built-in) member/2 and setof/3 defined in the following 2 slides (a quick review)

  18. setof/3 • See • http://www.swi-prolog.org/pldoc/doc_for?object=section(2,'4.29',swi('/doc/Manual/allsolutions.html')) • SWI Prolog built-in:

  19. setof/3 • Example:

  20. member/2 • See • http://www.swi-prolog.org/pldoc/man?predicate=member%2F2

  21. Semantics • fjpSlides4.pdf • Slide 7

  22. Semantics • fjpSlides4.pdf • Slide 8

  23. Semantics • fjpSlides4.pdf • Slide 9

  24. Semantics • fjpSlides4.pdf • Slide 10

  25. Semantics: Implementation • Desired implementation: The extra argument returns a Prolog query that can be evaluated against the database Note: we are bypassing the (explicit) construction of the syntax tree Imagine if the Penn Treebank was labeled using a semantic representation

  26. Semantics: Implementation • Let’s write the semantic grammar to handle “Jack is hungry” • first, let’s introduce a bit of notation (lambda calculus) • λ = function • λx.x+1 denotes a function that takes an argument x and computes value x+1 • (a period separates the argument from the function body) • (λx.x+1)(5) means apply 5 to the lambda function • substitute 5 in place of x and evaluate • answer = 6

  27. Semantics: Implementation Syntax: setof(X,hungry(X),S), member(jack,S) jack setof(X,hungry(X),S)

  28. Semantics: Implementation • Semantic grammar:

  29. Semantics: Implementation • Semantic grammar:

  30. Semantics: Implementation • More examples of computation: • fjpSlides4.pdf • Slide 10

  31. Semantics: Implementation • More examples of computation:

  32. Semantics: Implementation • More examples of computation:

  33. Semantics • fjpSlides4.pdf • Slide 11

  34. Semantics: Implementation • Scope of negation: wide or narrow narrow wide

  35. Grammar: version 3 g3.pl

  36. Grammar: version 3

  37. Evaluation • Check our computer implementation on… • fjpSlides4.pdf • Slide 12

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