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Elaborazione del linguaggio naturale

Elaborazione del linguaggio naturale. Fabio Massimo Zanzotto. Part five. Feature Structures. Where we are?. Target of the analysis: interpret NL sentences with respect to a sort of anambiguos internal laguage

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Elaborazione del linguaggio naturale

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  1. Elaborazione del linguaggio naturale Fabio Massimo Zanzotto FMZ

  2. Part five Feature Structures FMZ

  3. Where we are? Target of the analysis: • interpret NL sentences with respect to a sort of anambiguos internal laguage • Natural Languages is a ambiguous and social beast vs. Formal Languages is unambiguous and “top-down” decided • What’s a language model: • treating infinite sentences with generative machinaries with a finite set of rules FMZ

  4. How we proceeded so far... ASSIOM: A syntactic interpretation of a sentence helps in understanding its semantics Let’s build a syntactic model for NL!! • Analysis of the chomsky hierarchy • Use of Context-free formalisms and related parsing algoritms • CYK • DCG (in prolog) FMZ

  5. How we proceeded so far... OBSERVATION: NL is more difficult that what we may think Let us renounce to the total grammaticality!!! • Model offering Partial Analysis • CYK • Chart parsing and early algorithm FMZ

  6. Our Aim Lines of development Grammatical Representation Power: Build a formalism/model able to give the possibility of reducing the unnecessary interpretations Grammar Use: Build a formalism (and an associated algorithm) able to represent partial analysis FMZ

  7. Our Aim Lines of development Grammatical Representation Power: • CFG (context free grammars)  DCG Grammar Use: • CYK • Chart and Early Algorithm FMZ

  8. Observing natural language Toy Examples: ... La vecchia porta la sbarra ... ... Il vecchio porta la sbarra ... ... Flying planes can be dangerous ... ... Flying planes is dangerous ... FMZ

  9. A sample Grammar (introspectively produced) S  NP VP | S SBAR | SBAR S SBAR  CongSub S S  S CongCoord S | S, S CongCoord S NP  NP SBAR VP VerbX NP | VerbX NP PP VerbX  Verb | Modal Verb NP  Art Noun | Art Adj Noun | Noun | Verb Noun | NP PP PP  Prep NP FMZ

  10. Observations • The sample grammar is insufficient!! • Spurious interpretations are produced for unambiguous sentences • Loosing the eternal struggle between coverage and induced ambiguity NP  Art Noun | Art Adj Noun | Noun | Verb Noun | NP PP ... the old man carries apples ... A) the old man (VP carries apples) B) the old man (NP carries apples) FMZ

  11. Necessary extensions • Introducing notions like: • gender: masculine, feminine • number: singular, plural • person (for verbs) • time (for verbs) • mood (for verbs) FMZ

  12. Grammar Adding number (Sing, Plur) NPSing  ArtSing NounSing NPPlur  ArtPlur NounPlur VPSing  VerbXSing NP | VerbXSing NP PP VPPlur  VerbXPlur NP | VerbXPlur NP PP S  NPSing VPSing | NPPlur VPPlur FMZ

  13. Grammar Adding number (Sing, Plur) and gender (Mas, Fem) NPMasSing  ArtMasSing NounMasSing NPFemSing  ArtFemSing NounFemSing NPMasPlur  ArtMasPlur NounMasPlur NPFemPlur  ArtFemPlur NounFemPlur VPSing  VerbXSing NP | VerbXSing NP PP VPPlur  VerbXPlur NP | VerbXPlur NP PP S  NPMasSing VPSing | NPFemSing VPSing | NPMasPlur VPPlur | NPFemPlur VPPlur !!Rules are uncontrollably proliferating!! FMZ

  14. Feature Structures FMZ

  15. What do we desire? Adding number (Sing, Plur) and gender (Mas, Fem) NPMasSing  ArtMasSing NounMasSing NPFemSing  ArtFemSing NounFemSing NPMasPlur  ArtMasPlur NounMasPlur NPFemPlur  ArtFemPlur NounFemPlur NP_Gen:X_Num:Y  Art_Gen:X_Num:Y Noun_Gen:X_Num:Y FMZ

  16. Feature Structures Feature structures (information containers) are: • Sets of attribute-value pairs • a value of an attribute may be: • a final value (i.e., an element from a set) • a feature structure Cat: np Agreement: Gen: mas Num: sing FMZ

  17. Feature Structures Formally if F is a feature structure, • F is a set of pairs (f,v) • given (f,v)F • v is a final value • v is a feature structure FMZ

  18. Feature Structures: Lexicon • nouns • forma_superficiale • lemma • genere • numero • verbs • forma_superficiale • radice • coniugazione: are, ere, ire • genere: mas, fem • numero: sing, plur • persona: 1,2,3 • modo: indicativo, congiuntivo, imperativo • tempo: presente, passato,... • verso: attivo, passivo FMZ

  19. Lexicon: examples forma_superficiale: mangeremo radice: mangi coniugazione: are numero: plur persona: 2 modo: indicativo tempo: futuro FMZ

  20. Lexicon: examples forma_superficiale: mangerebbe radice: mangi lemma: mangiare coniugazione: are numero: sing persona: 3 modo: condizionale tempo: presente FMZ

  21. Lexicon: examples forma_superficiale: uomini lemma: uomo numero: plur genere: mas FMZ

  22. How to use the lexicon? “l’uomo mangierebbe pere” that may be seen: [forma_supericiale: l’] [forma_supericiale: uomo] [forma_supericiale: mangierebbe] [forma_supericiale: pere] forma_superficiale: mangierebbe radice: mangi lemma: mangiare coniugazione: are numero: sing persona: 3 modo: condizionale tempo: presente FMZ

  23. Comparing feature structures: subsumption • A Feature Structure F1 subsumes F2 (F1 F2) if all the information that is in the F1is also in F2 Formally, F1 F2 se e solo se v=v’ oppure (f,v) F1(f,v’) F2. vv’ FMZ

  24. After the lexicon and the subsumption “l’uomo mangierebbe pere” that may be seen: [forma_supericiale: l’] [forma_supericiale: uomo] [forma_supericiale: mangierebbe] [forma_supericiale: pere] forma_superficiale: mangierebbe radice: mangi lemma: mangiare coniugazione: are numero: sing persona: 3 modo: condizionale tempo: presente forma_superficiale: uomo lemma: uomo numero: sing genere: mas FMZ

  25. What if? “l’uomo mangierebbe pere” that may be seen: [ forma_supericiale: l’] [ forma_supericiale: uomo] [ forma_supericiale: mangierebbe , forma_fonologica: xxxx ] [ forma_supericiale: pere] forma_superficiale: mangierebbe radice: mangi lemma: mangiare coniugazione: are numero: sing persona: 3 modo: condizionale tempo: presente Subsumption is not sufficient! FMZ

  26. Unification Unification is a partial operation between two feature structures so that the new feature structure contain all the information of the two F1F2 is so that: • F1 F1 F2 • F2 F1 F2 • if H has the property F1  H and F2  H then F1 F2 H FMZ

  27. Unification Example forma_superficiale: mangierebbe radice: mangi lemma: mangiare cat: verbo coniugazione: are numero: sing persona: 3 modo: condizionale tempo: presente = forma_superficiale: mangierebbe forma_fonologica: xxx  forma_superficiale: mangierebbe radice: mangi forma_fonologica: xxx lemma: mangiare cat: verbo coniugazione: are numero: sing persona: 3 modo: condizionale tempo: presente = FMZ

  28. Unification Unification between two feature structures may not exist. forma_superficiale: mangierebbe radice: mangi lemma: mangiare cat: verbo coniugazione: are numero: sing persona: 3 modo: condizionale tempo: presente forma_superficiale: mangia forma_fonologica: xxx  FMZ

  29. Coindexing What if we want to apply this rule? cat: s cat: nome numero: [1] cat: verbo numero: [1] persona: 3 forma_superficiale: mangierebbe radice: mangi lemma: mangiare cat: verbo coniugazione: are numero: sing persona: 3 modo: condizionale tempo: presente forma_superficiale: uomo lemma: uomo cat: nome numero: sing genere: mas FMZ

  30. Feature Structures in Prolog • feature structures will be represented as a open list of attribute value pairs • : (the colon) will be used to form attribute value pairs es. [number:sg, person:3 | _ ] [cat:np, agr:[number:sg, person:3 | _ ] | _ ] FMZ

  31. Unification in Prolog unify0(Dag,Dag) :- !.     unify0([Feature:Value|Rest],Dag) :-       val(Feature,Value,Dag,StripDag),        unify0(Rest,StripDag).val(Feature,Value1,[Feature:Value2|Rest],Rest) :-    !,    unify0(Value1,Value2).val(Feature,Value,[Dag|Rest],[Dag|NewRest]) :-    !,    val(Feature,Value,Rest,NewRest). FMZ

  32. Where we worked today? Lines of development Grammatical Representation Power: Build a formalism/model able to give the possibility of reducing the unnecessary interpretations Grammar Use: Build a formalism (and an associated algorithm) able to represent partial analysis FMZ

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