LIN6932 Topics in Computational Linguistics

1. LIN6932 Topics in Computational Linguistics Lecture 7 Hana Filip LIN 6932

2. Overview Admin Stuff: All have access to their lin6932 class account? • Historical background • Definition of formal grammars • Chomsky Hierarchy • Computational complexity of natural language LIN 6932

3. Historical Background • In 1949, Shannon and Weaver published The Mathematical Theory of Communication, showing that statistical approximations to English based on Markov processes could be used to encode English efficiently for transmission in noise. Tasks like machine translation could be solved by treating e.g. Russian as a noisy encoding of English. • In 1957, Chomsky published Syntactic Structures, showing that natural grammars could not be exactly captured by such methods. It seemed to follow that machine translation could not be modeled as a noisy channel (although the machines were too small to actually try any of this). • Chomsky made a point of being open to the idea that STATISTICS could guide grammatical processing. • Nevertheless, most work in computational linguistics switched to linguistically informed high-level SYMBOLIC representations of syntax and semantics, and small knowledge domains. LIN 6932

4. Historical Background Given the grammar of a language, one can study the use of the language statistically in various ways; and the development of probabilistic models for the use of language (as distinct from the syntactic structure of the language) can be quite rewarding. Chomsky, 1957:17, note 4 LIN 6932

5. Historical Background • Around 1988, the machines got big enough to try both techniques. • Surprisingly, low level statistical approximations such as Markov processes worked better than linguistically informed representations on almost all tasks, such as speech recognition, parsing free text, and MT itself. • Most work in computational linguistics switched to linguistically uninformed, low-level statistical approximations and machine learning. • Around 2000, the process of putting linguistics, statistical models, and machine learning, back together again. LIN 6932

6. Historical Background • Chomsky’s 1957 book Syntactic Structures, together with certain more technical papers from around the same time, is one of the most important documents in linguistics and cognitive science. • The theory in detail has been completely superseded. • Nevertheless, surprisingly many of the formal devices that it includes recur, particularly in the most modern descendents, including: kernel sentences (aka lexicalized elementary trees or categories etc.); generalized or “double-based” transformations (aka Merge, combinatory rules, tree-adjunction, etc.; affix-hopping); the role of statistics in natural language processing. LIN 6932

7. Historical Background Affix Hopping as PF Merger and VP Ellipsis* LIN 6932

8. Historical Background One crucial ingredient of the theory was a hierarchy of language types, now known as CHOMSKY’S HIERARCHY, each type characterized by a class of rules that are sufficient to specify all languages of that type, an automaton which is sufficient to recognize whether a sentence is from a given language of that type, and a class of languages including all those of classes lower in the hierarchy as a proper subset. Question: What is the expressive/generative power of natural languages? Where are human languages located on this hierarchy? LIN 6932

9. Formal Grammar (Review) A formal grammar G is quadruple G = <N, T, S, R> N a finite set of nonterminal symbols T a finite set terminal symbols S start symbol (S  N) R rules (‘productions’) • Rules take the form , “ rewrites as ”, where ,  are strings of symbols from the infinite set of strings (TN)* and  must contain at least one non-terminal symbol. Other conditions on the rules are imposed for particular classes of grammars. LIN 6932

10. Chomsky Hierarchy Other conditions are imposed for particular classes of grammars. The Chomsky hierarchy: Types of grammars defined in terms of additional restrictions on the form of the rules: Type 0: No restriction. Each rule is of the form   , and  ≠. Type 1: Each rule is of the form A  , where A N and ≠. Type 2: Each rule is of the form A  . ( may be .) Type 3: Each rule is of the form A aB or A a. Common names: Type 0: Unrestricted rewriting systems (Turing Equivalent) Type 1: Context-sensitive grammars. Type 2: Context-free grammars. Type 3: Right-linear, or regular, or finite state automata / finite state grammars. LIN 6932

11. Chomsky Hierarchy LIN 6932

12. Chomsky Hierarchy of classes (or families) of languages, originally defined by the form of the rules needed to generate the languages in those classes, but which can also be characterized at least in part by "dependencies" between elements that appear in the strings that make up the languages of those classes. The smallest infinite class of languages in the Chomsky hierarchy is the class RL of regular languages. These are the languages that can be represented by regular expressions. The next larger class of languages in the Chomsky hierarchy is the class CFL of context-free languages. Every regular language is also a context-free language, but the converse is not true. LIN 6932

13. Chomsky Chierarchy TYPE O: UNRESTRICTED REWRITING SYSTEMS no restrictions on the rules: L(G) = {w  T* | S * w} a set of strings composed of terminal symbols derived from S “*” is the reflexive and transitive closure of “” “*” can be constructed as a set of operators each of which is obtained by applying  successively 0 or more times. • Every recursively enumerable language can be described by a rewriting system: an algorithm that "enumerates" the strings of the language, I.e., this means that its output is simply a list of the members of L: w1, w2, w3, ... . If necessary, this algorithm may run forever. • Recursively enumerable languages are languages for which there is a decision procedure for determining for any arbitrary string that it is a well-defined string in the language, but not necessarily for determining for any arbitrary string not in the language that it is not. • Membership in a type-0 language is undecidable. LIN 6932

14. Chomsky Chierarchy CONTEXT-SENSITIVE GRAMMARS • Subclass of type-0 grammars • Restriction: all rules take the form  A   length()  length() where AN, , ,   (TN)*,  ≠  Membership in a context-sensitive language (CSL) is decidable. CLSs are languages for which there is a decision procedure for determining whether an arbitrary string does or does not belong to the language. LIN 6932

15. Chomsky Chierarchy • Not all decidable languages are context-sensitive (but most are) LIN 6932

16. Chomsky Chierarchy CONTEXT-FREE GRAMMARS • Subclass of context-sensitive grammars • Restriction: rules take the form A   where AN,   (TN)+ • Membership in context-free language (CFL) is decidable LIN 6932

17. Chomsky Chierarchy REGULAR GRAMMARS • Subclass of context-free grammars • Restriction: all rules take the form A  a or A  aB where A,B  N and a  T Membership is decidable. RGs are expressively equivalent to finite state automata, or Markov process. Automata viewed as either generators or acceptors. Grammars viewed as either generators or acceptors. LIN 6932

18. Chomsky Chierarchy REGULAR GRAMMARS A  a or A  aB where A,B  N and a  T LIN 6932

19. Chomsky Chierarchy Strong and Weak Equivalence • Two grammars are said to be “weakly equivalent” if they generate the same language or string set: cp. categorial grammars and phrase structure grammars. • Two grammars are “strongly equivalent” if they assign the same tree(s) to each string in the same language. • All grammars at a given level in the hierarchy have strongly equivalent grammars at higher levels, but not vice versa. • A grammar or class of grammars is said to be “strongly adequate” to the capture of a language or class of languages if it assigns the “right” trees to strings. The “right” tree is the one we need for semantic interpretation. • Weakly equivalent grammars which assign the “wrong” trees are said to be only “weakly adequate.” LIN 6932

20. Chomsky Chierarchy Categorial GrammarPhrase Structure Grammar John loves Mary S n <n, <n,t>> n NP VP n <n,t> N V NP t (semantic type of a sentence) John loves Mary combinatorially transparent categories LIN 6932

21. NL and Chomsky Hierarchy Where are natural languages located? Expressive power of human languages • The first three chapters of Chomsky 1957 show that human languages fall outside the lowest level of Regular/Finite State languages, and are at least at the level of Context Free Languages. • The proof requires a distinction between ideal linguistic capacity, now known as Competence and the Performance mechanism that actually processes sentences. • Competence allows sentences that are so long or convoluted that none of us will live long enough or have enough memory to process them. Performance cannot cope with them—but clearly this limitation is accidental, not a fact about English. • Syntactic Structures goes on to suggest that the level of human grammars is still higher in the hierarchy. It raises (but does not answer) the question of which level is just high enough to contain all human languages. LIN 6932

22. NL and Chomsky Hierarchy Where are natural languages located? Typical argument for the complexity of NL: • Find a recursive construction C in a natural language L (English) • Assume that the construction type in question is theoretically unbounded: i.e., in theory, speakers could go on producing ever longer instances of the construction. • Argue that the competence of speakers admits unlimited recursion (while the performance certainly poses an upper limit; competence vs. performance distinction!) • Reduce C to a formal expression of known complexity in language L’ via a homomorphism (a structure-preserving mapping) • Make a case that L must be at least as complex L’ • Extrapolate from this one instance to all human languages: if there’s this one construction C in this one language that has this complexity, then the human language faculty must allow this in general. LIN 6932

23. NL and Chomsky Hierarchy NL is not regular: Chomsky’s 1957 original argument Structure of his argument: Consider 3 hypothetical languages: 1. ab, aabb, aaabbb (anbn) 2. aa, bb, abba, baab, aaaa, bbbb, aabbaa, abbbba, … (palindromic) 3. aa, bb, abab, baba, aaaa, bbbb, aabaab, abbabb, aababaabab (copy language) • It can be shown that these are not regular languages LIN 6932

24. NL and Chomsky Hierarchy The Pumping Lemma (a technique for proving that certain languages are not regular). If L is an infinite finite automaton language (FAL) over alphabet A, then there are strings x,y,z A* such that y ≠ and xynz L for all n ≥ 0. Why: machines for infinite languages must have loops. The string y in the lemma corresponds to a string accepted during a traversal of a loop. Note that lemma does not say ‘iff’. LIN 6932

25. NL and Chomsky Hierarchy The Pumping Lemma (a technique for proving that certain languages are not regular). Example: L = {anbn | n ≥ 0 } If L were a FAL, then, by Pumping Lemma, there would be x,y,z A* such that y ≠ and xynz L for all n ≥ 0. Assume that such x,y,z exist so the string xyz is in L. But by definition of L it should be in the form anbn for some n. What’s y in this case? It can’t be empty, so it would consist of (1) some number of a’s, or (2) some number of b’s, or (3) some number of a’s followed by some number of b’s. But it is easy to see that in any of those cases the strings xyyz, xyyyz, etc. could not belong to L. But the pumping theorem is not always useful for showing a language to be non-regular. LIN 6932

26. NL and Chomsky Hierarchy NL is not regular - at least context-free power: Chomsky’s 1957 original argument: LIN 6932

27. NL and Chomsky Hierarchy Therefore, Chomsky claims, English cannot be regular “It is clear, then that in English we can find a sequence a+S1+b, where there is a dependency between a and b, and we can select as S1 another sequence c+S2+d, where there is a dependency between c and d … etc. A set of sentences that is constructed in this way … will have all of the mirror image properties of (2) which exclude (2) from the set of finite languages.” (Chomsky 1957, p.21) Note: Chomsky writes “finite languages”, but he means “regular languages”. LIN 6932

28. NL and Chomsky Hierarchy Chomsky’s argument: because English contains these constructions, which are not regular, English is not regular. As stated, the argument is fallacious. LIN 6932

29. NL and Chomsky Hierarchy How to state the observation correctly LIN 6932

30. NL and Chomsky Hierarchy Similar point about center-embedding/nested dependencies the cats that the dogchases miau the dependency between the dog and chases nests within the dependency between the cats and miau Assume the following homomorphism: a = {the cat, the dog, the rat, …} b = {chase, miau, bite, bark, …} Then this is an instance of anbn Chomsky’s argument: • Any useful syntactic analysis will relate the nouns to their corresponding verbs. • No FSA is capable of keeping track of center embeddings of arbitrary depth (which would be required since the grammatical subset of L is infinite). No FSA can provide a useful syntactic analysis for center-embedding. Therefore, since English has such constructions, English is non regular language. LIN 6932

31. NL and Chomsky Hierarchy LIN 6932

32. NL and Chomsky Hierarchy The language anbn corresponds to the context-free grammar S  a S b S  a b It gives rise to the following tree for the string aaabbb LIN 6932

33. NL and Chomsky Hierarchy Dissenting view 1: • all arguments to this effect use center-embedding/nested dependencies,cp. the cats that the dog chasesmiau the dependency between the dog and chases nests within the dependency between the cats and miau • humans are extremely bad at processing center-embedding • notion of competence that ignores this is dubious • natural languages are regular after all LIN 6932

34. NL and Chomsky Hierarchy Dissenting view 2: • Any *finite* language is a regular language. • If you don't distinguish performance and competence, then English as a language certainly couldn't contain any sentence longer than the number of words a human being could utter in a lifetime. (This assumes human lifetimes are finite, but that seems uncontroversial.) • This may be a HUGE number, but it is definitely finite, and so without the distinction English is formally a finite language and therefore regular. LIN 6932

35. NL and Chomsky Hierarchy Are natural languages context-free? • history of the question: Chomsky’s 1957 conjecture that natural languages are not context-free • In the 60’s and 70’s, many attempts to prove that NL is not context-free • Pullum and Gazdar 1982 (Generalized Phrase Structure Grammar): - all these attempts have failed- for all we know, natural languages (conceived as string sets) might be context-free • Huybregts 1984, Shieber 1985: - proof that Swiss German is not context-free, cross-serial dependencies • Culy 1985: proof that Bambara (a Northwestern Mande language spoken in Mali) ) is not context-free LIN 6932

36. NL and Chomsky Hierarchy • Nested and Crossing Dependencies Context-free languages -- unlike regular languages -- can have unbounded dependencies. However, these dependencies can only be nested, not crossing. Example: anbn has unlimited nested dependencies: context-free The copy language has unlimited crossing dependencies: not context-free LIN 6932

37. NL and Chomsky Hierarchy • Nested and Crossing Dependencies Bar-Hillel and Shamir (1960): • English contains copy-language (crossing dependencies) • Cannot be context-free John, Mary, David, ... are a widower, a widow, a widower, ..., respectively. Claim: the sentence is only grammatical under the condition that if the nth name is male (female) then the nth phrase after the copula is a widower (a widow) LIN 6932

38. NL and Chomsky Hierarchy • Nested and Crossing Dependencies suppose the claim is true, intersect English with regular language L1 =(Paul|Paula)+are(a widower|a widow)+respectively Result: Copy language L3 {ww|w(a|b)+} English  L1 = L2, homomorphism L2  L3 John, David, Paul, …  aa widower  a Mary, Paula, Betty, …  ba widow  b are, respectively   LIN 6932

39. NL and Chomsky Hierarchy • Result: Copy language L3 {ww|w(a|b)+} Copy language is not context-free Hence L2 is not Hence English is not LIN 6932

40. NL and Chomsky Hierarchy Cross-serial dependencies in Dutch Huybregt (1976) • Dutch has copy-language like structures • thus Dutch is not context-free dat Jan Marie Pieter Arabisch laat zien schrijven that jan marie pieter arabic let see write ‘that Jan let Marie see Pieter write Arabic’ LIN 6932

41. NL and Chomsky Hierarchy Counterargument Crossing dependencies only concern argument linking, i.e., semantics As far as plain strings are concerned, the relevant fragment of Dutch has the structure NPnVn which is context-free LIN 6932