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Beesley 2001 The lexc Language Prepare to partition your brain to learn a whole new formalism.

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Presentation on theme: "Beesley 2001 The lexc Language Prepare to partition your brain to learn a whole new formalism."— Presentation transcript:

1 Beesley 2001 The lexc Language Prepare to partition your brain to learn a whole new formalism.

2 Beesley 2001 The lexc language “lexc” stands for “LEXicon Compiler” lexc is a high-level, declarative programming language lexc is different from regular expressions and from xfst the syntax is different the assumptions are different the special characters are different the interfaces are different BUT, the lexc compiler produces STANDARD Xerox networks these networks are fully compatible with networks from xfst you can sometimes choose to use lexc or xfst for building a network This is all fertile ground for confusion!

3 Beesley 2001 Why a Separate lexc Language? Lexc is intended for use by lexicographers. Regular expressions in xfst are often hard to read, especially big ones Typing spaces between all the letters, e.g. e l e p h a n t, to be concatenated in xfst is a nuisance, especially if you need to type 40,000 words You can also write {elephant} in xfst regular expressions, but that’s a nuisance too Lexc is more efficient for compiling large natural-language lexicons (it optimizes the union operation) Lexc has better error messages But remember: lexc is just another formalism for defining finite-state languages and relations you can (and will) use lexc and xfst together in building significant applications

4 Beesley 2001 The lexc Source File: Multichar_Symbols The lexc compiler and the xfst regular-expression compiler have completely opposite assumptions about multicharacter symbols: In xfst Regular Expressions, the default is to treat a string of symbols written together, e.g. %+Noun or cat, as a single symbol. Concatenation of separate symbols is indicated by manually separating symbols with white space, e.g. [ c a t ], or by using the curly-brace notation, e.g. {cat}. In lexc, in contrast, the default is to treat strings, e.g. cat, as a concatenation of three symbols. Any multicharacter symbols must be explicitly declared at the top of the source file.

5 Beesley 2001 Multichar_Symbols declaration Multichar_Symbols +Noun +Verb +Adj +Adv +Sg +Pl +1P +2P +3P ^FEAT1 ^FEAT2 The Multichar_Symbols statement is formally optional and is placed at the top of your lexc source file. You can declare as many multicharacter symbols as you find necessary or useful. The compiler uses this declaration to separate the strings of your lexc program into symbols. You are strongly encouraged to include a non-alphabetic character like the plus sign or the circumflex to help the multicharacter symbol stand out visually.

6 Beesley 2001 The Body of your lexc Program The body of a lexc program is composed of LEXICONs. There should be one LEXICON named Root. It corresponds to the Start State in the resulting Network. If you don’t define a LEXICON Root, lexc will try to use the first LEXICON in the file as the Start State. LEXICON Root dogN ; catN ; birdN ;

7 Beesley 2001 Entries in a LEXICON Each defined LEXICON must have at least one entry. An entry consists of two parts and is terminated with a semicolon data continuation-class ; The data part has to fit one of four formats: stringe.g. dog upper:lowere.g. swim:swam e.g. empty e.g.

8 Beesley 2001 upper:lower Entries The upper:lower entries are the simplest way to specify portions of the network where the upper-side and lower-side differ. They are especially useful for irregularies/suppletions. Multichar_Symbols +Verb +Past +Noun +Sg +Pl LEXICON Root swim+Verb+Past:swam # ; go+Verb+Past:went # ; child+Noun+Pl:children # ; ox+Noun+Pl:oxen # ;

9 Beesley 2001 upper:lower Entries In upper:lower entries, you can overtly indicate where the epsilons should go. Multichar_Symbols +Verb +Past +Noun +Sg +Pl +Nom LEXICON Root poder+Verb:pod0rFutCond ; Danger: the lexc upper:lower notation is not quite the same as the regular-expression colon notation.

10 Beesley 2001 Regular Expressions in lexc Any data written as a regular expression must be surrounded with angle brackets, e.g. CC ; Inside angle brackets, you revert to all the assumptions suitable for xfst regular expressions, including the treatment of multicharacter symbols vs. concatenation of symbols. This is fertile ground for confusion and errors.

11 Beesley 2001 Continuation Classes The Continuation Class is just the name of a defined LEXICON or #, indicating end-of-word (a final state). Multichar_Symbols +Noun +Sg +Pl LEXICON Root dogN ; catN ; LEXICON N +Noun+Sg:0# ; +Noun+Pl:s# ;

12 Beesley 2001 Thinking About lexc LEXICONS A LEXICON should hold a coherent class of morphemes The entries in a lexc LEXICON are unioned together by the compiler; the order of the entries in a LEXICON is not significant. Think of LEXICONs as potential “targets” Entries “point at” a LEXICON via the ContinuationClass But each entry in a LEXICON could itself point to a different ContinuationClass During development, you may have to subdivide lexicons Avoid having copies of the same material (if possible) You may change an entry in one place and forget to change the copy

13 Beesley 2001 Formally Speaking Lexc syntax is a kind of right-recursive phrase-structure grammar. Phrase-structure grammars can in general describe languages beyond finite-state power, including languages with balanced parentheses. But with the right-recursive limitation, a phrase-structure grammar can define only finite-state languages. Lexc can describe only finite-state languages. Lexc descriptions compile into finite-state networks.

14 Beesley 2001 Lexc Idiom: Optional Morphemes via By-Pass LEXICON Vroot kantV ; dirV ; don V ; pens V ; LEXICON V AdLex ; Vend ; LEXICON AdLex adVend ; LEXICON Vend as# ; is# ; os# ; us # ; u # ; i # ;

15 Beesley 2001 Lexc Idiom: Optional Morphemes via “Escape” Entries LEXICON Vroot kantAdLex ; dirAdLex ; don AdLex ; pens AdLex ; LEXICON AdLex adVend ; Vend ; ! escape LEXICON Vend as# ; is# ; os# ; us # ; u # ; i # ;

16 Beesley 2001 Lexc Idiom: Loops LEXICON Nroot LEXICON Plur katN ;jCase ; ! Opt. plural ending hundN ;Case ; elefantN ; LEXICON NLEXICON Case egN ; ! loopn# ; ! Opt. case ending etN ; ! loop# ; inN ; ! loop Nend ; LEXICON Nend oPlur ;

17 Beesley 2001 Stem compounding (loops) in lexc LEXICON NrootLEXICON Plur katN ;jCase ; ! Opt. plural ending hundN ;Case ; elefantN ; LEXICON NLEXICON Case Nroot ;n# ; Nend ;# ; LEXICON Nend oPlur ;

18 Beesley 2001 Special Characters in lexc Overall, there are far fewer special characters in lexc than in regular expressions. In lexc, the following are special: SpecialLiteralized :used in upper:lower notation %: ;terminates an entry %; ends a regular expression %> 0denotes the empty string (epsilon) %0 ! introduces a comment line %! #continuation-class for end-of-word %# % literalizing prefix %

19 Beesley 2001 Lexc source files Lexc sources files are ascii, typically edited with xemacs Lexc programs for natural language can get very large Typically 8000 to entries for verbs Tens of thousands of entries for nouns and proper nouns

20 Beesley 2001 The lexc interface Invoke the lexc interface by simply entering ‘lexc’ at the UNIX prompt. unixprompt% lexc You communicate with the interface using lexc commands. Type ‘?’ to see all the possible commands. Invoke ‘help commandname’ to see some terse online documentation. Enter ‘quit’ to leave lexc and return to the operating system. lexc: quit

21 Beesley 2001 The Three lexc Registers To understand lexc commands, you must understand that they refer to and operate on networks held in three registers, visualized as SOURCE RULES RESULT Typically used to store a lexicon FST. Typically used to store a rule FST or FSTs. Typically used to store the result of composing the rule FST(s) under the source FST.

22 Beesley 2001 Basic lexc commands SOURCE RULES RESULT compile-source filename compiles the lexc source code in filename and stores the resulting network in SOURCE read-rules filename reads the binary file filename and stores the network(s) in RULES. The binary file may be from xfst or twolc. compose-result composes the RULE FST(s) under the SOURCE FST and stores the resulting FST in RESULT

23 Beesley 2001 Some Other lexc Commands read-source load a pre-compiled binary network into the SOURCE register save-source filenamestore network in SOURCE to binary file save-result filenamestore network in RESULT to binary file lookup word(equivalent to the xfst ‘apply up’) lookdown word(equivalent to the xfst ‘apply down’) result-to-sourcemove the network from the RESULT register to SOURCE Enter ‘?’ to see all the lexc interface commands.

24 Beesley 2001 Using lexc and xfst together Write a lexc source file (e.g. mysrc.lex) using xemacs or a similar editor Write suitable alternation rules (in xfst or even twolc). Compile them and save the network(s) to file, e.g. to myrul.fst Then from the lexc interface: lexc: compile-source mysrc.lex lexc: read-rules myrul.fst lexc: compose-result lexc: save-result mylang.fst

25 Beesley 2001 Using lexc and xfst together Lexc lexicons build “words” (strings) using union and concatenation Entries within a LEXICON are unioned (the order of entries is not significant) The LEXICON Root corresponds to the start state The special # continuation class corresponds to final states Other continuation classes translate into concatenation By Xerox convention, upper-side strings consist of a baseform and “tags” By convention, a surface (or more surfacy) form appears on the lower-side The surfacy forms generated by lexc may still be rather abstract, hyper-regular, or “morphophonemic”. They may sometimes contain multicharacter symbols. Replace Rules (perhaps a whole cascade of them) map from the surfacy strings produced by lexc to real surface strings; rules are applied using composition. Composition can also be used to “filter” out various kinds of overgeneration.

26 Beesley 2001 A Typical Finite-State System Filters (xfst) Core Lexicon (lexc) Orthographical or Phonological Alternation Rules (xfst).o.

27 Beesley 2001 A System may be a Union of Subsystems Nouns (lexc)Verbs (lexc)Adjs (lexc)Numbers (lexc) Noun Rules Verb Rules.o. define final NounFST | VerbFST | AdjFST | NumberFST ; Then, in xfst:

28 Beesley 2001 Review: Outputs and Inputs Unix Pipe: cat | sort | uniq -c | sort -rnb > myfile.out The output of one routine is the input to the next NOT reversible Cascade of Replace Rules: read regex [ N -> m || _ p ].o. [ p -> m || m _ ] ; Reversible/bidirectional relation apply down: the output of the first rule is the input to the second the lower side of the top rule is the upper side of the bottom rule

29 Beesley 2001 Review: Up and Down In xfst (regular expressions)In lexc a:bswim+Verb+Past:swam %+Pl:supper:lower [ a.x. b ] a -> b a <- b

30 Beesley 2001 Review: Xerox Conventions Upper (lexical) language:baseform+Tag+Tag+Tag Lower (surface) language:orthographical-string The surface language is usually determined for you by the standard orthography. The lexical side language, and all intermediate languages, have to be defined by the linguist writing the grammar. (mapping via rules)

31 Beesley 2001 Review: Up and Down with Composition baseform+Tag+Tag+Tag surfacy-form A rule that refers to tags on its lower side.o. A rule that refers to a surfacy form on its upper side.o. An FST defined using lexc

32 Beesley 2001 Review: Think in terms of Languages and Relations Lexical Language Core Lexicon FST Surfacy Language Rule1 Intermediate Language Rule2 Intermediate Language Rule n Final Surface Language

33 Beesley 2001 Other Important Topics in The Book Composition is Our Friend Modify a common “core” network to handle –Multiple orthographies –Multiple dialects –Multiple registers Testing with the Finite-State Calculus Bulk testing against corpora Regression testing/comparison Testing against wordlists Testing the well-formedness of the upper-side strings

34 Beesley 2001 Advanced Features “Flag Diacritic” features and feature unification Simplify lexc descriptions Help keep transducers small The compile-replace Algorithm Useful for non-concatenative morphology –Reduplication –Semitic Interdigitation

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