Two-Stage Constraint Based Sanskrit Parser Akshar Bharati, IIIT,Hyderabad

Brief outline Dependency Paninian framework vibhakti-karaka correspondence karaka frames (basic + transformation) Source groups, demand groups Constraints Three basic constraints Constraints as Integer programming equations

Notions from Paninian Framework – a)Karaka relations It uses the notion of karaka relations between verbs and nouns in a sentence. The notion of karaka relations is central to the Paninian model. The karaka relations are syntactico-semantic (or semantico-syntactic) relations between the verbals and other related constituents in a sentence.

Notions from Paninian Framework – Demand Frames For the task of karaka assignment, the core parser uses the fundamental principle of ' akanksha' (demand unit) and ' yogyata' (qualification of the source unit). Ex: CAwraH vixyAlayam gacCawi (student) (school) (go) Verb Frame for this form of “gacCawi”

Demand Frame Gam1: arc-label necessity vibhakti lex-type src-pos arc-dir K1m1n l ds K2m2n l ds K3m3n l ds K5m5n l ds

Constraint Based Parsing Computational Paninian Model Integer Programming with basic constraints For each mandatory karakas in a karaka chart there should be exactly one outgoing edge labelled by the karaka from the demand group For each of the desirable or optional karakas in a karaka chart there should be at most one outgoing edge labelled by the karaka from the demand group There should be exactly one incoming arc into each of the source group

Parser Two stage strategy Stage I (Intra-clausal relations) Dependency relations marked Relations such as k1, k2, k3, etc. for each verb Stage II (Inter-clausal relations & conjunct relations) Conjuncts and relative clauses

Steps in Parsing Morph, POS tagging, Chunking SENTENCE Identify Demand Groups Load Frames & Transform Find Candidates Apply Constraints & Solve Final Parse Is Complex NO YES STAGE - II

Morph,Chunked,Tagged data (( 1 (( NP 1.1 CAwraH NN )) 2 (( NP 2.1 vixyAlayam NN )) 3 (( VGF 3.1 gacCawi VM ))

CAwraH vixyAlayam gacCawi

Demand Frame Gam1: arc-label necessity vibhakti lex-type src-pos arc-dir K1m1n l ds K2m2n l ds K3m3n l ds K5m5n l ds

k1 k2 CAwraH vixyAlayam gacCawi

Sanskrit Example CAwraH vixyAlayam gacCawi

Steps (Stage II) Identify New Demand Groups Load Frames & Transform Find Candidates Apply Constraints & Solve FINAL PARSE Repair Output of STAGE - I

Example – Relative Clause vaha puswaka jo rAma ne mohana ko xI hE prasixXa hE that book which Ram ERG. Mohana DAT. gave is famous is ‘The book which Ram gave to Mohana is famous’

Output after Stage - I xI puswaka mohanarAma k2 k4 k1 _ROOT_ jo hE k1 prasixXa k1s main vaha

Identify the demand group xiyA ‘give’ Main verb of the relative clause

Identify the demand group, Load and Transform DF jo ‘which’ transformation (special) Transforms the demand frame of the main verb of the relative clause arc-label necessity vibhakti lextype src-pos arc-dir oprt nmod__relc m any n r|l p insert

Karaka Frame vaha puswaka jo rAma ne mohana ko xI prasixXa hE | that book which Ram ERG. Mohana DAT. gave famous is ‘The book which Ram gave to Mohana is famous’ Main verb of relative clause arc-label necessity vibhakti lextype src-pos arc-dir oprt nmod__relc m any n r|l p insert Transformed frame for xe after applying the jo trasformation New row inserted after transformation

Possible candidates vaha puswaka jo rAma ne mohana ko xI hE prasixXa hE | nmod__relc

Output after Stage - II xiyA hE vaha puswaka mohana rAma k2 k4 k1 _ROOT_ jo hE k1 prasixXa k1s nmod__relc main

Example II – Coordination rAma Ora siwA kala Aye | Ram and Sita yesterday came ‘Ram and Sita came yesterday’

Output of Stage - I rAma _ROOT_ Aye k1 siwA Ora kala k7t dummy main

For Stage – II (Constraint Graph) rAma _ROOT_ Aye k1 siwA Ora kala main k7t ccof

Candidate Arcs rAma _ROOT_ Aye k1 siwA Ora kala main k1 ccof

Solution Graph rAma _ROOT_ Aye siwA Ora kala k7t main k1 ccof

Parse tree Aye kalaOra k7t k1 _ROOT_ rAma siwA ccof main Output after Stage II

Results for Hindi

Results CBP: Results when only the first parse is considered CBP’’: When best parse of the first 25 parses are considered CBP was tested on 220 sentences These are the results published in IALP-2008

Work Progress in Sanskrit Existing Constraint Based parser for Sanskrit can parse simple sentences. Over 2000 demand charts Two stage parsing needs more development Experiments performed with 268 simple sentences Re-ranking of parses is not done,only the first parse is considered for results Results not very accurate due to data problems

Results in Sanskrit Labelled attachment score: 540 / 1213 * 100 = % Unlabeled attachment score: 876 / 1213 * 100 = % Label accuracy score: 566 / 1213 * 100 = %

Treebank requirement Proper Gold tagged,chunked and dependency marked data for Sanskrit will improve the efficiency of the parser Annotation with proper tools It will also help us in using machine learning methods to train statistical parsers for Sanskrit

Further work on Constraint Based Parsing. Extension of the parser using treebank data Hybrid approaches Soft Constraints Pruning of the graph in data driven parsers using Constraint Graph Allow learning of the parser from the treebank data Better performance

What we expect From Data (( 1 (( NP 1.1 CAwraH NN )) 2 (( NP 2.1 vixyAlayam NN )) 3 (( VGF 3.1 gacCawi VM ))

THANKS!!