Constraint Grammar

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En français

Constraint Grammar is a tool that can be used to POS-tag ambiguous text. There are free constraint grammars developed outside the Apertium project for: Norwegian (the Oslo-Bergen tagger), Sámi languages (from Giellatekno), Faroese (also from Giellatekno), Finnish (by Fred Karlsson).

Terminology

See also: Apertium stream format
Apertium equivalent: ^words/word<n><pl>/word<vblex><pres><p3><sg>$
  • baseform — the lemma of a word.
  • reading — a single analysis of a word.
Apertium equivalent: ^word<n><pl>$

Basic Rule Format

Sets

Sets are defined like this: 

LIST VERB = vblex vbser ;     # matches <vblex> or <vbser>
LIST NSG = (n sg) ;           # matches <n><sg>
LIST TO = "to" ;              # matches the lemma "to"
LIST CASE = (n nom) (n acc) ; # matches <n><nom> or <n><acc>

Context Patterns

Context patterns look like this: 

([LOCATION][MODIFIERS] [PATTERN])

PATTERN can be a lemma, set of tags, or the name of a set.

Symbol Meaning
0 The current word
1 The word following the current word
-1 The word preceding the current word
2 The word 2 words after the current word
C Every reading this position must match the pattern (normally only 1 has to)
* In that position or further in that direction 
(0 (v))      # the current word must have a verb reading
(1 VERB)     # the following word matches the set "VERB"
(-1 "to")    # the previous word must be "to"
(2C (n))     # every reading of the word after the next one must be a noun
(1* (pr))    # the current word has a preposition after it
(-2* (pron)) # there is a pronoun at least two words before the current word


Rules

Rules look like this: 

SELECT [FORM] IF [CONTEXT] ;
REMOVE [FORM] IF [CONTEXT] ;

Where FORM is a lemma, set of tags, or the name of a set and CONTEXT is a set of patterns. 

SELECT VERB IF (1 (det)) ;    # prefer reading from set "VERB" if following word has <det> tag
REMOVE (n) IF (-1 (adv)) ;    # disprefer <n> reading if preceding word has <adv> tag
SELECT (p1) IF (-1* (prn p1)) # prefer 1st person reading for verbs
               (0 (v)) ;   # preceded at any distance by a 1st person pronoun

Note on parenthesis

The use of parentheses to distinguish between tags and lists/sets seems to be the main confusing point for people learning CG. If we have the morphological tags tag1 and tag2, then we can have rules like this: 

LIST set1 = tag1 ;
LIST set2 = (tag1 tag2) ; # matches a word with both tag1 and tag2
LIST set3 = tag1 tag2 ;   # matches a word with tag1 or tag2
LIST word = "hello" ;

SELECT:1a (tag1) (1 word) ;
SELECT:1b  set1  (1 word) ;   # equivalent to 1a

SELECT:2a (tag1 tag2) (1 word) ;
SELECT:2b  set2       (1 word) ;   # equivalent to 2a

SELECT:3a tag1 (1 word) ;
SELECT:3b tag2 (1 word) ;
SELECT:3c set3 (1 word) ;   # equivalent to 3a and 3b combined

SELECT:1c  set1  (1 ("hello")) ; # equivalent to 1a (or 1b)

Languages using CG in Apertium

and many others. The following languages currently (2014-06-27) have CG's of over 100 rules:

When is CG needed?

Currently some of the CG rules written in the above language pairs may be written as forbid rules in the TSX format used by apertium-tagger. If the rules for your language pair can be written in the .tsx format, you can go for an easier design without a CG module in that language pair.

Editor support

  • CG-3 IDE – the official vislcg3 CG IDE
  • Gedit syntax highlighting (also for any other editor that uses gtksourceview)
  • Emacs emacs mode for editing and testing CG grammars (highlighting + IDE-like features)

See also

External links

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