Difference between revisions of "Morphological disambiguator"
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=== Constraints === | === Constraints === | ||
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| + | Think of constraints as a series of heuristics to whittle down the number of analyses until you have as little ambiguity as possible. | ||
The most commonly used constraints are to either '''remove''' a reading under a certain condition or '''select''' a reading under a certain condition. The syntax is e.g., <code>REMOVE <target> [contextual tests] ;</code>. | The most commonly used constraints are to either '''remove''' a reading under a certain condition or '''select''' a reading under a certain condition. The syntax is e.g., <code>REMOVE <target> [contextual tests] ;</code>. | ||
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Contextual tests come together in ()s. All tests must match in order for the constraint to apply. Inside the parentheses, the first definition is a position: <code>0</code> means the current cohort, <code>-1</code> means the previous cohort, etc. The rest of the test is what should be matched. If you want to match any member of the <code>Nominals</code> set in the previous position, it would be <code>(-1 Nominals)</code> You can match tag sequences by adding an extra set of parentheses, e.g. <code>(1 (n pl))</code> would match a plural noun in the following position. You can match specific baseforms with quotation marks, e.g. <code>(1 ("house" n))</code> would match any form of the noun "house" in the following position. | Contextual tests come together in ()s. All tests must match in order for the constraint to apply. Inside the parentheses, the first definition is a position: <code>0</code> means the current cohort, <code>-1</code> means the previous cohort, etc. The rest of the test is what should be matched. If you want to match any member of the <code>Nominals</code> set in the previous position, it would be <code>(-1 Nominals)</code> You can match tag sequences by adding an extra set of parentheses, e.g. <code>(1 (n pl))</code> would match a plural noun in the following position. You can match specific baseforms with quotation marks, e.g. <code>(1 ("house" n))</code> would match any form of the noun "house" in the following position. | ||
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Revision as of 07:28, 23 February 2017
Contents
Why we need disambiguation
Imagine you have a word that has two different tagsets, e.g.
^houses/house<n><pl>/house<v><tv><p3><sg>$ ^this/this<det><dem><sg>/this<pron><dem><sg>$
Normally your analyser will randomly choose one of these. Now imagine you have a sentence where the wrong analysis is chosen (in this case, twice!):
^The/The<det><def>$ ^motel/motel<n><sg>$ ^houses/house<n><pl>$ ^this/this<pron><dem><sg>$ ^dog/dog<n><sg>$^./.<sent>$
The goal of a disambiguator is to choose the correct analysis based on the surrounding words.
Using Constraint Grammar to disambiguate
Constraint Grammar (CG) is a formalism that can be used to formalise context sensitive constraints to select or remove analyses from the list of possible analyses.
The structure of a CG file
At the top you define delimiters. You can then add lists (which are kind of like sets in twol). After that you can have any number of constraints. You can also put lists and constraints in sections: all the constraints in each section are applied simultaneously, but the sections are applied in order and lists are always global.
Constraints
Think of constraints as a series of heuristics to whittle down the number of analyses until you have as little ambiguity as possible.
The most commonly used constraints are to either remove a reading under a certain condition or select a reading under a certain condition. The syntax is e.g., REMOVE <target> [contextual tests] ;.
Targets can be defined as list names—like Nominals—or individual tag names, in parentheses—e.g., (n pl). This target identifies the reading to select or remove.
Contextual tests come together in ()s. All tests must match in order for the constraint to apply. Inside the parentheses, the first definition is a position: 0 means the current cohort, -1 means the previous cohort, etc. The rest of the test is what should be matched. If you want to match any member of the Nominals set in the previous position, it would be (-1 Nominals) You can match tag sequences by adding an extra set of parentheses, e.g. (1 (n pl)) would match a plural noun in the following position. You can match specific baseforms with quotation marks, e.g. (1 ("house" n)) would match any form of the noun "house" in the following position.
Example above
Useful commands
Getting output before disambiguation
Getting output after disambiguation
Calculating ambiguity
Seeing which constraints are doing what
The assignment
- Add words
- Measure ambiguity
- Identify ambiguous form
- Write rule
- Measure ambiguity
More resources
- Constraint Grammar on the Apertium wiki
- Apertium and Constraint Grammar on the Apertium wiki
