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user:zeman:interset:how-to-write-a-driver [2007/09/26 20:34] zeman Link. |
user:zeman:interset:how-to-write-a-driver [2007/10/01 15:37] zeman Replacing arrays. |
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====== How to write a driver ====== | ====== How to write a driver ====== | ||
- | Perl is the language to write a driver. A driver is a simple Perl module (.pm). It should implement the following functions: decode(), encode(), list(). | + | Perl is the language to write a driver. A driver is a simple Perl module (.pm). It should implement the following functions: |
- | Input/output tag can be any string. If the information is stored in several kinds of tags, they can be passed in one string, using some unique delimiters. We recommend " | + | <code perl>use tagset:: |
+ | |||
+ | The input/output tag can be any string. If the information is stored in several kinds of tags, they can be passed in one string, using some unique delimiters. We recommend " | ||
Empty feature value means " | Empty feature value means " | ||
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If the tagset encodes features separately (e.g., each character is a value of a particular feature): The decoder should be tolerant to unexpected combinations of features (or should be able to be tolerant if asked for it). | If the tagset encodes features separately (e.g., each character is a value of a particular feature): The decoder should be tolerant to unexpected combinations of features (or should be able to be tolerant if asked for it). | ||
+ | |||
+ | |||
===== encode() ===== | ===== encode() ===== | ||
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This function has one argument, a reference to a hash of features (feature names are hash keys to the feature values). The function returns a string - the tag. | This function has one argument, a reference to a hash of features (feature names are hash keys to the feature values). The function returns a string - the tag. | ||
- | The encoder should be able to process all possible values from the central pool. If the tagset does not recognize a value, the most appropriate substitute should be chosen. | + | The encoder should be able to process all possible values from the [[features|central pool]]. If the tagset does not recognize a value, the most appropriate substitute should be chosen. |
- | Since any feature can in theory have an array of values instead of a single value, the encoder should either be prepared to arrays (more precisely: array references) anywhere, or call tagset:: | + | Since any feature can in theory have an array of values instead of a single value, the encoder should either be prepared to arrays (more precisely: array references) anywhere, or call '' |
- | **WARNING: | + | **WARNING: |
===== list() ===== | ===== list() ===== | ||
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**Note:** This approach cannot encode situations where some combinations of feature values are plausible and some are not! For instance, if positions [2] and [3] in a tag encode gender and number, respectively, | **Note:** This approach cannot encode situations where some combinations of feature values are plausible and some are not! For instance, if positions [2] and [3] in a tag encode gender and number, respectively, | ||
+ | |||
+ | |||
+ | |||
+ | |||
+ | ===== Replacing feature values with defaults ===== | ||
+ | |||
+ | The encoder' | ||
+ | |||
+ | - A table of replacement values for each value, ordered by precedence. There is a default table in '' | ||
+ | - The list of all tags in the tag set (implemented by the '' | ||
+ | |||
+ | Building the list of permitted values is expensive (all tags must be decoded!) and you should do it only once when your driver initializes. In your '' | ||
+ | |||
+ | <code perl> | ||
+ | use tagset:: | ||
+ | BEGIN | ||
+ | { | ||
+ | # Store the hash reference in a global variable. | ||
+ | $permitvals = tagset:: | ||
+ | } | ||
+ | ... | ||
+ | $replacement = tagset:: | ||
+ | </ | ||
+ | |||
+ | If an array is a permitted value, all member values are permitted. | ||
+ | |||
+ | If an array is checked, all member values must be permitted in order for the array to be permitted. Otherwise, the array is pruned and the replacement is a subarray where only permitted values are kept. If no member values are permitted (hence the pruned subarray would be empty), the replacement is a single value, the highest-priority replacement of the first element of the array. If the original array was empty (which should never happen but we ought to be careful anyway), the single empty value is checked and possibly replaced. | ||
===== Common problems ===== | ===== Common problems ===== |