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user:zeman:interset:how-to-write-a-driver [2007/09/26 20:41]
zeman Link. 		user:zeman:interset:how-to-write-a-driver [2007/10/01 13:53]
zeman use tagset::common;
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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: ''decode()'', ''encode()'', ''list()''. The ''tagset/common.pm'' module contains some code you may have use for, so your driver module should start with
  
-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 "\t" (horizontal tab, ASCII 9) as delimiter. If desirable, the input/output tag can be even a multi-line XML!+<code perl>use tagset::common;</code> 
 + 
 +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 "\t" (horizontal tab, ASCII 9) as delimiter. If desirable, the input/output tag can be even a multi-line XML!
  
 Empty feature value means "unknown". It is even not known, whether this feature would be relevant. If we know that a feature is irrelevant, we can set it to "n/a" (not applicable; although not mentioned explicitly, this value is allowed for all features). However, I am not sure whether this should be used at all. While something can be irrelevant in one tagset, we can hardly say that it is not relevant in any tagset. So, since we are setting a value in a universal "tagset", we probably better leave the value empty or even set it to an appropriate default. Empty feature value means "unknown". It is even not known, whether this feature would be relevant. If we know that a feature is irrelevant, we can set it to "n/a" (not applicable; although not mentioned explicitly, this value is allowed for all features). However, I am not sure whether this should be used at all. While something can be irrelevant in one tagset, we can hardly say that it is not relevant in any tagset. So, since we are setting a value in a universal "tagset", we probably better leave the value empty or even set it to an appropriate default.
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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).
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 ===== encode() ===== ===== encode() =====
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 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. 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::single_values() to get rid of the arrays (or some of them).+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::single_values()'' to get rid of the arrays (or some of them). See [[#Alternative values]] for more details.
  
-**WARNING:** Before modifying the contents of ''%f'', you should make a //deep// copy of it. You cannot assume that the user of the driver will not need the values in ''%f'' after encoding.+**WARNING:** Before modifying the contents of ''%f'', you should make a //deep// copy of it. You cannot assume that the user of the driver will not need the values in ''%f'' after encoding. If you have called ''single_values()'', it made the copy for you.
  
 ===== list() ===== ===== list() =====

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