Alternative Specifications¶

Sometimes the format in which the tuples of a specific relation are represented can vary according to the query made to the source, but without it being possible to determine a priori which of the output formats is going to be used.

Example: In many electronic shops, when a search offers just one product as a result, it jumps directly to the format of product detail page instead of the usual search results page. However, before launching the search it is not possible to know if this will return one or several products as a result, whereby it is not possible to determine a priori which pattern should be used to extract the tuples from the relation.

DEXTL provides a function to deal with this type of case, which consists in defining DEXTL elements to extract tuples from all the possible document formats and separate them with the symbol || to indicate which are alternative elements. The DEXTL program will try the top element first. If it does not find tuples that match, it goes on to try the second and so forth. Given that the program goes to the element in the top position first, it is a good idea to put the element that represents the most frequent format in said position.

Thus, for the above example, a specification of the following type could be constructed:

{
    //Pattern of the search result page
    .....
}
||
{
    //Pattern of the product detail page
    .....
}