XPDL Syntax Tips

This section contains several XPDL syntax tips to reduce typing effort and write shorter rules.

Tip 1

If a ruleset consists of a single rule that has no child rules and outputs the whole query match, it is enough to write only the query. For example, instead of standard rule form as in Figure 1, one could write only the query as in Figure 2.

xpdl simpl rule
Figure 1. Standard rule form
Rule fragment 
rule: dict_cities
 {
 /* for example, New York, Munich, Rome */
 query: {dictword(GeoAdministrative, "category = city")}:m

 result: City = $m
 }
xpdl simpl query only
Figure 2. Short rule form
Rule fragment 
dictword(GeoAdministrative, "category = city")

In this case, the rule body as well as the result section are generated automatically. Note that in this mode it is impossible to change built-in rule and result names ("xpdl_generated_rule" and "Match" respectively) or to add attributes to the result.

Tip 2

The "$&" expression can be used in the result section to refer to the complete text matched by the rule query which not enclosed in the named group (Figure 3).

xpdl simpl ref amp
Figure 3. Using "$&" expression
Rule fragment 
rule: dict_cities
 {
 /* for example, New York, Munich, Rome */
 query: dictword(GeoAdministrative, "category = city")

 result: City = $&
 }

Tip 3

The result section at the top of the file is automatically added to the rules that do not have result sections. This can be useful to avoid copy-pasting if several rules contain the same result. For example, a ruleset in Figure 4 is equivalent to a ruleset in Figure 5.

xpdl simpl def results
Figure 4. Shortened result section
Rule fragment 
result: City = $m

 rule: dict_cities
 {
 /* for example, New York, Munich, Rome */
 query: {dictword(GeoAdministrative, "category = city")}:m
 }

 rule: context_cities
 {
 /* for example, town of Rajpura, city of Ziyun */
 query: phrase(0, form(city, town), of, {case(title)}:m)
 }
xpdl simpl def results2
Figure 5. Standard rule form
Rule fragment 
 rule: dict_cities
 {
 /* for example, New York, Munich, Rome */
 query: {dictword(GeoAdministrative, "category = city")}:m

 result: City = $m
 }

 rule: context_cities
 {
 /* for example, town of Rajpura, city of Ziyun */
 query: phrase(0, form(city, town), of, {case(title)}:m)

 result: City = $m
 }