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14.8. Adding Constraints to Facts

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Table 14.3. Adding Constraints to Facts
Name Description Example
Expressing LHS conditions
The DSL facility allows you to add constraints to a pattern by a simple convention: if your DSL expression starts with a hyphen (minus character, "-") it is assumed to be a field constraint and, consequently, is is added to the last pattern line preceding it.
In the example, the class Cheese, has these fields: type, price, age and country. You can express some LHS condition in normal DRL. 
Cheese(age < 5, price == 20, type=="stilton", country=="ch")
DSL definitions
The DSL definitions given in this example result in three DSL phrases which may be used to create any combination of constraint involving these fields. 
[when]There is a Cheese with=Cheese()
[when]- age is less than {age}=age<{age}
[when]- type is '{type}'=type=='{type}'
[when]- country equal to '{country}'=country=='{country}'
"-"
The parser will pick up a line beginning with "-" and add it as a constraint to the preceding pattern, inserting a comma when it is required. 
There is a Cheese with
        - age is less than 42
        - type is 'stilton'
 
Cheese(age<42, type=='stilton')
Defining DSL phrases
Defining DSL phrases for various operators and even a generic expression that handles any field constraint reduces the amount of DSL entries. 
[when][]is less than or equal to=<=
[when][]is less than=<
[when][]is greater than or equal to=>=
[when][]is greater than=>
[when][]is equal to===
[when][]equals===
[when][]There is a Cheese with=Cheese()
[when][]- {field:\w*} {operator} {value:\d*}={field} {operator} {value}
DSL definition rule n/a 
There is a Cheese with
   - age is less than 42
   - rating is greater than 50
   - type equals 'stilton'
In this specific case, a phrase such as "is less than" is replaced by <, and then the line matches the last DSL entry. This removes the hyphen, but the final result is still added as a constraint to the preceding pattern. After processing all of the lines, the resulting DRL text is: 
Cheese(age<42, rating > 50, type=='stilton')

Note

The order of the entries in the DSL is important if separate DSL expressions are intended to match the same line, one after the other.
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