The aim of openVALIDATION's grammar is to be as natural as possible. To get even closer to this goal, it is possible to declare domain-specific or semantic operators in the rules.
the age of the applicant MUST be ATLEAST18
The rule sounds quite natural, but not quite. The rule could sound much more natural if it were expressed as follows:
the applicant MUSTNOT be YOUNGER than 18
That sounds a lot more natural. The problem is that even if openVALIDATION would provide an alias for the operator < called YOUNGER, at the end openVALIDATION does not know which attribute of the schema should be used for the comparison operation. We have to tell openVALIDATION that somehow. We do this by defining a user-defined, domain-specific operator:
age and smaller
the applicant must not be younger than 18
We can create a domain-specific comparison operator using a new global element as operator. We can then use this in our rules. In our example, we have created a new comparison operator called younger. The operator itself consists of a concrete comparison operator, namely less and the attribute age of the schema. The keyword and in between is optional.
Domain-specific or semantic operators can be created in advance, e.g. together with the variables, and then serve as a semantic basis for the actual validation rules.