User-definable error messages for integrity constraints

 

ConceptBase provides a couple of errors messages in case of an integrity violation. These errors messages refer to the logical definition of the constraint and are sometimes hard to read. To provide more readable error messages, one can attach so-called hints to constraint definitions. These hints are attached as comments with label hint to the attribute that defines the constraint.

Consider the salary bound constraint above. A hint could look like:

Employee!SalaryBound with
 comment
   hint: "An employee may not earn more than her/his manager!"
end

It is also possible to attach hints to meta-level constraints. In this case, the hint text can refer to the meta-level variables occuring in the meta-level constraint. These variables will be replaced by the correct fillers when the meta-level constraint is utilized in some modeling context.

Assume, for example, we want to have a symmetry category and attach a readable hint to it:

Proposition with
  attribute
    symmetric: Proposition
end

RelationSemantics in Class with
  constraint
 symm_IC: $ forall AC/Proposition!symmetric C/Proposition x,y/VAR M/VAR
                     P(AC,C,M,C) and (x in C) and (y in C) and
                     (x M y)  ==> (y M x) $
end

RelationSemantics!symm_IC with
 comment
   hint: "The relation {M} of {C} must be symmetric,
          i.e. (x {M} y) implies (y {M} x)."
end

Note that the references to the meta variables M and C are surrounded by curly braces, and that these meta variables are also occurring in the meta-level constraint. Now, use the symmetric concept in some modeling context, e.g. to define that the marriedTo attribute of Person should be symmetric:

Person with
  attribute,symmetric
   marriedTo: Person
end

At this point of time, ConceptBase will find the hint text for the symmetric constraint and will adapt it to the context of C=Person and M=marriedTo. When an integrity violation occurs, the substituted hint

    "The relation marriedTo of Person must be symmetric,
     i.e. (x marriedTo y) implies (y marriedTo x)."

will be presented to the user. An example violation is:

bill in Person with
  marriedTo m1: eve
end

eve in Person end

One can also define a hint for the meta-level constraint that refers only to a (non-empty) subset of the meta variables. If a hint for a meta formula cannot be substituted as shown avove, ConceptBase will not issue the hint but rather the text of the generated formula.