Constrained Objects

EXPRESS uses constraints to ensure that data conforms to the intended semantics of the information model. This lesson covers the constraint mechanisms available in the language.

Local Constraints

WHERE Rules

WHERE rules constrain individual entity instances:

ENTITY resistor;
  resistance : REAL;
WHERE
  positive_resistance : resistance > 0;
END_ENTITY;

UNIQUE Rules

UNIQUE rules enforce uniqueness of attribute values:

ENTITY component;
  part_number : STRING;
  package_type : STRING;
UNIQUE
  unique_part : part_number, package_type;
END_ENTITY;

Global Constraints

Global rules can span multiple entities and reference multiple instances:

RULE unique_product_ids FOR (product);
  LOCAL
    ids : SET OF STRING := [];
  END_LOCAL;
  REPEAT i := 1 TO SIZEOF(QUERY(p <* product | TRUE));
    IF p[i].id IN ids THEN
      RETURN(FALSE);
    END_IF;
    ids := ids + p[i].id;
  END_REPEAT;
  RETURN(TRUE);
END_RULE;

Derived Attributes

Attributes can be computed from other attributes:

ENTITY board;
  width : REAL;
  height : REAL;
DERIVE
  area : REAL := width * height;
END_ENTITY;

Inverse Attributes

Inverse attributes constrain relationship cardinality:

ENTITY assembly;
  components : SET [1:?] OF component;
END_ENTITY;

ENTITY component;
  parent : assembly;
INVERSE
  belongs_to : assembly FOR components;
END_ENTITY;

Functions and Procedures

EXPRESS functions encapsulate reusable logic:

FUNCTION positive_real(val : REAL) : BOOLEAN;
  RETURN(val > 0);
END_FUNCTION;

Functions can be called from WHERE rules, derived attributes, and other functions.