JOIN operator

The JOIN operator creates a property that implements a composition.

Syntax

[JOIN] mainProperty(expr1, ..., exprN)

Where mainProperty is defined as:

propertyId

"[" operator "]"

"[" expression "]"

Where "[" and "]" are ordinary brackets.

Description

When the main property is given in brackets — as a context-independent property operator or an expression — the property is built anonymously at the place of use, so an intermediate property need not be declared via the = statement.

An operator or expression in brackets may reference external parameters — the parameters already declared in the context where the JOIN operator is used. They are passed to the anonymous property automatically; its remaining parameters follow the same rules as those of a property defined with = without explicit parameters.

Parameters

• propertyId

  Property ID of an existing property.

• operator

  A context-independent property operator in brackets.

• expression

  An expression in brackets.

• expr1, ..., exprN

  List of expressions supplying the arguments of the main property. The number of expressions must equal the main property's parameter count; the list is empty when the main property has no parameters. The external parameters referenced in brackets are not counted and are not listed among the arguments.

Examples

f = DATA INTEGER (INTEGER, INTEGER, INTEGER);
g = DATA INTEGER (INTEGER, INTEGER);
h = DATA INTEGER (INTEGER, INTEGER);
c(a, b) = f(g(a, b), h(b, 3), a);

count = DATA BPSTRING[255] (INTEGER);
name = DATA BPSTRING[255] (INTEGER);
formatted(INTEGER a, INTEGER b) = [FORMULA BPSTRING[255] ' CAST($1 AS TEXT) || \' / \' || CAST($2 AS TEXT)'](count(a), name(b));

Sometimes it’s convenient to define the main property with an expression to simplify the source code and make it more understandable.

CLASS Triangle;
cathetus1 = DATA DOUBLE(Triangle);
cathetus2 = DATA DOUBLE(Triangle);

hypotenuseSq(triangle) = cathetus1(triangle)*cathetus1(triangle) + cathetus2(triangle)*cathetus2(triangle);

// a similar property set using composition
hypotenuseSq2(triangle) = [ x*x + y*y](cathetus1(triangle), cathetus2(triangle)); 

External parameters referenced in brackets are not listed among the arguments.

CLASS Book;
name = DATA STRING[100] (Book);
CLASS Order;
CLASS OrderDetail;
order = DATA Order (OrderDetail);
book = DATA Book (OrderDetail);
quantity = DATA INTEGER (OrderDetail);

// k is an external parameter, so the only argument is the value of x
scaledQuantity(OrderDetail d, INTEGER k) = [x*k](quantity(d));

showQuantities (Order o)  {
    // o is an external parameter, so the only argument corresponds to the BY expression;
    // this argument declares the new parameter b, and the loop iterates over the books of the order
    FOR INTEGER q = [GROUP SUM quantity(OrderDetail d) IF order(d) = o BY book(d)](Book b) DO
        MESSAGE name(b) + ': ' + q;
}