1.7.44 CQL: Predicate-generated Attribute Values

It is possible to generate compile time attribute values by specifying a predicate which is executed when the CQL statement is compiled.

The predicate must return the value you want as its last argument. You specify the predicate where you would normally put the attribute value. The predicate is specified with its output argument missing.

Example - Using domain allowed values in a query.

In the following CQL statement the predicate cql_domain_allowed_value/3 is called within findall/3 at compile time to generate a list of domain values that restrict favourite_colour to be’ORANGE’or’PINK’or’BLUE’, or’GREEN’.

colour('ORANGE').
colour('PINK').
colour('BLUE').
colour('GREEN').

{[],
 se_lt_x :: [d-findall(Value,
                       permissible_colour(Value)),
             a-UserName]}

Note how findall/3 is actually called by specifying findall/2.

There is not much point using predicate-generated attribute values in compile-at-runtime CQL as you can always call the predicate to generate the required values outside the CQL statement.