We've discussed filters and transforms as applying to specific kinds of set expressions: those returning classes, or those returning methods. This concept is expressed more formally in the language with expression types. A set expression has a type, which represents the type of objects that the expression returns (this type is distinct from any type within the analyzed system.) BBQ supports a hierarchy of types that is fully described in the reference: it includes: classes, methods, fields, method calls, field references, types (Java types), arguments, packages, and strings and numbers in expressions.

Most transforms can only be applied to certain types: for example, methods in can only be applied to a set expression with type class, and the resulting expression has type method. The reference page for each transform lists the types to which the transform may be applied, as well as the type that will result from applying the transform.

Similarly, most filters may only be applied to certain types, which are listed on the reference page for the filter. From the definition of filters you can see that the output type of a filter is always identical to its input type.

When you combine two set expressions with an operator, the resulting expression is assigned the most specific type that is a base type of both the constituent expressions. Similarly, BBQ has rules for determining the applicable and result types of complex transforms and filters. You probably don't need to know the details of the rules, but BBQ uses them to determine if the expressions you write make sense. For example, the transform calls to intersection fields in can't work because it is combining the two transforms calls to and fields in;the calls to transform applies to methods while the fields in transform applies only to classes. There is no set expression type to which both transforms could sensibly be applied. If you are using BBQ and receive an error message about Incompatible types, you've probably made this type of mistake in writing your set expression. The complete type resolution rules are listed in the reference.