This method is intended to test on attribute values. Similar and more powerful results can be obtained using class ==.. It's usage is to be preferred. The following code objects and Prolog fragments all test whether the point<-x of the point object @p equals 3:

Prolog:

get(@p, x, 3)              % Uses Prolog unification
send(@p, has_value, x, 3)

Code fragments:

message(@p, has_value, x, 3)
@p?x == 3

object->not_has_value is equivalent to object->has_value, but returns the inverse result. object->is_on is equivalent to object->has_value: @on and object->is_off is equivalent to object->has_value: @off.

See also

- object->is_on
- object->is_off
- class \==
- object->not_has_value
- class ==