Using the fragment
SHARES: Foo USES STRING bar
one can gain access to (some) of the parameters of other thorns / implementations.
Cactus will refuse to compile if no thorn for the implementation is compiled in. However at runtime no check is made that an implementation is actually active letting one access parameters without an active thorn to them.
Cactus should tread this the same way as a missing active implementation when INHERITing in interface.ccl.