Quintus' derived module systems have no means to separate the lookup
module (for finding predicates) from the calling context (for qualifying
meta arguments). Some other Prolog implementations (e.g., ECLiPSe and
IF/Prolog) distinguish these operations, using
setting the calling context of a goal. This is provided by SWI-Prolog,
currently mainly to support compatibility layers.
- @(:Goal, +Module)
- Execute Goal, setting the calling context to Module.
Setting the calling context affects meta-predicates, for which meta
arguments are qualified with Module and transparent
It has no implications for other predicates.
For example, the code
asserta(done)@worldis the same as
asserta(world:done). Unlike in
world:asserta(done), asserta/1 is resolved in the current module rather than the module
world. This makes no difference for system predicates, but usually does make a difference for user predicates.
Not that SWI-Prolog does not define
@as an operator. Some systems define this construct using
op(900, xfx, @).