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 @/2
for 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
predicates (see
module_transparent/1).
It has no implications for other predicates.
For example, the code asserta(done)@world is 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, @).
