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|Overruling Module Boundaries|
The module system described so far is sufficient to distribute programs over multiple modules. There are, however, cases in which we would like to be able to overrule this schema and explicitly call a predicate in some module or assert explicitly into some module. Calling in a particular module is useful for debugging from the user's top level or to access multiple implementations of the same interface that reside in multiple modules. Accessing the same interface from multiple modules cannot be achieved using importing because importing a predicate with the same name and arity from two modules results in a name conflict. Asserting in a different module can be used to create models dynamically in a new module. See section 6.13.
Direct addressing of modules is achieved using a
explicitly in a program and relies on the module qualification mechanism
described in section 6.5.
Here are a few examples:
?- assert(world:done). % asserts done/0 into module world ?- world:asserta(done). % the same ?- world:done. % calls done/0 in module world
Note that the second example is the same due to the Prolog flag
The system predicate asserta/1
is called in the module
world, which is possible because
system predicates are visible in all modules. At the same time,
calling context is set to
world. Because meta
arguments are qualified with the calling context, the resulting call is
the same as the first example.
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.
For example, the code
asserta(done)@world is the same as
asserta(world:done). Unlike in
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, @).