Understanding comment in pl-comp.c

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1

The following line in this comment in pl-comp.c has me confused.

This is the case, for predicates for which the body is defined from another module as the head and the predicate is not a meta-predicate.

It has me thinking the head of a predicate is in one module and the body is another, but I don’t see how that can be right.

Here is the entire comment:

Now compile the body. After the I_ENTER, we check whether we need to insert an I_CONTEXT instruction to change the context. This is the case, for predicates for which the body is defined from another module as the head and the predicate is not a meta-predicate. In principle, we could delay this until we decided there may be a meta-call, but this will harm automatic update if a predicate is later defined as meta-predicate.

Not that this is also the case for predicates that have previous clauses that have an I_CONTEXT because we need to reset the context.

2

I think this was discussed a while back. This happens for the code below. While p/1 lives in module b, the body of this particular clause executes in the context of the module where it defined: a.

module a

:- multifile b:p/1.

b:p(X) :- q(X).

q(X) :- ...
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