Confusion over functor versus predicate -- and predicates in general

[peter.ludemann]

There is some discussion that might help you here:

How does prolog tell the difference between atoms and zero arity predicates?

true is a predicate, but ?- atom(true). true. ?- functor(true, X, 0). X = true. How can it be atom and functor at the same time? From here, they should be different entities. Can you explan me what the functor really is? Are there strict Prolog language specification which I can refer to?

What is meant by "meta-level" in logic programming?

Yes, but call/n can be defined as a first-order thing (just add a corresponding call/n clause for every predicate in the program), so I don’t think of it as something special. Others might disagree. “Thunk” is, I suppose an old-fashioned word for what is now called “closure” (although the definition of “closure” is somewhat variable, it seems) – basically, a pointer to code plus some environment. See also delimited continuations, which I don’t fully understand. So, it might be useful to have…