Meta-Predicates in Prolog

Meta-predicates, also called higher-order predicates, let us dynamically call and inspect predicates and help us write short and correct programs. Among the most useful meta-predicates are the call/N, maplist/N and foldl/N families of predicates. More information: www.metalevel....

[@michaelkohlhaas4427]

*Your videos are unbelievable. Treasure trove!*

[@skc909887u]

Such a powerful underutilised language . !!

[@abeard1]

Agree, totally different aproach than mainstream one

[@p.z.8355]

I dont understand the start of the video. What is the query in 0:33 ? Is the first predicate equal to Goal(X) :- X=1+2. ?

[@ThePowerOfProlog]

The goal is a conjunction, using the built-in predicate ','/2 with two arguments:
The first conjunct is a goal that uses the predicate (=)/2. It is true if Goal is unifiable with the term (X #= 1+2), i.e., the compound term #=(X, +(1,2)). Since Goal is a variable, this is the case, using the substitution Goal = (X #= 1+2). Note that the only predicate that is invoked at this point (besides conjunction) is the built-in predicate (=)/2.
The second conjunct is Goal. This is equivalent to the goal call(Goal), which - operationally speaking - invokes Goal. In this case, at this point of the execution, Goal is the term (X #= 1+2), and so invoking Goal in this case is equivalent to posting the goal X #= 1+2 directly.
However, note that the predicate (#=)/2 is invoked dynamically in this case, and does not appear as a goal at compilation time anywhere in the query.
This example shows that invoking goals dynamically "ships" as built-in feature of Prolog: We can simply form an arbitrary goal G we want to invoke, denoted as a Prolog term, and then invoke it by posting G, or - equivalently - the goal call(G).

[@douglasmennella4525]

I'm a little confused by the term "partial goal". Isn't the first argument to call/2 in call(=(X),a), the functor =/1 and not =/2 with the second arg missing? Does call/2 rewrite this =/1 predicate to be =/2?

[@douglasmennella4525]

I guess this is the definition of a partial goal, but the pendant in me wants to know if this is strictly speaking re-writing of a predicate with fewer arguments. I suppose I should simply understand that as an implementation goal.

[@ThePowerOfProlog]

@@douglasmennella4525 In the goal call(=(X), a), the first argument is a term with principal functor = and arity 1. There is no connection between the term =(X) and any predicate that - one could say: coincidentally - has the same name as the functor of this term, such as the predicate (=)/2 or any other predicate with predicate name =. Only when call/N actually calls the predicate that is indicated by the first argument (and additional arguments specified via call/N) does a connection become apparent, in the sense that the predicate that is indicated by the given name and arguments is called. The term =(X) in itself is not intended as a goal, and there is not even a predicate (=)/1 defined in this example. We say it is a partial goal, because it becomes the intended goal when an additional argument is added to this term.

[@douglasmennella4525]

@@ThePowerOfProlog Thanks for replying so many years after the original post. I think I follow. I was latching on hard to the idea that =/1 has no relation to =/2 so much so that I missed the intent.