What is the meaning of predicate “simple/1” in Prolog (SWI-Prolog)

Question

I run into problem while reading a book. I see a program use predicate "simple" ( I guess simple/1 ). I don't know what is the meaning of this predicate, I can't find it with ?-help(simple) in the console. But when I tried with some queries in console, it worked something like:

5 ?- simple(p(x)).
false.

6 ?- simple(mia).
true.

7 ?- simple(Mia).
true.

8 ?- simple(f(Mia)).
false.

I guess it is some sort of predicate to determine if the argument was Terms(or Variables) or Complex Terms.

Answer 1

The swi-prolog manual has the following definition:

simple(@Term) is semidet Term is atomic or a variable.

the definition is in the quintus prolog compatibility library; in the quintus prolog documentation the definition is:

simple(+Term)

Term is currently instantiated to either an atom, a number, a database or a variable.

in any case, simple/1 is true if the argument is a simple term (not sure what the quintus manuals means by database; possibly a handler for an ODBC connection i guess)

Answer 2

translated to ISO predicates:

simple(T) :- var(T) ; atomic(T).

var/1 it's the most basic metaprogramming device, because it's impossible to predicate (i.e. execute code, binding variables) about any clause without instancing the variables, that are many times the essential part we are interested to.