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

General
1

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?

2

A functor is just the name and arity of a compound term. For example, the compound term atom(true) has the functor atom/1, meaning the name of the term is atom and its arity is one.

By the same logic, the term true has functor true/0, meaning the name of the term is true, and its arity is zero.

A predicate also has a functor. The head of a predicate (the part before the :- operator) determines a predicate’s functor. So if you defined the predicate:

hello_world :- print("Hello World"), nl.

That predicate would have the functor hello_world/0, the name of the predicate is hello_world and the arity is zero.

Hope that helps.

3

I don’t think there’s any problem with true/0 being a control predicate.

Can use functor/4 to see difference (atom vs compound), for SWI-Prolog -- Compound terms with zero arguments

?- P = true, functor(P, F, A, T).
P = F, F = true,
A = 0,
T = atom.

vs:

?- P = true(), functor(P, F, A, T).
P = true(),
F = true,
A = 0,
T = compound.
4

Compounds and atoms (or any other atomic like integer or a string) are different entities when you look at them from some angles, for example:

?- atom(foo(bar)).
false.

?- compound(foo(bar)).
true.

?- compound(foo).
false.

?- atom(foo).
true.

Some predicates that work for compound terms just define atomics as having themselves as a name and having arity 0. This is useful and doesn’t break anything. Try with strings or maybe floats:

?- functor(2.5, Name, Arity).
?- functor("hello", Name, Arity).

See also the SWI-Prolog glossary of terms:

functor
Combination of name and arity of a compound term. The term foo(a, b, c) is said to be a term belonging to the functor foo/3 . foo/0 is used to refer to the atom foo.

1 Like
5

Prolog uses “ambivalent syntax”. The same symbol can be a predicate symbol, or a function symbol of any arity (this includes constants). Such a symbol is called “atom”.