Terms

Terms are the data values of Prolog. PyLog supports the standard term types:

• Atoms: a, foo, [], 'Hello', '@<'
• Integers: 0, 42, -3
• Variables: X, Y, _, _Tmp, _G123
• Compound terms: point(1,2), edge(a,b), tree(Left,Key,Right)
• Lists (syntactic sugar): [], [a,b,c], [H|T], [a,b|T]

Atoms

• Unquoted atoms start with a lowercase letter and may contain letters, digits, and _.
• Use single quotes for atoms with spaces or special characters: 'hello world', '->'.
• [] is the empty list; it’s also an atom. The list sugar desugars to the functor '.'/2 internally.

Variables

• Variables start with an uppercase letter or underscore: X, Value, _Tail.
• _ is the anonymous variable; each occurrence is distinct and never binds across occurrences.
• Variable scope is a single clause (fact or rule). Repeated occurrences of the same variable name within a clause must unify to the same value.

Compound terms

• A compound term has a functor name and a fixed arity: functor/arity.
• Examples: point(1,2) has functor point and arity 2; tree(L,K,R) has functor tree and arity 3.
• Use functor/3 and arg/3 to inspect, or =../2 (univ) to convert between a term and list form.

Lists

• [a,b,c] is shorthand for .(a,.(b,.(c,[]))).
• [H|T] matches a nonempty list with head H and tail T.
• Common patterns:

?- [H|T] = [a,b,c].
H = a, T = [b, c].

?- [X,Y|Rest] = [1,2,3,4].
X = 1, Y = 2, Rest = [3, 4].

Quoted atoms and operators

• Operators like ,, ;, ->, =.. are atoms in canonical form when quoted, e.g. ',', ';', '->'.
• The reader uses precedence and associativity to parse operator expressions into the underlying term structures. See Basics → Operators.

Examples

Unification builds and deconstructs terms:

?- X = point(1, 2).
X = point(1, 2).

?- point(X, 2) = point(1, Y).
X = 1, Y = 2.

?- edge(a,b) =.. L.
L = [edge, a, b].

?- T =.. [edge, a, b].
T = edge(a, b).