Domains and in/2

Use in/2 to post an integer domain for a variable. Domains restrict possible values and drive propagation.

Domain forms

• Interval: Low..High (inclusive bounds)
• Union: D1 \/ D2 (domain union)
• Finite set: {1,3,5}

Examples

?- X in 1..9, Y in {1,3,5}.
true.

?- X in 5..5.
% X is fixed to a single value
true.

Combining domains

Posting multiple in/2 goals for the same variable intersects domains.

?- X in 1..10, X in {2,4,6,8,10}.
true.
?- label([X]).
X = 2 ; X = 4 ; X = 6 ; X = 8 ; X = 10.

Unions and holes

Use union \/ to create non‑contiguous domains.

?- X in (1..3) \/ (7..9).
true.
?- label([X]).
X = 1 ; X = 2 ; X = 3 ; X = 7 ; X = 8 ; X = 9.

Good practices

• Post domains early to maximize propagation.
• Use the tightest bounds you know; wide domains increase search.
• Prefer intervals/unions for performance over large explicit sets.