CLP(FD) Introduction

Constraint Logic Programming over Finite Domains (CLP(FD)) lets you state relations between integer variables and prune their possible values before search.

You post domain limits with in/2, add constraints such as #=, and then search with label/1 to obtain concrete values.

Example:

?- X in 1..9, Y in 1..9, X + Y #= 10, label([X,Y]).
X = 1, Y = 9 ;
...
X = 9, Y = 1.

PyLog also supports CLP(FD) reification operators that relate a Boolean to a constraint's truth, for example B #<=> (X #= Y) or B #==> (X #< 5). See CLP(FD) → Reification.