A.9.13 Enabling monotonic CLP(FD)

In the default execution mode, CLP(FD) constraints still exhibit some non-relational properties. For example, adding constraints can yield new solutions:

?- X #= 2, X = 1+1.
false.
?- X = 1+1, X #= 2, X = 1+1.
X = 1+1.

This behaviour is highly problematic from a logical point of view, and it may render declarative debugging techniques inapplicable.

Set the Prolog flag clpfd_monotonic to true to make CLP(FD) monotonic: This means that adding new constraints cannot yield new solutions. When this flag is true, we must wrap variables that occur in arithmetic expressions with the functor (?)/1 or (#)/1. For example:

?- set_prolog_flag(clpfd_monotonic, true).
true.
?- #(X) #= #(Y) + #(Z).
#(Y)+ #(Z)#= #(X).
?- X #= 2, X = 1+1.
ERROR: Arguments are not sufficiently instantiated

The wrapper can be omitted for variables that are already constrained to integers.