[zmonx]

In a sense, yes, these modules are written in Prolog. However, that is not the full story: CLP requires special interface predicates that the underlying Prolog system must provide. You cannot implement CLP "on top" of just any Prolog system with reasonable performance and correctness.

Devising and implementing such suitable interface predicates is quite hard, and only a very small number of Prolog systems have succeeded with this so far.

For example, the SWI interface for CLP is too limited in practice, and its constraint solvers have elementary mistakes also because of the limitations of the interface predicates it provides.

In contrast, SICStus Prolog provides a much more general interface to attributed variables that supports all widely used constraint solvers (finite domains, Boolean variables, rational numbers etc.) with good performance and no known mistakes.

The tutorial you link to has several rather severe shortcomings, and I cannot recommend it for learning CLP(FD). Please see my profile page if you are interested in Prolog resources.