Constraint
logic programming is generally proposed as a more natural alternative for arithmetic, in Prolog, than Prolog's native arithmetic functions.
It's a biig discussion and I'm not the one to make the argument for CLP, but, for example, in straight-up Prolog arithmetic is implemented using the is/2 predicate, which looks a bit like this:
A is 1 + 2.
Where A is the result of the addition of 1 and 2. In CLP on the other hand, particularly in the CLP library for reasoning over integers, CLP(FD), the addition above looks like this:
X #= 1+2.
Note that here, "#=/2" ("equals") is a
constraint, so the statement above is true, iff the expression on the left-hand side of the #= is equal to the right-hand side of it. Which lets you do things like this:
?- 3 #= Y+2.
Y = 1.
Where trying to do the same thing with is/2 will give you an error (because Y is not sufficiently instantiated). And because of how constraint programming works, you can also do arithmetic with
ranges, like so:
?- X #= A + B, A in 1..3, B in 4..6.
X in 5..9,
A+B#=X,
A in 1..3,
B in 4..6.
In fact, what comes out the other end of the query above (the bit after the query prompt, "?") is a set of
new constraints, where the result of adding A in [1,3] and B in [4,6] is a number, X, in [5,9].
You can find more information in the CLP(FD) library documentation, here:
http://www.swi-prolog.org/man/clpfd.html
Or, if Markus Triska (the author of the library) is around, he can probably elucidate its use a bit better than I can :)