alt Hacker NewsIt doesn't scale that well because integers are their own, opaque thing in Prolog, and the is predicate is unidirectional. However, there's no inherent reason this has to be the case: you can construct your own representation of the integers from the Peano axioms, and recover bidirectionality of addition. (You'll want to be using some typed variant of Prolog with the "unary tree of units" optimisation, otherwise integers take O(n) space in memory, but it's possible in the language.) Prolog works how it does, because it's (mostly) backwards-compatible with a facts database used for symbolic AI research, where all atoms were opaque.
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“you can construct your own representation of the integers from the Peano axioms”
This is how the Idris prelude defines nat, the type of natural numbers (with some magic making it actually fast). I think that’s very cool.
There are many ways to recover relational behaviour. Libs such as clp(fd) (superseded by clp(Z)), clp(BNR) and others.
Modern Prolog libraries have handled the issues of unidirectional integer predicates, so the old ways of handling numeric values is not relevant for most problems.
Check out the CLP(ℤ) library by Markus Triska, who is also the author of Scryer Prolog. It defines new comparison predicates that lets you use bidirectional logical programming on standard integers with standard math operations.
https://github.com/triska/clpz