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In reality, and especially in larger systems, do people really use the database a lot, in Prolog? Feels ilke debugging or even understanding someone else’s code would be difficult, at scale. Though I suppose if done right it wouldn’t be that different than using a SQL db.
Like @emiruz I use functional-type constructs a ton in Prolog, and only rarely use the database – usually just as a convenient cache.
CLP is neat but I’m not sure I’ve ever had a need.
I just took delivery of the Frühwirth CHR book, but I guess that’s not a core part of the language => doesn’t quite count as an essential difference.
So I’m with @meditans – to me the essential difference, and the reason I still use Prolog today, is thinking of the computation as search.
That said, I must say that working in Prolog, or at least with SWI, has another benefit for me: direct, straightforward, declarative mechanisms, with solid defaults, for doing even basic things, e.g. filesystem interaction. On the surface this isn’t even an issue of the conceptual core of the language – it’s just nice APIs that sometimes remind me of the simplicity and directness of commands in BASIC when a child.