TAtalideon·Jun 21, 2020
This is something that unfortunately doesn't have a straightforward answer. Prolog ended up being very influential, but its very nature meant it was never going to become mainstream.
Prolog was intended to be a way to program using logic. It did this in a way by encoding facts in a manner not dissimilar to a relational database; logical statements about the world that resembled functions using Horn clauses (which you can think of as mathematical functions, but applied to logic); and unification, which is what links all this together. Unification is computation via the idea that you have a set of data, a bunch of logical statements, and you want to find everything in the data that satisfies those logical statement.
If you know SQL, Horn clauses should sound a lot like materialised views.
As to why it never really went mainstream, it's a ma
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tter of wrong place, wrong time. It didn't help that the language needed a checkpointing mechanism (referred to as 'cuts', and represented by '!') because the unification mechanism could get confused and do much more work than it needed to, so developers needed to put cuts in place to tell the interpreter where it could avoid backtracking to try more solutions within a Horn clause.
If you want to get a taste for it, maybe give this a try: https://github.com/norswap/prolog-dry
Prolog is one of a small number of languages that people should learn even if they never use it in a practical way.